SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com
Edition: Model Aviation - 2004/05
Page Numbers: 44,45,46,47,48,49,50,52,54,56,58
SEVERAL YEARS AGO we first heard of
model-aviation enthusiasts using Lithium-Ion
batteries to power their electric motors in RC
model aircraft. The Lithium-Ion cells came
from the cell-phone industry, which meant
that the supply was abundant. But early on,
modelers began to realize that Lithium-Ion
cells weren’t so adaptable for hobby use.
Along came the Lithium-Polymer cells, or Li-
Poly, and the rest is history.
These new Li-Poly cells have proven to be
a boon to the electric-power enthusiast. They
offer substantial weight savings and
considerably more capacity. Under proper
care, these new cells are much safer to use
than Lithium-Ion cells. The bottom line is that
your electric-powered model can weigh less
and fly for a longer period of time (longerduration
motor run).
That’s the good news, but using these new
battery cells has become a problem for many
modelers and has caused serious safety
concerns. The Li-Poly cell characteristics are
completely different (from Ni-Cd and NiMH
cells). The chargers and charging techniques
are completely different. There are also safety
issues associated with the use of these cells
about which the user must know.
Li-Poly cells are currently more expensive
than the traditional Ni-Cd and NiMH cells.
First-generation Li-Poly cells were more
limited in load current capabilities, but that
aspect has improved greatly in the last year.
In this article I will describe the Li-Poly
cell itself, and then I will go into charging and
discharging and provide application
suggestions for using this new form of electric
power. I will also revisit my little Scratch-One
RC Electric sailplane design which was
presented in the November 2003 and January
2004 “From the Ground Up” installments. I’ll
show you exactly how I replaced a NiMH
battery pack with one consisting of Li-Poly
cells. That way you will be able to relate
easily to the weight reduction along with the
increased capacity (longer flight time!).
Li-Poly Cell Characteristics: The basic Ni-
Cd or even NiMH battery cells can reach
approximately 1.4 volts at full charge and are
usually stated as having a nominal or average
voltage of 1.2 volts per cell. Lithium-Ion and
Li-Poly cells (which have the same basic
electrical characteristics) are considered fully
charged at 4.2 volts, and most experts
consider the nominal or average working
voltage to be 3.7 volts per cell.
The preceding can make life more difficult
for the electric flier. When using Ni-Cd or
NiMH with, say, seven cells, if you wanted to
add power you could add one cell, which is
only adding 1.2 volts to the total pack voltage.
The same is true if you wanted to reduce the
power and remove a single cell. The battery
pack’s total voltage would only go down 1.2
volts.
When you are dealing with Li-Poly cells,
the increments are one cell equals 3.7 volts,
two cells equals 7.4 volts, three cells equals
11.1 volts, etc. Tailoring your motor to this
kind of power source can be a challenge. The
7.4 volts might not be enough voltage, and the
11.1 might be too much.
Your only recourse to maintain the
allowable power (wattage) to your motor is to
select a different gear-reduction drive and/or a
different propeller size, which will, in turn,
change your motor current.
Cell Weight: Consider some weight
comparisons. A Sanyo 1250 mAh pack
consisting of seven Ni-Cd cells weighs 11.3
ounces (nominal voltage 8.4). A comparable
1200 mAh pack of eight NiMH cells weighs
7.2 ounces (nominal voltage 9.6). A 1200
mAh pack of three Li-Poly cells weighs 2.4
ounces (nominal voltage 11.1). For essentially
the same capacity (1200 mAh), the packs’
weights went from 11.3 or 7.2 ounces down to
2.4 ounces for the Li-Poly. That’s 80% less
weight for the same capacity.
At this point some of you battery experts
will tell me that you can get eight 1800 mAh
NiMH (AA-size) cells into the same 7.2-
ounce pack. That is true! I could also tell you
that a three-cell 2000 mAh Li-Poly pack
weighs 5.1 ounces and is still lighter, but the
gap in this regard closes. As you go higher in
capacity and in discharge-rate capability, the
Li-Poly cell clearly comes out the winner.
We get interesting fallout from this
wonderful weight savings. During all those
years flying with electric power, we had
heavy batteries to help us balance our
airplanes (obtain the correct CG). Now we
have much lighter batteries, and the result in
many situations is that our models end up tailheavy.
I’ve had to lengthen the nose moment arm
on several new designs to make sure it would
balance when using the lighter-weight Li-Poly
batteries. It’s something to consider for the
future!
Load Current Capability: The amount of
load that a Li-Poly cell can handle is not as
great as what you can expect from Ni-Cd or
NiMH cells, but the ability of Li-Poly cells to
44 MODEL AVIATION
Introduction to
Lithium-Polymer Batteries by Bob Aberle
Li-Poly cells from few of many sources (L-R): 145 mAh
FMA/Kokam—one of the smallest—can fly 1-ounce indoor RC
electric model for roughly 10 minutes; Balsa Products 600 mAh
cell (notice metal [foil-like] output tabs); purchased, assembled
pack of two 700 mAh FMA Direct/Kokam cells; two-cell 700 mAh
E-Tec pack; much larger assembled two-cell 2000 mAh Balsa
Products pack. Li-Poly cells are available from 40 to 3270 mAh;
much higher-capacity cells are due out soon.
At left is eight-cell 720 mAh NiMH-cell pack weighing 3.8 ounces.
Similar-capacity Ni-Cd pack would weigh even more. At right is
two-cell 700 mAh FMA Direct/Kokam Li-Poly pack of roughly
equal capacity, yet it weighs only 1.3 ounces. The eight-cell NiMH
pack will have initial starting voltage of more than 8.0; Li-Poly
pack will be near 7.4 volts. That will prompt slightly less power
but something you can easily tolerate.
8 cell 720 mAh NiMH
Weight: 3.8 ounces
2 cell 700 mAh Li-Poly
Weight: 1.3 ounces
1 X 145
FMA/KOKAM
1 X 600
BALSA PR
2 X 700
FMA/KOKAM
2 X 700
E-TEC
2 X 2000
BALSA PR
accept motor current loads is increasing
every day.
For instance, Ni-Cd and NiMH cells can
handle loads all the way up to 20-25C. A
NiMH pack rated at 3000 mAh, powering an
electric motor at 50.0 amps, would have a
load of 50 ÷ 3.0 = 17C.
When the Li-Poly battery cells came onto
our hobby market in mid-2002, the cells could
barely supply 2C or 3C loads; that was hardly
enough power except for the smallest and
lightest-weight RC models. But in the past
year and a half, these Li-Poly cells’ load
capability has grown to 4-, and then 6-, and
then up to 8C capability.
Within the past few months I have
received and tried several new cells from
FMA Direct/Kokam that can handle a 20C
load. That would put the Li-Poly cells on par
with the older Ni-Cd and NiMH cells. We are
only at the leading edge of this new battery
technology.
Series/Parallel Connections: In all the years
we have employed Ni-Cd or NiMH cells, we
have been told to only place them in series.
Doing that added to the battery pack’s total
voltage. These cells’ capacity was defined by
what was stamped on the cell case. If it read
2000 mAh capacity and you had eight cells
connected in series, the resulting pack would
have been 1.2 x 8, or 9.6 volts, and the rated
capacity would have been 2000 mAh.
Now the highest-rated Li-Poly cell is
approximately 3270 mAh. In time these
ratings will increase, but for the immediate
future these larger-capacity Li-Poly cells are
still expensive.
Currently we can easily fly electricpowered
models of up to Speed 400 size
using two or three Li-Poly cells in series with
a capacity rating of 1500-2000 mAh. If we
want to fly larger electric-powered models
using, say, 20-, 40-, or 50-size brushless
motors at upward of 50.0-amp current drain,
we have to resort to a different battery
configuration.
The person who flies larger-size electricpowered
models is connecting the Li-Poly
cells first in series and then in parallel. By
placing the cells in series, he or she adds
voltage at the rate of 3.7 volts per cell. By
placing these series strings in parallel, he or
she adds the capacity.
As an example, place three 1000 mAh Li-
Poly cells in series, for 3.7 x 3 = 11.1 volts.
Make a second set of three 1000 mAh Li-
Poly cells, and then place both series strings
in parallel. You end up with 11.1 volts at
2000 mAh capacity (2 x 1000). You also have
a total of six cells.
An illustration included with this article
shows the concept of series/parallel battery
configurations. This is a story in itself, and I
will pick it up in depth in a subsequent article.
Charge Retention: Li-Poly battery cells
have marvelous charge retention. Ni-Cd and
NiMH cells drop off peak charge in a couple
of weeks, forcing you to recharge often or use
a trickle charge to maintain them at full
capacity. Li-Poly cells only lose
approximately 1% or 2% of their charge in a
six-month period.
That is a real convenience. It is also why
it is recommended that you not resort to
trickle-charging Li-Poly batteries. Unlike Ni-
Cd and NiMH cells, Li-Poly cells could
May 2004 45
An interesting point arises with these
NiMH cells in AA-size configuration. Both
cells—1200 (L) and 1800 mAh—are same
size and weight, but one has considerably
more capacity. This type of NiMH cell can
come somewhat close to Li-Poly from
weight and capacity standpoint, but Li-
Polys will still be the leader!
Weight comparison of similar-capacity cells. L-R: Ni-Cd cell at 1.5 ounces, NiMH cell at
close to 0.9 ounce, Li-Poly cell at 0.8 ounce. Remember that two-cell Li-Poly pack will
have roughly same voltage as six NiMH cells. That would be 5.4 ounces for six NiMH as
opposed to 1.6 ounces for two Li-Polys.
New FMA Direct/Kokam 1500 HCT Li-Poly
cell. Label with “3.7V” means nominal
voltage of single cell. This cell can take
loads up to 8C. PC board strip has been
added to top of batteries—FMA
Direct/Kokam’s excellent idea to eliminate
need for soldering to metal battery tabs
directly. With PC boards it is easy to
solder wire for pack-assembly purposes.
1800 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Ni-Cd cell
Weight: 1.5 ounces
Nominal 1.2 volts
1200 mAh NiMH cell
Weight: 0.9 ounce
Nominal 1.2 volts
1200 mAh Li-Poly cell
Weight: 0.8 ounce
Nominal 3.7 volts
46 MODEL AVIATION
Sirius Lithium charger’s LCD screen confirms—at top—that you
are charging two cells at 750 mA current. At bottom left it tells
you that charger is in constant current mode and that voltage of
two-cell pack is up to 8.30 volts (maximum will be 8.40 volts). At
lower right is power put into battery during charging session. In
this case, battery pack was close to full charge in first place
because it is up to 8.30 volts, but only 31 mAh was put into pack
rated at 700 mAh. You must manually set number of cells and
charge current with this unit.
Model 109’s LCD screen provides many interesting parameters
during charge or discharge process. The 0.75A is charge current
you must manually set. The 2C2 indicates that two Li-Poly cells
have been selected; “C” stands for charge, “2” means mode 2 (of
three modes the charger cycles through). The 8.31V is voltage
under charge heading for maximum on this two-cell pack of 8.40
volts. At bottom left is time on charge (1 minute, 11 seconds).
Right indicates that 0.007 Ah (7 mAh) of power has gone into
battery. Again, voltage is so high so early into charge because
this was nearly fully charged pack.
Improved FMA Direct/Kokam Lipo-402 charger includes optional
proprietary automatic cell-select mode. Pencil points to added
Auto Detection feature. If you attach three-cell Li-Poly pack to
this unit, it will automatically set cell count to three. You must
still set charge current using shorting plug inserted along right
side of case. Lipo-402 can handle as many as four cells at six
discrete charging levels up to 1500 mA.
Contents of Bob’s charging station in his basement shop are
crucial to safely charging Li-Poly batteries (or any hobby
batteries). Charger and battery being charged are placed on fireresistant
16-inch-square piece of tile from The Home Depot. Also
shown are First Alert smoke/fire-detector alarm and First Alert
fire extinguisher (model FE10G). A safety warning from FMA
Direct concerning the use of Li-Poly batteries is posted on the
wall. (The company provides one with every pack it sells.)
AstroFlight Model 109 Lithium charger can handle more cells and
at higher current than Sirius charger, and it has discharge
capability. Model 109 also detects number of cells in Li-Poly
battery pack and automatically sets charger to that number, but
you must still set charge current.
Great Planes Triton charger is multifunction, meaning that it is
capable of peak-detect charging Ni-Cd and NiMH cells/batteries
and can charge Li-Poly and even lead-acid types. Here, LCD
screen is set for Ni-Cd cells. Make sure you don’t attempt to peak
charge a Li-Poly battery.
easily be overcharged in a so-called “trickle” mode, so please don’t do
it!
Expected Life: The life span of a Li-Poly cell is roughly 600 cycles,
at which point the capacity might only be down to 80% of the
original. That statement is qualified by the fact that the batteries must
be treated properly while charging and discharging, but at least you
get the idea of the life-span possibilities.
Appearance and Identification: In the past you might have mistaken
a Ni-Cd battery cell for a NiMH battery cell; both are cylindrical and
come in a variety of lengths. Li-Poly batteries, on the other hand, are
flat, aluminum foil pouches and are easy to identify; they are
generally rectangular and thin. The thin profile makes them especially
easy to install in many types of models.
The output tabs are mounted on one end of each cell. Each cell or
pack (more than one cell) is usually identified with the voltage and the
capacity. Not every cell I have seen identifies the manufacturer or
supplier by name.
Several manufacturers have been thoughtful enough to print
charging instructions directly on the battery pack, indicating the
number of cells to set (if you use a manual charger) and the
recommended charge current which you must also set. This is
especially helpful when employing multiple-cell packs, and even
more so when employing series/parallel pack assemblies.
A sort of cell-identity convention has been created for the Li-Poly
battery packs. A single cell is referred to as 1S, two cells in series is
2S, three cells in series is 3S, etc. When packs are configured in series
and parallel configuration, you will see “3S4P,” meaning that three
cells are placed in series, and then four of the three-cell strings are
placed in parallel.
Assembling Cells Into Packs: The output tabs on the first series of
Li-Poly cells were made from aluminum, which made them difficult
to solder. Early on, the manufacturers welded nickel-plated strips to
these tabs, which made soldering much easier, but many modelers still
accidentally shorted out the tabs while attempting to assemble cells
into packs.
FMA Direct/Kokam first came up with a printed board material
that is attached to the battery cell tabs. Then the modeler only has to
attach interconnecting wiring to the pads on the PC board. This makes
for the easiest possible pack assembly.
FMA Direct has gone even further with a connector strip that
allows you to connect sets of Li-Poly packs into a parallel
May 2004 47
When you charge a Li-Poly battery at the field, bring a small
snack tray or table. Place a 16 x 16-inch ceramic tile on top. Place
charger and Li-Poly battery being charged on top of tile. Run
charger input cables to battery terminals in car. Never place
charger and battery being charged on top of car’s engine block.
Yellow battery pack on underside of Bob’s Scratch-One electricpowered
sailplane/trainer is eight-cell 1100 mAh NiMH used in
articles featured in November 2003, January 2004 MA.
Bob chose two new FMA Direct/Kokam 1500 HCT Li-Poly cells to
illustrate conversion from NiMH to Li-Poly.
Since selected Li-Poly cells were loose, Bob had to assemble
them into a pack. He used Anderson Power Pole connectors, red
and black No. 16-gauge wire, heat-shrink tubing, and doublestick
tape. You can buy this type of pack assembled with
connector attached.
configuration to increase the basic capacity. This gets the job done
without the need for soldering.
Also, FMA Direct/Kokam is supplying ready-made Li-Poly packs
in which the individual cells are separated with a thin foam tape to
allow for ventilation between cells. Most of the Li-Poly cell
assemblies then receive a final heat-shrink wrap. Many other
manufacturers appear to be following their lead in this regard.
There is one more important point when assembling your Li-Poly
battery packs. All individual cells should be at the same potential
before construction. The best way to accomplish this is to individually
charge all the cells fully before assembly.
The worst-case scenario is to assemble, say, three cells when two
are fully charged and one is totally discharged. When you get these
cells connected, it is possible that the third cell will never be charged
properly.
Cost of Cells: The cost of the first few Li-Poly cells were on the
expensive side; almost the cheapest available was the little 145 mAh
cells at $10 each. Indoor RC models can operate on one or two of
these size cells, so an expenditure of $20 wasn’t out of sight.
Parking-lot flyers might use two 700 mAh Li-Poly cells at roughly
$30. The equivalent seven- or eight-cell NiMH pack might be in the
same ballpark. But when you get up to a three-cell 1900 mAh pack to
operate a Speed 400 motor, the cost can go up to $60-$70. At that
point the price is far higher than the cost of an eight-cell NiMH pack.
When you get into parallel cell connections for increased capacity,
the cost can increase greatly. An expert modeler is currently
48 MODEL AVIATION
Total of six Li-Poly cells
Each rated at 1000 mAh
Configurations: 3S2P (three in series, then two set
in parallel)
Set charger to three cells (not six)
Set charge current to 2000 uA (since the capacity
doubled in parallel)
Example of Series/Parallel
Battery Configuration
Double-stick tape is used to join two cells into a pack. The idea
is to let air circulate between them.
Sort of schematic shows how the two 1500 Li-Poly cells are
connected in series to supply a nominal 7.4 volts.
May 2004 49
The two cells have been joined with the double-stick foam tape.
Notice the gap, or airspace, between the cells. At top is original 5.9-ounce eight-cell 1100 mAh NiMH battery
pack. At bottom is 2.7-ounce two-cell 1500 mAh Li-Poly pack.
Roughly half the weight with more capacity results in a much
lighter Scratch-One that can fly considerably longer.
New FMA Direct/Kokam two-cell 1500 Li-Poly pack looks lost
inside original Scratch-One battery compartment. Pack was
moved forward in interest of maintaining same CG (balance)
point.
An AstroFlight digital meter is used to check the motor current
with the new Li-Poly battery pack.
AstroFlight’s Model 101 Whatt-Meter. Battery is connected to
right-side terminals (“Source”). Left-side terminals (“Load”) go to
motor. Here, only the battery has been connected, which is what
you must always do first. Current will be displayed at top left.
Voltage is at top right. Power in watts is at lower left, and
capacity going into or out of battery in Ah is at lower right.
This two-cell 145 mAh Li-Poly battery pack should have been
charged at 1C, or roughly 145 mA current. Bob accidentally left
his charger on 750 mA after charging a different pack. Much
higher current quickly turned little cells into “swollen sausage.”
Accidental misuse of Li-Poly cells generally doesn’t do any more
than this. The cells were ruined, but there was no explosion or
flames.
50 MODEL AVIATION
employing a total of 36 cells in a
series/parallel hookup that generates 37.8
volts at 7800 mAh capacity. He operates a
50-size brushless motor at as much as 50.0
amps current, but he paid close to $600 for
those 36 Li-Poly cells. I predict that a year
from now that cost might be half of what it is
today.
Charging: The techniques for charging Li-
Poly batteries are important. If done wrong,
you could damage the cells. Worse than that,
you might cause a fire. You should only use a
charger intended for charging Li-Poly battery
cells. Don’t ever attempt to use a peak-detect
charger (as you would for Ni-Cd or NiMH
cells) on Li-Poly batteries. You can’t do that!
With Li-Poly battery cells, the charger
starts at a constant current and the voltage
begins to rise. When the voltage gets close to
4.2 volts per cell, the charger essentially
switches over to a constant voltage type.
From that point on, the voltage never goes
higher than the 4.2 volts per cell.
To do that, the current is greatly reduced
or is even pulsed on and off until the battery
reaches full charge. If you unknowingly used
a regular peak-detect charger, you would
quickly exceed the maximum voltage per cell
and permanently damage the cell. You might
even cause a fire. (Editor’s note: Please
reread the preceding three paragraphs!)
As of this writing, several excellent
battery chargers on the market reliably charge
Li-Poly cells and packs. I’m using two
chargers at the moment: the Peak Electronics
Sirius Charge and the AstroFlight Model 109
Lithium Charge.
If you usually fly smaller models, with up
to Speed 400-size motors, the Peak charger is
a good choice. It can handle as many as three
Li-Poly cells and at currents of up to 1500
mA. I like this charger because it has an LCD
screen that tells you all the charging
parameters. I’m not a big fan of colored
LEDs used as status indicators.
The AstroFlight 109 Lithium charger can
handle as many as nine Li-Poly cells at as
much as 7.5 amps of current, so it is an
excellent choice if you plan on flying larger
electric-powered models. It has an LCD
screen that displays all the necessary charging
parameters, and it has a discharging
capability, which is nice.
Both chargers operate from a 12.0-volt
supply that you must purchase separately, or
you can power them from a 12.0-volt car
battery. Each charger costs roughly $130. The
AC supply would be an extra, one-time cost.
Another interesting charger for roughly
the same price is the Great Planes Triton,
which can handle as many as four Li-Poly
cells at up to 2.5 amps. It has an LCD screen
that monitors all the important parameters,
and it can discharge.
The Triton also offers peak-detect
charging capabilities for Ni-Cd and NiMH
battery cells. You must set it to the type of
battery being charged. I prefer a stand-alone
charger that handles only one type of battery
cell; however, the Triton is an excellent,
reliable charger and is a good buy.
While I was writing this article I obtained
an improved version of the Kokam Lipo-402
charger from FMA Direct. It can handle as
many as four cells and at six discrete current
settings up to 1500 mA. It uses tiny shorting
plugs to select the charge current and
employs colored LEDs as indicators.
The Lipo-402 now includes an optional
proprietary automatic cell select mode. If you
have the charger set for “Auto Detection” and
attach a three-cell Li-Poly battery pack, it will
set the cell count to “3” automatically. This
charger is $99.95.
Back to how the Li-Poly batteries are
charged. The recommended charge rate for all
Li-Poly cells at this time is 1C; the battery’s
stated capacity is exactly the same as the
charge rate. If your Li-Poly battery is rated at
1500 mAh, you will charge it at 1500 mA
current.
At that rate, the average Li-Poly cell will
get to approximately 90% of its full rated
capacity in roughly one hour. The remaining
10% might take an additional hour simply
because the current has been greatly reduced
to maintain the maximum voltage of 4.2 per
cell.
However, many of us have learned that
the extra 10% charge may not be worth the
wait. You get so much more from the Li
Poly, even at 90% of full charge, that the
other 10% necessary to reach full charge
isn’t worth the extra time.
The critical point in charging Li-Poly
batteries is to set the charger correctly for its
task. On the Peak Electronics charger, you
must first select the number of cells being
charged. Then select the charge current level
at the 1C rate. At that point the charge will
be initiated. When complete, the message on
the LCD screen will read “Full.”
Some inexpensive chargers use tiny
shorting plugs to select cell count and charge
current. Just plugging into the wrong
location could damage a battery. Again, you
must carefully set up your charger.
The AstroFlight 109 automatically selects
the number of cells. I’ve used this charger
many times, and it has never been fooled.
Attach a three-cell pack, and the number “3”
will come up on the LCD screen to confirm
it. Again, you must rotate an “amp adjust”
knob to establish the proper charge current.
This is why I mentioned that some battery
manufacturers are providing charging
instructions directly on the pack label.
When selecting cell count, remember that
you count the number of cells in series. Let’s
say you have three cells in series, each with a
1000 mAh capacity rating. You connect an
identical pack of three cells in parallel with
the first pack, and you have a total of six
cells. You set the charger to the cells in
series, which is three. Since the capacity is
1000 mAh, in parallel it is 2000 mAh, so the
current will be set at 2000 mA.
If that seems confusing, you know why I
believe that it is so important for the
manufacturers to provide this information on
the pack itself. If you had set the charger for
six cells in this instance, you would have
been in trouble.
Another extremely important aspect of
charging Li-Poly batteries is to remove them
from your model before you charge them. If
you set the charger wrong or if there is an
internal problem with the cells, or cell, you
don’t want them catching fire inside your
model; all that balsa wood will kindle any
fire.
Never leave a Li-Poly battery unattended
while it is being charged. The maximum
time to fully charge a Li-Poly pack may only
be one to two hours; be prepared to stay in
your shop during that time. I follow this
advice and get a great deal of building done
in the meantime.
Make sure you set up a safe area in your
shop as a “charging station.” I purchased a
large ceramic tile (16 x 16 inches) for my
bench top. Ceramic countertop (such as
Corningware) that is used in kitchens is also
an excellent material. I place my charger and
the battery being charged on this tile.
Next to this area I have a fire
extinguisher. I like the extinguishers with
gauges that let you know when it is time to
recharge. They only cost approximately $12.
In the same area I mounted a smoke/fire
detector. I bought one of the best ones that
let you test the battery. I replace the 9.0-volt
battery in the alarm every year when I
change my clocks back to Eastern Standard
Time.
Surprisingly I don’t see many modelers
12.0-volt charging Li-Poly batteries at the
flying field. You get so much more capacity,
and therefore flying time, that the need for
field charging is diminished. Many of us
bring out two charged packs, put in three or
four flights on each, and then call it a day.
But you definitely can charge these
batteries at the field. Just remember to
remove the battery from the model. I suggest
that you bring a small folding table with you
and place the charger and battery on it. I
would not leave them on a car fender, or
even inside the engine compartment, as
many modelers have been known to do.
Discharging: As I mentioned, Li-Poly
battery cells should not be discharged to less
than 2.5 volts per cell. To be slightly more
conservative, most of us use 3.0 volts per cell
as our minimum.
Many of the newer motor speed
controllers (ESCs) have voltage cutoff points
that are set for use with Li-Poly batteries.
The cutoff will operate at 3.0 volts (one Li-
Poly cell), 6.0 volts (two cells), or 9.0 volts
(three cells).
A few warning devices now on the
54 MODEL AVIATION
market activate an LED when the minimum
voltage per pack is obtained. FMA Direct is
about to release a device that will work well
with the older ESCs that did not have the
voltage cutoff set for Li-Poly batteries. This
add-on unit will sense the proper minimum
battery voltage and cut the motor off as a
clear indication that it is time to land.
Many of my models tend to want to land
before the minimum voltage cutoff point is
reached. I fly until the model wants to land.
Then once on the ground, I measure the
battery voltage under a load.
To get a quick load, you can usually turn
the motor on for a few more seconds while
inserting an AstroFlight current/voltage meter
in series in the battery circuit. That will give
you an idea of how low the voltage actually
went.
The battery chargers that include
discharge capability are set in the range of 2.5
to 3.0 volts per cell, so that will never prove
to be a problem. But keep in mind that if you
regularly go below that minimum, your
batteries are going to drop out of service fast.
Safety Issues: It’s sad, but in this new era of
Li-Poly batteries there have been several
stories of battery-cell or -pack fires. With
every new technology there is an associated
learning process. Modelers are quickly
learning all the new skills needed to handle
these wonderful new batteries, but while
learning there have been incidents and some
fires have occurred.
Most of these events have been caused by
inappropriate charger settings or failure of the
chargers themselves. There have only been a
few incidents of cells failing on their own
(such as internally shorting out while a motor
was running in flight!).
The original Lithium-Ion cells were sealed
in such a way that during misuse they could
explode as well as catch fire. The newer Li-
Poly cells are contained in a soft metal case
which can expand during misuse and, as a
result, are more likely to catch fire or flame
than explode. That is why it is so important
that the Li-Poly battery be removed from the
model during charging. All that balsa and
material makes for a wonderful fire. A
ceramic tile or insulated box will prevent any
fire from spreading.
A story I have heard involved a modeler
whose model crashed. He placed the damaged
airplane in the trunk of his car, where a short
while later the battery caught fire, and the car
was next! If your model suffers a crash, have a
fireproof, insulated box handy, and put the
battery in that box. I have a Sentry-brand
small security box stored in the trunk of my
car. It’s overkill maybe, but why not be totally
safe?
Understanding and using your charger
carefully can eliminate most of the problems.
In an attempt to be as safe as possible,
several Li-Poly manufacturers have plans to
include safety devices built into individual
cells or in assembled multiple-cell packs.
Such devices, known as Protective Circuit
Modules (PCM), have been built into almost
every Lithium-Ion battery cell or pack
because they are essentially a sealed metal
can, and they might explode. These PCMs
sense excess heat, voltage, or current (or
combinations thereof), and open the circuit
should any preset condition be exceeded.
Of course, being totally safe can often
make you sorry from a model standpoint.
Having a safety device operate in your shop
while the battery is under charge is a good
thing. However, sensing a problem in-flight
and opening the battery circuit stops your
motor and your RC system! Therefore, a
safety device in-flight could also result in a
crash and a damaged model and/or property.
All these factors are going to have to be
addressed!
Soon FMA Direct will offer a device
called Safety Guard. Unlike the PCM, the
Safety Guard does not go inside the battery
pack. It is placed in series between the
charger and the battery pack. If it senses a
problem, it will open the circuit as a “safety.”
But when it does that, the battery pack is on
the ground and removed from the airplane.
That sounds like an interesting concept, and I
look forward to trying it when it becomes
available.
Applications: Replacing Ni-Cd and NiMH
batteries in model aircraft with Li-Poly
batteries has initially become everyone’s
problem. It isn’t a simple swap-one-cell-for
another routine because of the different
voltage.
A popular GWS Lite Stik parking-lot
flyer is normally flown on a six-cell Ni-Cd
pack or a seven-cell NiMH pack. The motor
current is approximately 2.0 amps (2000
mA), and two Li-Poly cells will roughly
replace those seven or eight cells. For
capacity you could use 700, 1000, or 1200
mAh Li-Poly cells. The resulting pack would
weigh much less and have much more
capacity. I’ll give you exact replacement
figures in a moment.
You don’t want to greatly increase the
voltage when going from Ni-Cd/NiMH
power to Li-Poly cells. Let’s say you went to
three Li-Poly cells in the preceding example.
The voltage would go up to more than 11.0
volts. The current might go from 2.0 up to 3.0
amps and more. That extra current might melt
your tiny motor in a single flight. Being able
to measure motor current and power
(wattage) is essential to the safe use of Li-
Poly batteries.
Many Li-Poly battery suppliers are kind
enough to include battery applications in their
literature and on their Web sites. Horizon
Hobby is doing this with its FMA/Kokam
and Thunder Power Li-Poly batteries. For
each Li-Poly pack you are given an idea of
what it can power in terms of model type and
motor type (such as Parking Lot, E-3D
Vertical, Speed 280, Speed 400, Hacker B-2,
etc.).
FMA Direct supplies the Kokam brand of
Li-Poly batteries from Korea. On its Web site
under Support, the AN000002 pdf file (0.99
megabytes/48 pages) titled “The Kokam
USA Lithium Battery System” by FMA
Direct Inc. President Fred Marks is an
excellent reference for every aspect of Li-
Poly batteries.
Dave Radford of E-Tec has an excellent
Web page—www.aircraft-world.com/
prod_datasheets/lipoly.htm—that contains a
considerable amount of application
information. I’m sure that more of this type
of data will begin to show up as the electricpowered
modeler gets further into Li-Poly
batteries.
Scratch-One Li-Poly Retrofit: In my portion
of MA’s “From the Ground Up” series I
presented a simple model design which I felt
a beginner could easily build from scratch or
from raw materials. I called this electricpowered
RC sailplane/trainer the Scratch-
One. It was featured in two parts—in the
November 2003 and January 2004 issues—
with plans and assembly instructions.
As originally presented, this airplane
employed an eight-cell NiMH battery pack
with a rated capacity of 1100 mAh. The
motor was a Speed 400 running direct drive
with a starting current of approximately 12.0
amps. That current quickly drops off to
roughly 10 amps and realistically goes even
lower when the motor is throttled to slow the
airplane in flight. The Scratch-One’s all-up
weight with the specified motor and battery
was 16.9 ounces.
My goal was to swap a new Li-Poly
battery pack for the NiMH pack. I decided to
try two Li-Poly cells, which will give me a
nominal 7.4 volts rather than go to three cells,
which would have put the voltage up to more
than 11.0 volts. The 7.4 volts is admittedly
lower than the approximately 8.5 volts
obtained from the eight NiMH cells.
For capacity I decided on the newly
released FMA/Kokam 1500 HCT (high
discharge) cells, which claim to take loads as
high as 8C (eight times the battery’s
capacity). So in this case that will be 8 x 1500
= 12000 mA (or the same as 12.0 amps). That
will be the upper limit for the cells and the
maximum current I will experience.
On the other hand, I will have a lighterweight
battery; therefore, I will have a
lighter-weight model. It is more than likely
that I will be flying mostly at half throttle at
roughly 5.0-7.0 amps of motor current. At
that rate, motor run time might increase to 20
minutes or more.
The eight-cell 1100 NiMH battery weighs
5.9 ounces, and the two-cell 1500 Li-Poly
pack weighs 2.7 ounces. The weight savings
is 3.2 ounces, which brings the Scratch-One
down to just 13.7 ounces (16.9 less 3.2).
At the same time, the motor current came
down somewhat (because of the lower
voltage), but not enough to have any real
effect on the flight performance. In
combination with the capacity increasing, the
motor run time almost doubled.
The NiMH pack cost $32.95, and this new
1500 Li-Poly pack cost $41.75. That’s not
very much more! At the reduced weight and
58 MODEL AVIATION
increased battery capacity, my Scratch-One
can fly at a variety of speeds and for a
considerably longer period of time.
In addition, with the centrally located
battery position, the weight reduction had
little effect on the CG. The lighter wing
loading makes for more thermaling and even
longer flight times. The extra $8 for the Li-
Poly pack was worth it!
Judging by the amount of questions that
came in from readers about my “Battery
Basics” article in the October 2003 MA, I’ll
bet this article will set records. I’ll try to use
the forum of my new FAQ (Frequently Asked
Questions) department to answer as many of
these as I can.
The foreseeable future of electric-powered
RC models lies with Li-Poly battery
technology. I’m a realist, and I have to think
that there is even more new technology on the
horizon that will allow us to enjoy even more
of the advantages of clean, quiet electricpowered
flight. MA
Bob Aberle
[email protected]
Manufacturers/suppliers:
Advanced Energy Technology (Thunder
Power Li-Poly batteries)
(702) 228-8883
Air Craft Inc. (E-Tec chargers, Li-Poly
batteries)
00-81-948-21-1045 (Japan)
www.aircraft-world.com
AstroFlight Inc. (Model 109 Lithium charger)
(310) 821-6242
www.astroflight.com
Balsa Products (chargers, Li-Poly batteries)
(732) 634-6131
www.balsapr.com
Batteries America (Li-Poly batteries)
(800) 308-4805
www.batteriesamerica.com
Bishop Power Products (Chargers,
FMA/Kokam Li-Poly batteries)
www.b-p-p.com/
FMA Direct (Chargers, Kokam Li-Poly
Batteries, Safety Guard)
(800) 343-2934
www.fmadirect.com
Hobby Lobby International (chargers,
FMA/Kokam Li-Poly batteries)
(615) 373-1444
www.hobby-lobby.com
Horizon Hobby Inc. (Chargers, FMA/Kokam
Li-Poly batteries)
(217) 403-3279
www.horizonhobby.com
New Creations R/C (chargers, Li-Poly
batteries)
(936) 856-4630
www.newcreations-rc.com/
Peak Electronics Inc. (Sirius Lithium charger,
Li-Poly batteries)
(858) 679-4952
www.SiriusElectronics.com
Radical R/C (B) (chargers, Li-Poly batteries)
(937) 256-7727
www.radicalrc.com
Tower Hobbies (Triton charger, FMA/Kokam
Li-Poly batteries, items identified with Great
Planes and Hobbico)
(800) 637-6050
www.towerhobbies.com