IN THE SEPTEMBER column I looked
inside a laptop-computer Lithium pack to
marvel at the complexity of the circuitry it
contained. Did you ever wonder what the
inside of a Li-Poly pack looked like?
These exploratory excursions should
only be attempted by trained professionals
and not tried at home. No healthy cells
were sacrificed in this examination.
I had a “violated” (propeller strike—
don’t ask) 900 mAh, 3S pack. I removed
the violated cell and made it into a 2S
pack that checked out to spec. Not
wanting to waste an opportunity, I
proceeded with an autopsy.
I started by discharging to 0 at
approximately 100 mA. It had been several
days since the pack had been cut in a crash,
so there was only roughly 20 mA in the
pack. The sweet smell you get with
ruptured packs had almost gone away also.
The rubber gloves came on and I made
a cut in the Mylar envelope. A few deft
slices with a No. 11 blade gained me access
and I folded the Mylar back, revealing the
Polymer separator seen in Figure 1.
The unfolding began, as shown in
Figure 2. By the time I had 5 feet of
Polymer material laid out along with 41
individual plates, as in Figure 3, I came to
the conclusion that I didn’t want to go into
the Li-Poly manufacturing business. I
would like to see the machine that puts
these together.
It is interesting that I encountered no
violent reaction; there was no reaction at
all. It just laid there. The Polymer separator
measured on the order of .0007 inch thick,
and the plates were approximately .002
inch. There was slightly more than 6 feet of
separator and 42 plates.
Your battery guru tells you to
occasionally check the capacity of your
flight and transmitter packs so you will
know how they are holding up. Say you
reallocate some of your hobby financial
resources and buy the Nifty Deluxe Super-
Cycler and Battery Maintenance System.
Only then do you find that when you plug it
into your transmitter, you cadischarging the pack through the charge jack. The manufacturer
claims that this is for safety purposes, but why doesn’t it do it in
the low-end units where the users are more apt to mess up?
You are left with physically removing the pack and mating with
its connector (if you can find it). On some transmitters this is a
reasonably simple task; on others it involves major surgery to get to
it, and then finding a mating plug becomes another issue.
For those little white connectors on the battery pack, a male
Deans mini or JST plug is a solution. See Figure 4.
I recommend bypassing the diode with a single strand of servo
wire. This will allow you to discharge the pack through the charge
jack while providing short protection as a fuse; a single strand will
handle no more than 2 amps.
You can find details about performing this operation at the Peak
Electronics Web site at www.siriuselectronics.com/. Click on
“Diode Sheets.” If you are not handy with a soldering iron, get
someone who is to do the work for you. Making the modification
may void your warranty; some manufacturers are put off by
modelers trying to “improve” their equipment.
Battery Fakes: A modeling friend was proud of the purchase he
made: a package of 24 Duracell AA cells
for $4. This was a great deal, with the best
price for alkaline AA cells at 50¢ each.
Then I noticed something strange
about the cells; you could deform them
just by squeezing. Then I looked closer at
the name and it was “Dinacell”—not
Duracell—but the packaging was nearly
identical. Even Duracell’s Bethel,
Connecticut, address was replicated.
My curiosity was piqued. I put one of
these counterfeit cells to the test with my
West Mountain Radio CBA II and then
sacrificed a new Duracell for comparison.
You can see in Figure 5 that the Dinacell
gave roughly 300 mAh compared to the
Duracell at 2400—eight times as much.
A bit of digging revealed that these
bogus cells from China are flooding the
US, particularly through flea-market
outlets. Rip-offs are showing up dressed as
Energizer cells as well.
Great minds have concluded that
unbalanced packs (cells at different states
of charge) are a major contributor to the
early failure of Li-Poly cells. Knowing
that 4.2 volts is the maximum level you
should charge to, it makes sense that if onen’t make it
work.cell is not coming up in voltage as fast as
others in the pack, the good cells will have
to make up for the slacker by going higher
than 4.2. The more cells you have in series,
the more critical the problem becomes.
Lithium-pack distributors have come to
our rescue with balancing chargers. The
only way to balance cells in a pack is to
have access to each cell. Any device
claiming to balance the cells in a pack that
does not have access to each cell is simply
blowing in your ear.
Until now the procedure has simply
been to provide taps to each cell and then,
before charging, measure each cell voltage.
If the cells are out of balance, charge them
individually. There has to be a better way.
Several charger providers are offering
balancing chargers to address the problem.
I’ve had the opportunity to do some beta
testing of the Skyvolt balancing system,
shown in Figure 6, offered by FMA Direct.
This particular charger focuses on the
larger-battery user—up to six cells in series
at 2000-9600 mAh. Its 10-amp capability
allows the cells to charge to 90% capacity
in 20 minutes.
All Skyvolt packs from FMA Direct
come standard with the unique Safe Charge
Connector, which is the power connector
and the voltage-monitoring connector. The
Safe Charge Connector is used in
conjunction with FMA Direct’s Discharge
Protection Module (DPM), shown in Figure
7.
The DPM monitors each cell voltage
and shuts off the motor circuit when any
cell reaches a safe cutoff voltage, further
enhancing the life of the pack by
preventing overdischarge of one or more of
the cells. You still need your ESC because
it is controlled by the DPM.
There is more! This may be unique to
the Skyvolt system; FMA Direct provides
software you can download from its Web
site (www.fmadirect.com) that allows you
to monitor the charge status with your PC.
The viewing screens keep you informed
of the charge progress and the amount of
charge returned to the pack. The Skyvolt
charger does not need the computer
interface for field charging because it does
the whole routine from its internal
programming with a couple LEDs to
indicate where it is in the cycle.
Although other manufacturers are
providing their own balance connectors,
one would have to make an adapter to use
the FMA Direct balancing chargers and
make sure they do not exceed that pack
manufacturer’s charge rates.
Approaches vary. The Skyvolt system
monitors each cell during the charge cycle
and stops the charge on that particular cell
when it reaches the optimum voltage.
Others look at the individual cells,
discharge other cells to the lowest cell’s
voltage to bring them into balance, and
proceed with the charge from that point.
I am not too comfortable with the latter
approach because it assumes that the
charge acceptance and capacity are the
same for each cell. I have not had the
chance to check one of these out, so I can
only report what chatter on the Internet has
revealed.
I could take up my next 10 columns
expounding on the Skyvolt system. But
since FMA Direct has copies of its manuals
on its Web page’s support section, you can
get all the details and diagrams there.
I’ve seen three packs from different
sources with a balancing connector. I wish
these people would standardize on one
connector and pinout for tapping the packs.
Reader Writes: Carl Gotch of Phoenix,
Arizona, wrote:
88 MODEL AVIATION
“For optimum reliability and
longevity, how should the various Ni-Cd,
NiMH, and Li-Poly battery packs be
stored; i.e., discharged, fully charged, or
somewhere in between? Can Li-Poly
packs self-discharge to the point where
they are no longer serviceable (2.7
volts/cell or so)?”
It doesn’t matter what the state of
charge is when you store Ni-Cd or NiMH.
Just check the voltage before charging
them again; if it is less than 1.2 per cell,
you may have a problem.
Lithium-cell manufacturers
recommend storage between 40% and
60% charged. They have not been around
long enough for us to really have a
handle on storage, so we will have to go
with this.
Logic dictates that these cells can selfdischarge
to a point where they are
probably damaged and cannot be
recharged, but the self-discharge is
measured in terms of many months to
years. How long has that little Lithium
coin cell been holding up your computer
memory?
Generally the self-discharge of the Li-
Ion battery is 5% in the first 24 hours after
charge. Then it reduces to 1% -2% per
month thereafter.
I’ve been getting some interesting reports
about CL going electric. Some top
competition fliers are opening some eyes,
and at least one will participate in the CL
World Championships. I’ll have more
about this in future columns.
Are you getting bored waiting for epoxy
to set? Visit MA’s Sport Aviator at
www.masportaviator.com/. It’s a good Ezine
where you will find a particularly
good article about electric flight safety by
Bob Aberle.
If you don’t have an E-mail connection,
drop me a note at The Battery Clinic,
12219 NW 9th Ln., Newberry FL 32669.
It’s my flying season, so it may take me a
day or so to respond to you. MA
For reasons known only to the
manufacturer, it has placed a diode in series
