How is your LiPo holding up? - 201209

Do your LiPos seem to have lost
some of their punch? LiPos
deteriorate over time. But how
much and how do you keep track of
it? I discovered a helpful technique on
YouTube.
For those of you who are computer
challenged, I’ll attempt to describe the
technique. First, you need a wattmeter
device—the kind used to check the
performance of our power systems. Then,
you will need a load. The one on YouTube
uses a pair of 12-volt halogen bulbs wired
in parallel to give an approximate 100-
watt load with a 3S LiPo pack.
Plug your battery into the wattmeter
and note the voltage after approximately
10 seconds. Write this down, then
connect the load for 10 seconds and
record the voltage and current.
Subtract the loaded voltage from the
unloaded voltage and divide that number
by the current (in amps). Multiply the
number you get by 1,000 to receive the
milliohm internal resistance and record it.
Periodically check your pack in the
same manner (same load) and you can
see if, or how much, your pack’s internal
resistance is changing. These tests should
be done at roughly the same ambient
temperature for best results.
The fi rst voltage should be with
a smaller load, such as 1C. This will
give a more consistent reading than
simply using an open voltage to start.
Internal resistance of a LiPo pack is not
rate-dependent in most cases, but the
open-circuit voltage will give you a false
reading.
Give each pack a name or some
way it can be identifi ed so that as you
develop your history list, you can keep
everything straight. Using a spreadsheet
such as Excel, record the date, ambient
temperature (to be sure you are not
muddying up the water with temperature
extremes), battery identifi cation, starting
voltage, ending voltage, current, then
the milliohm calculation. You can try
with different temperatures later if that
interests you.
The YouTube video gives an excellent,
straight-forward description of the
process, plus a number of other things
you can do to better understand your
packs.
2.4 GHz Antenna Guide
An antenna guide is a nice way to
position 2.4 GHz antennas to get
maximum utilization. The installation
can be built into your model so that if
you do change receivers, you can always
get the correct antenna separation. Slip
the antenna wires
into the tubes you
have formed by
heating them.
Parallel Charging
of LiPos
I receive
frequent questions
about parallel
charging. Here are
my thoughts:
First, NiCd and NiMH should never
be charged in parallel because of their
charge-voltage characteristics. They are
recombinant systems, meaning that
any gases generated in overcharge are
recombined in the cell. This heat of
recombination is why the voltage drops
when the cells reach full charge.
If you are trying to charge packs in
parallel, one will reach full charge and
the voltage will attempt to drop. The
charger will supply more current to
this pack, possibly destroying it while
the other pack will be undercharged.
Lithium packs can be charged in parallel
as long as they have the same cell count.
They don’t have to be the same capacity.
Each pack will charge to whatever
the charger is set, then the charge will
terminate. The only problem with
parallel charging is
that few chargers
have provisions
for parallel charge
balancing.
It’s Seldom One Thing
There is the time-honored “the battery
failed.” I am amazed by the number of
people who rely on the battery voltage
reading alone. They never discharge a
pack to check the capacity delivered.
It may come as a shock to some, but
a fully charged 100 mAh pack will have
the same voltage reading as a 2000 mAh
pack. A pack only capable of delivering
100 mAh will get you far enough into
the air to let you develop the downward
momentum to bury your engine/motor
as it fails to sustain control.
If you have been crash free this far into
the fl ying season, you should check the
capacity of the pack to ensure that your
good fortune will continue. A charger
that will allow you to discharge the pack
and record the results is cheap insurance.
Don’t depend on what the charger
tells you the pack input was because this
has little bearing on the real capacity
capability. You can input 1000 mAh into
a nearly dead battery. The voltage will
read okay, but this pack will not work.
New Products
Bob Obemberger at Romco
Manufacturing (the makers of TruTurn
Spinners) has engineered a special
spinner for use by electrics. The spinner
is useful when the motor is cowled in
and does not receive cooling air.
The holes in the spinner are
engineered to allow the highest
pressure air into the cone, and the
specially cut backplate forces the air
back over the motor. The new line is
called Turbo-Cool and can be seen on
the TruTurn website, which is listed in
“Sources.”
J-B Weld Adds New Formulation
Most hobbyists have known the value
of having J-B Weld Steel Epoxy included
in their standard shop supplies. It has
been known as one of the strongest
bonding agents available. It also has a
drawback—a slow set time (4 to 6 hours)
and a full cure takes even longer.
There is now a new kid on the block
called J-B Kwik. It is the same price as
Steel Epoxy and also comes in a blister
pack. Kwik is available at most hardware
stores.
Like metal, J-B Kwik can be formed,
drilled, ground, tapped, machined, fi lled,
sanded, and painted. It stays pliable for
approximately two minutes after mixing,
sets in four minutes, and fully cures in
four hours. It is waterproof, petroleum-,
chemical-, and acid-resistant. Kwik also
resists shock, vibration, and extreme
temperature fl uctuations, and withstands
temperatures up to 300° F.
J-B Kwik is strong, non-toxic, and
safe to use. It bonds to virtually any
combination of iron, steel, copper,
aluminum, brass, bronze, pewter,
porcelain, ceramic, marble, glass, PVC
and ABS, concrete, fi berglass, wood,
fabric, and paper. The product will bond
with nearly any porous or non-porous
material.
That’s all for this month. I enjoy your
letters. No email connection? Drop me a
note at the address listed below.