The Battery Clinic 2010/07

86 MODEL AVIATION
EVEN AFTER SEVERAL years of use in
our hobby, there still seems to be confusion
about the “C rate” for Li-Poly batteries. Until
Li-Poly technology entered the modeling
scene, “C” stood for capacity of the battery.
It was a measure of how much energy could
be drawn out in a given time period.
Theoretically, a 1 amp-hour battery would
deliver 1 amp for 1 hour. I wrote
“theoretically,” because manufacturers
actually rate them at the five-hour (C/5)
discharge rate. This was felt to be the typical
application discharge.
All was well until power tools and other
energy hogs came into being. The rating
method by that time was well entrenched, so
there was no reason to change it.
The “C rate” of a Li-Poly battery is a
whole other beast that Li-Poly vendors (not
cell manufacturers) created to indicate the
maximum discharge that the pack could
tolerate without significant damage. It
became a “mine is better than yours” thing
that entertains marketing types who find real
numbers depressing.
Deciding what “significant damage” was
was left to the user, I guess, because the
vendors have been a bit remiss in defining it.
One said it was the maximum rate the pack
could be discharged at and not exceed 140º,
which theoretically was the temperature at
which bad things began to happen.
The definition of “bad” is basically left up
to the user. I do know, and many have
learned the hard (expensive) way, that the
harder you drive Li-Polys, the sooner they are
ready for the graveyard.
We see packs rated at 10C, 15C, 20C,
30C, and even some as high as 50C. This
implies that you can discharge the pack at
whatever the C is, multiplied by the actual
capacity. A 40C 2000 mAh pack could be
discharged at 80 amps. This would give you
approximately 60 seconds of flight time.
I expect that we will see the C ratings go
higher, even though it doesn’t make a whole
Red Scholefield | redscho@The Battery Clinic bellsouth.net
“C”: What is it all about?
These Li-Poly packs are extremely swelled, but they still have
good power-delivering capacity. Why would they expand under
normal use?
A123 discharge curves showing bad cell. The dip is because of
its heating up, even at a 2.3-amp discharge.
Above: Factory A123 packs have the
plus/minus leads coming from them, with
Deans and two red and black wires going
to a Molex-type plug. The cells have to be
balanced, as do Li-Polys.
Right: Have all the fun you can stand. And
watch out for those fence posts. Red
loves flying his Funster.
Also included in this column:
• The unanswered question
• AXi motor failures explained
• A123 failures discussed
• A123Racing charger questions
07sig3x.QXD_00MSTRPG.QXD 5/25/10 3:09 PM Page 86
generation of hydrocarbons. Decomposition
of solvents used in the manufacturing
process is also suspect.
If anyone has had better luck than I have
trying to get an explanation of the swelling
under normal use, what the mechanism is, I
hope you will share that information so I
can report it here.
AXi Motor Failure Explained: L.A.
Johnson, a technician who worked for
Hobby Lobby before retirement, offers the
following explanation of the AXi motor
failure that I covered in the March column.
He wrote:
“I need to tell you why you had the
problem with the AXI 2820/14 motor, and
how to cure the problem.
“The motor is a very early model. I can
tell because of the fact the magnets are not
keyed into the rotating end bell, and the fact
that the rotating end bell will not accept the
prop adapter that is used when the motor is
reverse mounted.
“First, let’s cover the field coils rotating
on the motor core.
“The field coils are machine wound, and
then they are ‘glued’ to the motor core using
an epoxy adhesive. The problem with doing
it that way is if the adhesive gets too hot,
you don’t get a complete glue line between
the two, the field coils will break loose and
rotate on the core under hard acceleration,
or ‘braking’ action.
“The cure for this was doing the same
thing with the core that you do when you
are trying to glue a dowel into a hole. You
cut a spiral groove around the dowel. This
allows the glue to spread over the whole
length of the dowel.
“All the later model AXi motors have a
spiral groove around the core so the glue
spreads over the whole length of the
core/field coil joint. In addition, the
manufacturer went to a higher temp
adhesive which also helped.
“Now, on to the magnets. AXi found that
the adhesive they were using on these early
motors was defective, and it did not have
enough sheer strength to hold the magnets
in place during heavy acceleration and
deceleration. They changed to a higher
lot of sense considering the flight time. You
might notice that you never see an 18C or
23C pack; that indicates that the C rating is
not an exact science, but rather an attempt to
keep up with your competition.
One statement can be made about the C
rate, assuming that the vendor has a
modicum of conscience. The higher the C
rate, the lower the internal resistance of the
pack. Low internal resistance is good,
because you get more voltage delivered to
the system and less heat generated in the
battery.
Heat is bad. Voltage is good.
For conservative flying, I try to use as
much battery as my model can carry without
significantly compromising its flying
characteristics to the point at which the
maximum discharge is considerably lower
than the C rating. I would rather use a 10C
4000 mAh pack with a maximum load of 30
amps than a 20C 2000 mAh pack. I get
much longer life from Li-Polys choosing
them that way.
What causes packs to swell? This question
still goes unanswered. I have been using Li-
Poly packs for nearly four years and still
can’t get a satisfactory response from
suppliers or manufacturers.
Misusing packs will do it. That includes
too high of a discharge, discharging a pack
to an extremely low level, or too fast of a
charge.
But what about packs that have not been
subjected to this? I have several, from
various manufacturers, that are swollen
significantly.
Some of those packs have fewer than a
dozen cycles, while others have more than
50. All showed evidence of swelling early.
These packs have always been charged
on a balancing charger at well less than the
C/2 rate. Discharges have never exceeded
roughly 8C. It is interesting that they still
have good power-delivering capacity,
despite the fact that they are swollen
significantly.
One source attributes the swelling to the
88 MODEL AVIATION
07sig3x.QXD_00MSTRPG.QXD 5/25/10 3:09 PM Page 88
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strength adhesive for the later motors.
“The mechanical keying you mentioned
in your column helped, but it was not the
total cure. One thing that was bad about the
mechanical keying was the fact that if one
of the magnet’s glue lines let go, the magnet
would break since it was keyed into the
rotating end bell.
“The major cause of the problem (other
than the age of the motor) is the fact that
there is no cooling on the motor, ‘and,’ it’s
a typical sailplane start, and stop on the
motor. ‘Hard’ acceleration and ‘brake’ stop.
A partial cure would be to use a motor
controller that has a ‘soft’ start, and a
‘delay’ braking.”
If you have encountered this problem
with your older AXi motor, contact Hobby
Lobby and inquire about its
replacement/repair policy for this defect.
A123 Failure: Dennis Vollrath of the
Racine Radio Control Club dropped me an
interesting e-mail on his first A123 cell
failure. He wrote:
“This 6S2P battery pack was first set up
as a 5S2P. Later on, two more cells were
added to make it a 6S2P pack for the
‘Revolver.’ One of those two cells failed.
“It’s pretty obvious which cell in the two
tests failed from the discharge plot.
“Something else, look how flat the
voltage discharge curve is for the ‘good’
cell. That’s why this battery pack could not
be balanced with the ‘Blinky’ balancer. The
two cells that were added were way out of
balance with the other existing 5 series
cells.
“So, my flying with this pack over a
period of several months resulted in those
two cells getting worse and worse, finally
killing one of the cells.
“Those modelers that are using the two
A123 packs for radio power probably will
not run into this issue, since their battery
pack was assembled at the same time.”
The dip noted in the bad cell’s discharge
curve is caused by the cell’s heating, even at
2.3-amp discharge. Heating is the result of
the high internal resistance that the cell has
developed.
A123 Racing Charger Question: Tom
Smith wrote the following.
“These factory packs (see photo) have
the plus/minus leads coming out of them
with Deans female connectors on them and
two red and black wires going to a Molex
type plug. The A123 Racing charger has the
corresponding male plus/minus Deans on it
and on the end plate of the charger is a
receptacle for the Molex type plug to plug
into.
“I understand the need to balance Li-
Poly batteries, but is it necessary with
A123s as well? We never did it with Ni-Cds
or NiMH cells. I would like to use the A123
Racing dedicated charger if possible. The
guy I bought it from said it charged each
pack in about 10 minutes.”
Tom, you need four wires to balance a
three-cell pack. Your A123Racing pack
seems to have these.
You can tell if it is actually for balancing
by measuring the voltage from the negative
(black) on the power lead to one of the
wires on the Molex connector. If you get a
reading of either 3.6 or 7.2 volts, that means
that the lead on the Molex connector is
going to one of the two cell interconnection
points, and that is how the balancing is
accomplished.
There’s nothing wrong with using the
A123Racing charger if it does, in fact,
balance the cells. You can check this by
probing with a voltmeter.
The reason why we need to balance
Lithium-technology packs is that they are a
constant potential charge system as opposed
to the constant-current charging system we
have with Ni-Cd or NiMH. When you
charge with a constant potential, it looks at
the overall pack voltage and terminates
when the pack is at 10.8 volts for the three
cells.
One cell, if low, could cause the others
to be driven to a higher voltage to reach the
overall cutoff voltage of 10.8. In charging
Ni-Cd or NiMH with constant current, each
cell is taken to its full state of charge and
then is able to accommodate any overcharge
as the others catch up, which takes them all
to the same state of charge. MA
Sources:
AXi motors:
Hobby Lobby
(866) 512-1444
www.hobby-lobby.com
A123 batteries
(617) 778-5575
www.a123systems.com
The Battery Clinic
12219 NW 9th Ln.
Newberry FL 32669
www.hangtimes.com/redsbatteryclinic.html
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