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Old 02-23-2008, 05:32 AM
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AEAJR
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LITHIUM BATTERY CHARGERS -
Balance chargers vs. external balancers

Lithiums are great, but they benefit from a little extra care. We have seen that packs
with two or more cells can get out of balance. That means that one cell tends to
rundown lower or tends to charge higher. Since charging through the power port
that connects to the ESC only reads the total pack voltage the charger will charge
the pack to the expected voltage. For Lipoly packs that would be 4.2V per cell.
Therefore, when charging through the power port, the charger will take a 3 cell Lipo to
12.6V, regardless of the individual cell voltages. But if one cell is low and one is
high, that could result in one cell perhaps being charged to 4.3V or one being charged to
4.1V, for example.

Over many cycles this difference will build up. The most benign outcome is a loss of
pack performance. A more serious outcome could be that the low cell will drop below
the critical 2.5V level on discharge and be damaged, rapidly degrading the pack.
The more serious issue could be that one cell gets seriously over charged getting
well above the desired 4.2V top charge. This can result in pack failure or can cause the
over charged cell to "vent with flame". This is ungood. :-O

Balancers

So, for the past 18-24 months we have seen a flood of pack balancers that will bring the
packs into balance to maintain an even charge across all cells. To use these balancers you
need a compatible balance plug on the pack. Assuming you have this arrangement,
a balancer can help prevent the above situation. If you are happy with your charger and
don't feel the need for a new one, a balancer is a good investment. They run from $20 to $50
with a variety of features.

The balancing benefit is significant but it need not be critical to every charge cycle. Packs
don't go out of balance THAT fast. It might happen over 10 cycles or 20 cycles and it builds
up over time. So using a regular charger that charges through the power plug is fine. If you
balance every few charges, that would be adequate. Just be sure to do it and you have to
have a way of being sure you are doing it across all your packs.

Note that a balancer can only drain power so it does reduce the overall charge level of the pack,
it does not bring up the low cells. But I don't think that is a big deal.


Balancing Chargers

There are two features being discussed here, charging and balancing.

Some are chargers combined with balancers. They charge the pack to the desired
level, then the built in balancer bleeds down the high cell and charging can continue.
This is a good combination. It saves you from having to do this with a separate device.
This type of charger provides the very significant value of keeping your packs in balance
automatically. This leads to longer life, and better performance. And it has some safety
benefits in that it prevents one cell from being over charged.

Balanced Chargers

Then there are balancing chargers that charge each cell individually during the charge
cycle. The CellPro 4S, for example, charges each cell individually during the charge cycle.
If one cell is a little slower than the others the charger compensates so higher rates can be
tolerated, or so the charger companies claim. The older CellPro 4S that I have has a safe
charge cycle that charges at up to 1.4C. This is a side benefit of the balanced charge process.
The newer Cell Pro 4S charges at up to 3C. If charging your packs faster,
safely, is important to you, then these types of balancing chargers are a good value. CellPro is not
the only one but it is a good example. So, from that respect, certain chargers, let's call
them balanced chargers, bring more benefits than just balancing.

Practical use

I have 5 lipo packs with CellPro balance taps. Most of the time I charge them on my CellPro
charger but I also charge them on my Triton charger and on an AC wall wart Lipo charger.
Only the CellPro balances, but the packs get on it every few cycles so they will be balanced
on the next charge cycle. And only the CellPro charges at the higher rate. The others are
limited to 1C and I will not push them.

Cold Weather Cycle

I don't know if this is a common feature but the CellPro 4S also has a cold weather cycle.
It actually detects the temperature of the surrounding air. If it is below a certain level, it only
charges the cells to about 95% of full charge. This has very little impact in practical use but
it provides a safety effect. If you were to charge a lipo pack at the field, say at 30 degrees,
then not use it and take it home, as it warmed the cell voltage would rise, potentially taking it
over the desired 4.2V level. I can not say how serious a concern this may be, but it seems
to make sense that it could present an unrecognized problem. This charger accounts for it
automatically. I am sure there must be others that do it as well.

I do feel the balancing chargers are better than balancers, BUT not enough that it should be a
big concern if you don't feel you want a second charger or the higher charge rates that some
of the newer ones can offer. But understanding the benefits of balancing IS important.

Other Reference Sources

CellPro discussion
http://www.rcgroups.com/forums/showthread.php?t=786192&page=2#post8863342
Notes on Lithium Batteries
http://www.rchobbies.org/lithium_battery_breakthrough.htm
safety warning on Lithium Batteries
http://www.modelaircraft.org/safetycom.asp
The Battery Clinic
http://www.rcbatteryclinic.com/
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