Q-86: I intend to purchase two sets of Li-Poly batteries from a local hobby shop. Each set has three cells in series. I plan to connect each three-cell pack in parallel to increase the capacity, so I will end up with what you would designate as a “3S2P” pack. The cells of this pack are rated a 2100 mAh capacity and have a load capability of 20C. When I use this pack in my aircraft—six cells all together—the total capacity will be 4200 mAh (2 X 2100). My question is, can I charge the entire pack as one 4200 pack, or must I charge them separately?
A-86: The basic answer is that you may charge them together. You would
treat it as three cells in series—“3S”—and you would also, capacity wise, treat
is as “2P” because each of the two sets of three cells are then connected in
parallel. When hooking up the packs in parallel you add the capacity. So two
packs of 2100 cells will, when connected in parallel, provide 4200 mAh capacity.
You would attach you charger to the two output leads coming from your battery.
Set the charger to three cells (unless it automatically sets) then select a 1C
charge current rate, which in this case would be 4.2 amps (based on 4200 mAh-rated
capacity. You would need a charger capable of this higher current level like the
Astro Flight 109 or the Great Planes Triton.
But the story goes a little further. As you get into larger capacity Li-Poly
packs and greater combinations of series and parallel hook-ups, you should get
into the habit of periodically checking cell voltage using a simple digital
voltmeter. I realize that getting into the terminals of each cell can be a
problem. Some vendors are now providing external connections for easy access to
the internal cells. If you are lucky to have such a pack, you want to make sure
that all cells in the pack stay within approx. 0.1 volt of each other at
completion of charge. If they get notably out of balance, you should resort to
charging each cell individually. You should be doing this sort of check on a
regular basis.
New chargers are being developed that will charge each cell in a pack on an
individual basis. As each cell reaches the maximum voltage under charge, the
charge to that cell is cut off. When the last cell in the pack reaches full
voltage, you have a fully charged and balanced pack. That will be the way of the
future!