Q-95: I’m building a small Spirit of Sam electric airplane and I have a question concerning battery packs. If I hook up four AAA batteries in series and RadioShack says that each battery cell is 300 mAh capacity, is the resulting pack going to have 1200 mAh capacity (4 times 300)?
A-95: This is a basic question, but one that will always be important.
If you take four AAA batteries (in this example either Ni-CD or NiMH) and
connect them in series (like one big link) you will have four times the nominal
cell voltage. For Ni-Cd or NiMH cells this would be four times 1.2 volts
(nominal) for a total of 4.8 volts. But when you connect these cells in series,
the capacity of the pack is still the same as a single cell in that pack, namely
300 mAh. The rule when connecting the same kind of cells in series is that the
voltage is added (for each cell), but the capacity stays the same as the rating
of the single cell.
If you connected these same four cells in parallel, the voltage would remain the
same at 1.2 volts (like for a single cell!), but the capacity would now be added
like four times 300 mAh for a total of 1200 mAh. If you need a four-cell battery
pack with 1200 mAh capacity, you would need to buy four cells each rated at 1200
mAh. Placing the cells in parallel, you would achieve the capacity, but not the
necessary voltage and hence, your model would never get off the ground.