I received an email on the Net recently asking me about why electric RC'ers and other RC applications don't often adopt Nickel-Metal-Hydride (NiMH) cells instead of Nickel-Cadmium (NiCad):
Q:Anyway, my question for you if you have the time - have you investigated NiMH batteries? They claim to have twice the capacity for comparable weight & size... seems like a free lunch to me... Opinions?
A: NiMH cells indeed have higher energy capacity per ounce. The show-stopper is that they also have very high internal impedances (resistance). This means two bad things:
These two problems require us to use special NiCad cells that are designed for very rapid discharge and charge cycles. These are torturous conditions, and to manage them we do pay a price in terms of capacity. But they're getting better. The "sub-C" class cells used to be 1200 mAH in capacity in 1985 when I started into RC and electrics. Now they're 2000 mAH, and they're more rugged and reliable. They're getting better, although it's the low-current-drain end of the battery market (NiMH, etc.) that's being developed the quickest due to laptop computers.
An aside: why is high cell resistance a problem? Ohm's law says
, where V is voltage, I is current, and R is
resistance. If the resistance of a cell is high, and the current draw
is high (i.e. I and R are large), then the voltage "lost" due to
internal heating will also be large. This means that
when you go to full throttle, you won't get much additional power because
all of the energy is going into heat. We can't change I much, because we
still need plenty of current to fly our planes, but we can choose cells
with small R so that the cells don't melt and we maintain high efficiency.