There are two sources of electrical energy in an aircraft: (1) The generator, which converts mechanical energy into electrical energy, and (2) the battery, which converts chemical energy into electrical energy. During normal engine operation, electrical energy is taken from the engine driven generator. The storage battery is used as an auxiliary source of power when the generator is not operating.

When the generators are operating at a speed too low to supply electrical energy for the airplane, electrical power is taken from the battery and the battery discharges, losing some of the chemical energy stored in it.

During flight, the airplane generator charges the battery over a long period of time and restores the chemical energy. Lead-acid and nickel-cadmium batteries are the types of storage batteries in general use.