The battery is an electrochemical device for converting chemical energy into electrical energy. Active materials within the battery react chemically to produce a flow of direct current whenever lights, radio, starter motor, or other current-consuming accessories are connected to the battery. This current is produced by chemical reaction between the active materials of the plates and the sulfuric acid of the electrolyte.

When the battery is supplying electrical current to the accessories, it is said to be discharging. When the engine is running, the generator supplies current to the battery and it is said to be charging.