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, cranking 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.

The battery performs four functions in the electrical system, First, it supplies electrical energy for the accessories when the engine is not running. Second, it supplies of energy to the cranking motor and ignition system as the engine is started. Third, it intermittently supplies current for lights, radio, heater and other accessories when the electrical demands of these devices exceed the output of the generator. Fourth, it acts as a voltage stabilizer for the electrical system.

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.