PURPOSE OF THE BATTERY
ENGINE IS OFF: Electricity from the battery is used to operate lighting, accessories, or other electrical systems when the engine is not running.
ENGINE IS STARTING: Electricity from the battery is used to operate the starter motor and to provide current for the ignition system during engine cranking. Starting the car is the battery's most important function.
ENGINE IS RUNNING: Electricity from the battery may be needed to supplement the charging system when the vehicle's electrical load requirements exceed the charging system's ability to produce electricity. Both the battery and the alternator supply electricity when demand is high.