The battery, starting and charging systems are interrelated by a continual cycle of converting chemical energy to mechanical energy and then back again. The starter motor uses mechanical energy to crank the engine and once the engine is running, it’s really a merry-go-round sort of relationship. The rotation of the engine drives the alternator, forcing electrical energy (current) into a battery, where it’s stored as chemical energy.
The chemical energy of the battery is then changed back to electrical energy when it supplies current to the starter motor and accessories. The cycle repeats itself as the engine’s mechanical energy again drives the alternator to recharge the battery so it can supply more current to the starter when needed.