At the point you turn your ignition key, a battery must provide a considerably high current for a short time, for example, lasting as much as 300 amps for only 15 seconds.

This process was designed to start a motor only once. The battery does not keep a car running, it’s the alternator that does that, generating the electricity required to keep the engine going and enabling other operating devices.

The starter’s job is finished once the car starts, and is dormant until the car needs starting again. The battery, after providing the current needed to start the car, is also needed to absorb sudden drops or surges in voltage. These are called voltage spikes. A minus 200 volt spike can occur once the car is started, then at other times large positive and negative spikes can occur. This fluctuation of electricity must go somewhere, so the battery acts as a buffer and soaks it up, basically shielding the car’s electrical system from damaged.