What does a car battery do in a car?

Answer 1

Powers the starter motor and all the other electrics in the car when the engine is not running. When the engine is running, the battery is not needed because the alternator of which works very much like a bicycle dynamo runs all of the electrics in the car. It also keeps the battery charged.

If fitted to a bicycle a dynamo will generate electricity to the bike lights when you peddle.

An alternator does the same thing but with engine power instead of peddle power. the only difference between a dynamo and an alternator is that an alternator alternates the current from AC to DC a dynamo does not.

Answer 2

Provides 12 volts of DC power.