An auto battery is used mainly for starting your engine. The starter draws current from the battery so that it can crank the engine and begin the combustion cycle. While the engine is running, the alternator is driven by a belt which in turn provides a power source that powers the electric components of the vehicle and also recharges the battery. When the engine is not running, the battery provides the power source for the electric components.

A battery gives off electricity due to chemical reactions inside. When the battery is connected to something in a circuit it works like this. One side of the battery is Positive where electricity flows out and into whatever it is connected to. The reason it needs to be connected to negative is because the electricity cannot just flow to a dead end.

In a car? The battery provides a current to all of the electrical demands of a car, and its most important task is powering the starter to start the car. It is recharged by the alternator when the car is started.

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chemical reaction inside the battery