The alternator belt or V-belt is turned by the engine, which spins the rotor. The rotor is a magnet or group of magnets inside copper wires that spins fast to create electricity used to charge the battery.
A battery itself only has enough power to keep a car running for a few miles and then needs to be re-charged. An alternator must have an output of 13.5 to 14.8 volts in order to maintain a 12-volt battery at 100 percent charge and run all of the vehicle's electrical needs. The modern alternator has three main parts: stator, rotor, diode and voltage regulator. The rotor consists of a magnet inside a nest of copper wires, which represent the stator. The diode is the part which changes the electricity from AC to DC current which is what the battery uses. The last component is the voltage regulator, a built-in part in modern alternators. The regulator limits the flow of electric to the battery to prevent the battery from going above 14.5 volts and being overcharged or ruined. In older cars the regulator was a black box located under the hood somewhere and was wired into the vehicles electrical system. As the battery loses power the regulator and alternator work to re-charge it.