The alternator is a generator for your vehicle’s battery. It creates an alternating current that is converted to direct current, because your car’s accessories run off of a 12V DC system because the battery produces 12V DC. Things like the stereo, lights, power windows and everything that requires electric power in your vehicle depends on the 12V produced from the vehicle’s battery and that battery requires the alternator to keep it fully charged. The Alternator is normally driven from the crank pulley. Current is produced when a conductor is passed through a magnetic field and this is what the alternator does when its pulley is turned by the crank of the engine. An alternator will provide a certain amount of charging voltage for the battery which is limited by its internal components because a battery should not be overcharged. The average car battery should measure 12.5V when at rest i.e. car is off. When the car is running at idle the battery should measure at least 13.4V. This is a good way to tell if the alternator is charging properly. Most of the components in an alternator can be replaced; this is why rebuilt, remanufactured and even used alternators are quite popular when having to replace an alternator. The new ones are normally quite expensive and may not be worth the extra cost for a lot of people, so the replacements seem to depend quite heavily on wallet thickness.