The alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.The modern type of vehicle alternators were first used in military applications during World War II, to power radio equipment on specialist vehicles. After the war, other vehicles with high electrical demands — such as ambulances and radio taxis — could also be fitted with optional alternators.

Alternators were first introduced as standard equipment on a production car by the Chrysler Corporation on the Valiant in 1960, several years ahead of Ford and General Motors.

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. Efficiency reduces dramatically at high speeds mainly due to fan resistance. At medium speeds efficiency of today's alternators is 70-80%. This betters very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60%.

Until the 1960s, automobiles used DC dynamo generators with commutators. As silicon diode rectifiers became widely available and affordable, the alternator gradually replaced the dynamo. This was encouraged by the increasing electrical power required for cars in this period, with increasing loads from larger headlamps, electric wipers, heated rear window and other accessories.

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