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The alternator, also known as an AC generator, is a device that converts mechanical energy from the engine into electrical energy. It is driven by a belt at two to three times the crankshaft speed by the engine crankshaft pulley. While the automobile's engine was running, the alternator produced electricity to recharge the battery and supply the electrical components with the amount of electricity required. In most automobiles, alternating current alternators are preferred over direct current dynamos in electric power generation performance and longevity. The alternating current produced by the alternator is rectified just before output since most of an automobile's electrical components require direct current.