An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current. Usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Alternators are used in modern automobiles to charge the battery and to power a car’s electric system when its engine is running. Automotive alternators use a set of rectifiers (diode bridge) to convert the AC output of the alternator to the DC used in vehicle’s electrical system .

Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water environment. Marine alternators are designed to be explosion proof so that brush sparking will not ignite explosive gas mixtures in an engine room environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits, the power is split between the engine starting battery and the domestic or house battery (or batteries) by use of a split-charge diode (battery isolator) or a mechanical switch (battery switch)

-adapted from Wikipedia