Alternator

An alternator is an electromechanical device that converts the mechanical energy produced by the motorcycle engine into electrical energy in the form of alternating current.

Most alternators use a rotating magnetic field around a conductor to produce current. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an induced EMF, as the mechanical input causes the rotor to turn.

The alternator on a motorcycle provides the power that recharges the main battery when the engine is running, therefore indirectly powering all electrical components on the motorcycle. The power produced is stored in the battery. Flat batteries occur when the motorcycle hasn't been running for a while, hence all charge is lost.