Vehicle alternator control device and method

A vehicle alternator control device (1) has a power transistor (11), a flywheel diode (12), a voltage control circuit (13), a primary power supply circuit (14), and a secondary power supply circuit (15). When the alternator 2 starts rotation, the secondary power supply circuit (15) drives the primary power supply circuit (14) when the alternator speed reaches a frequency (fs) equivalent to a speed higher than an engine idling speed. When the engine stops and the speed of the alternator (2) drops, the secondary power supply circuit (15) stops the primary power supply circuit (14) when the alternator speed reaches a frequency (fe) equivalent to a speed below the engine idling speed.