The present invention relates to a battery monitoring and charging system capable of determining when charging conditions are favorable, thereby adjusting the charging level to maximize fuel saving. In particular, the system increases the alternator field current to charge a battery during vehicle deceleration so that vehicle momentum, instead of engine power, is used to torque the alternator shaft. The system controls the alternator field current with a voltage regulator. The voltage regulator senses the charge level of the battery, the amplitude and direction of the battery current load, and vehicle operating conditions to provide the proper current into the alternator rotor for maximum fuel savings. The voltage regulator may be a non-microprocessor or a microprocessor controlled device.