Abstract

A method and an apparatus (10) for equalizing the state of charge among a plurality of series-connected batteries (B1-BN). A module (12A-12N) is connected in parallel with each of the batteries (B1-BN), respectively. Each module (12A-12N) contains a voltage divider circuit (40) so that the voltage across a battery may be measured to determine the state of charge, and a circuit (43) which selectively shunts charging current around a battery or applies a discharging current to a battery. A charging transistor (30) applies a charging current to the series of batteries (B1-BN) and, depending upon the state of charge of each battery, a portion of the charging current may be shunted around the battery by its associated module (12) so as to prevent overcharging of the battery. A controller (14) monitors the state of charge of each of the batteries and the temperature of each of the batteries and adjusts the portion of the charging shunted around an individual battery so as to rapidly equalize the state of charge among the different batteries (B1-BN). This process is automatically conducted whenever a charging current is applied to the batteries so that the batteries are maintained in an equalized condition.