Patent ID: 7728553

Claim:
A battery pack of a cordless power tool system including a power tool and a charger, the pack removably attachable to the tool and charger, the battery pack comprising: a housing, a plurality of battery cells within the housing, a battery electronic control unit within the housing for controlling the charger during a charge of the cells, and for controlling a field effect transistor (FET) during discharge operations to control current to a motor of the power tool, a first voltage monitoring unit for monitoring one or both of individual cell voltages and total stack voltage in the pack during charge and discharge operations, and a second voltage monitoring circuit for separately monitoring individual cell voltage to detect any cell reaching an over-voltage condition during a charge, wherein, upon detecting any cell reaching an over-voltage condition, the second voltage monitoring circuit, without interfacing with the battery electronic control unit, sends a signal to the charger to generate a disable signal in the charger for terminating charge current to the pack.