Battery pack for electric power tool

A battery pack for an electric power tool includes a control device (40) that determines the necessity of a balancing control to reduce difference in voltages of cells (11-15) of a battery (10) based upon voltages of the respective cells (11-15) measured by a voltage measuring device (28). The control device (40) controls a subject cell (11-15), which is subject to the balancing control to discharge via the discharging device (21-25) when the balancing control is determined to be necessary. A voltage measuring device (28) measures the voltages of the respective cells (11-15) when a battery charger detecting device (56) detects the connection of the battery charger (60) in a state where the battery (10) has not been connected to the battery charger (60) for a predetermined specified period of time or more and power feeding to the electric power tool has not been performed for the specified period of time or more.