A battery pack for an electric power tool includes a control device that determines the necessity of a balancing control to reduce difference in voltages of cells of a battery based upon voltages of the respective cells measured by a voltage measuring device. The control device controls a subject cell, which is subject to the balancing control to discharge via the discharging device when the balancing control is determined to be necessary. A voltage measuring device measures the voltages of the respective cells when a battery charger detecting device detects the connection of the battery charger in a state where the battery has not been connected to the battery charger 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.