A battery pack usable as a power source of an electric device, is disclosed, in which battery modules are in series; each battery module is configured to include battery cells in series; a discharge controller controls discharge power which is supplied from the battery modules to the electric device; a modulator modulates a voltage of each module between a high voltage and a low voltage which is higher than zero and lower than the high voltage; the high voltage and the low voltage are set to allow a load device of the electric device to operate by the high voltage, and to allow the load device not to operate by the low voltage; and the discharge controller operates by the battery modules, irrespective of whether each module outputs the high voltage or the low voltage.