According to one embodiment, a power supply device includes a fuel cell, a battery, a switch, and a controller. The fuel cell supplies electric power to an electronic device provided with a built-in battery. The battery is charged with electric power supplied from the fuel cell and supplies electric power to the electronic device. The switch switches the battery between charge mode and discharge mode. The controller controls the switch to switch the battery between the charge mode and the discharge mode. The controller calculates integrated discharge electric power and integrated discharge amount of the battery and, when the integrated discharge electric power reaches predetermined discharge electric power, switches the battery from the discharge mode to the charge mode. The controller calculates integrated charge amount of the battery and, when the integrated charge amount reaches the integrated discharge amount, switches the battery from the charge mode to the discharge mode.