Disclosed herein is a power supply device and a control method for the power supply device. The power supply device includes: a battery; an external interface electrically connected to the battery and electrically attachable to or detachable from an external device; a voltage converter provided between the battery and the external interface; a low-voltage circuit that branches from an interface connection circuit connecting the external interface and the voltage converter and is connected to a low-voltage load having a rated voltage lower than a voltage of the battery; an interrupt circuit provided in the low-voltage circuit and configured to electrically interrupt the low-voltage circuit and the interface connection circuit; and an electronic control unit configured to control operations of the voltage converter and the interrupt circuit. The electronic control unit operates the interrupt circuit when a voltage of the interface connection circuit exceeds the rated voltage of the low-voltage load.