POWER SUPPLY DEVICE

A control circuit of a power supply device forms a state in which an auxiliary power supply is connected via a charge circuit to a main power supply when a power switch provided on a power feeding path is turned on. The control circuit calculates an internal resistance of the auxiliary power supply based on a charging current. The control circuit forms a state in which the power supply device is disconnected from the main power supply and a power feeding target, and only the auxiliary power supply and a boost circuit are connected to each other when the power switch is turned off. The control circuit disconnects the auxiliary power supply from the boost circuit when a terminal voltage of the auxiliary power supply is less than a stop determination threshold value.