A power supply system includes a first-battery, a battery unit including a second-battery, and a generator. The battery unit is connected to the generator and the first-battery via a conductor. The battery unit includes a connection-terminal connected to the conductor, an output-terminal connected to a load, a first-switch disposed on a wiring connecting the connection-terminal and the output-terminal, a branch line branching off from the wiring between the first-switch and the output-terminal and connected to the second-battery, a second-switch disposed on the branch line between the wiring and the second-battery, a first-voltage-detecting unit detecting voltage of the connection-terminal, and a switch control unit controlling the first and second-switches. The power supply system further includes a second-voltage-detecting unit detecting electromotive force of the first-battery, and a fault-determining unit determining a fault of the conductor based on opening/closing states of the first and second-switches and results of the first and second-voltage-detecting units.