A power supply system including: a first battery connected to a first load; a second battery; a DC-DC converter connecting the first battery and the second battery; and a connection switching unit including a first switch configured to connect the first battery to a second load and a second switch configured to connect the second battery to the second load. The connection switching unit is configured to switch selectively to a first mode in which the first switch is closed and the second switch is opened and a second mode in which the first switch is opened and the second switch is closed.