In a power supply system (10) and a method for controlling the same, at least one battery from among a plurality of batteries (12a-12d) is designated as a charging-side battery, and the remaining batteries are designated as discharging-side batteries. Next, the difference in current between the current flowing from the discharging-side batteries and the current flowing into the charging-side battery is determined on the basis of currents (I1-I4) measured by a plurality of current measuring instruments (26a-26d). Next, the transformation rate (Tr) of a voltage transformer connected to the discharging-side batteries is determined on the basis of the determined difference in current.