In a power supply system and a method for controlling the same, at least one battery from among a plurality of batteries 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 measured by a plurality of current measuring instruments. Next, the transformation rate of a voltage transformer connected to the discharging-side batteries is determined on the basis of the determined difference in current.