When remaining capacity estimated by a calculator is higher than a control upper limit, a charge-disabled mode is selected in which charging operation is disabled. When remaining capacity is lower than a control lower limit, a discharge-disabled mode is selected in which discharging operation is disabled. When remaining capacity is higher than an upper limit of a given range having its center at a target value specified between the control upper and lower limits, an allowance mode is selected in which charging/discharging current limits are adjusted to allow a discharging amount to exceed a charging amount in a predetermined period. When remaining capacity is not higher than the upper limit of the range, a restriction mode is selected in which charging/discharging currents are restricted so that the remaining capacity is held within the range.