Power supply device and method for controlling charging/discharging operation of power supply device

When a remaining capacity estimated by a calculator 13 is higher than a control upper limit, a charge-disabled mode is selected in that charging operation is disabled. When lower than a control lower limit lower than the control upper limit, a discharge-disabled mode is selected in that discharging operation is disabled. When 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 that charging/discharging current limits are adjusted to allow a discharging amount to exceed a charging amount in a predetermined period. When not higher than the upper limit of the range, a restriction mode is selected in that charging/discharging currents are restricted so that the remaining capacity is held within the range.