When a time average of square values of charge-discharge current exceeds a predetermined reference value and when a state of charge of a battery is higher than a preset reference level, the procedure corrects a basic input limit set according to a battery temperature and the state of charge and sets the corrected basic input limit to an effective input limit. When the state of charge is lower than a preset reference level, the procedure of the invention corrects a basic output limit set according to the battery temperature and the state of charge and sets the corrected basic output limit to an effective output limit. This arrangement more adequately sets the effective input limit and the effective output limit, compared with a technique of simultaneously correcting the effective input limit and the effective output limit based on the temperature of the battery.