an apparatus is provided to control a power voltage on a power - supplying line extending from a generator and connecting to a battery and eclectic loads . the apparatus is mounted on a vehicle and comprises a detecting device , a calculator , and a controller . the detecting device detects pairs of voltage and current of the battery . the calculator calculating a control current on the basis of the detected pairs of voltage and current and a target voltage for the power voltage . the pairs of voltage and current are used to calculate an internal resistance and / or a regression line of the battery . the controller controlling a charge and discharge current of the battery on the basis of the control current so that the power voltage is controlled to the target voltage .