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.