If an input voltage to an output control device is above a lower limit value, the output control device operates in a constant voltage control mode for maintaining an output voltage at a substantially constant voltage. If the input voltage of the output control device falls below the lower limit value, the output voltage is controlled to be lower than the constant voltage in the constant voltage control mode, and the output control device operates in a lower limit value sustaining mode for maintaining the input voltage at the lower limit value. Then, the output control device shifts to a current increasing mode in which the output current rises as the output voltage falls.