In a vehicle-mounted power supply system, the automotive generator generates a voltage for charging a first battery. Power conversion circuitry is connected between the first battery and a second battery that is connected to an electrical load to operate in a first mode or in a second mode when power consumption of the load is lower or higher than a specified value, respectively. In the first mode, the power conversion circuitry maintains the voltage at the second battery at a substantially constant level and in the second mode, the voltage at the second battery varies with the voltage at the first battery. In response to a vehicle speed variation, the automotive generator is controlled so that its output voltage is varied at a first rate when power conversion is performed in the first mode and varied at a second rate lower than the first rate when power conversion is performed in the second mode.