A vehicle-mounted power supply system comprises a first storage battery, an automotive generator for generating electrical energy and charging the first storage battery with the generated electrical energy. A second storage battery is connected to an electrical load and a power converter supplies electric energy from the first storage battery to the second storage battery. A controller, or battery-monitoring unit, sets the power converter in a sleep mode when the vehicle engine is stopped, periodically detects the voltage of the second storage battery as a pseudo-open circuit voltage during the sleep mode of the power converter if current of the second storage battery is within a predetermined range, estimates a state of charge of the second storage battery from the detected pseudo-open circuit voltage and sets the power converter in a run mode when the estimated state of charge is lower than a predetermined value.