Power supply system and control method for the system

In a power supply system in which a load (22) is connected to a battery (17), the battery is efficiently charged without degrading the characteristics of the load. When a load (22) that deteriorates in characteristics if a voltage exceeding the rated voltage of the load is applied thereto is connected to the battery (during an on-state of the load), the battery (17) is charged with the rated voltage even if there is a need to charge the battery (17) with a charging voltage higher than the rated voltage. Alternatively, the battery (17) may be charged for a predetermined time at a voltage above the rated voltage and then at or below the rated voltage, or the voltage seen by the load may be converted to a voltage at or below the rated voltage while the battery (17) is being charged at a voltage above the rated voltage. When the load is off, the battery (17) is charged with a voltage higher than the rated voltage.