Dual-power-supply system

1. A dual-power-supply system using both a fuel cell and a chargeable secondary battery, comprising:

a differential voltage adding unit configured to add a differential voltage between a voltage of the fuel cell and a necessary target output voltage to the voltage of the fuel cell by using the secondary battery, thereby obtaining an output voltage of the dual-power-supply system; and

a control unit configured to detect the output voltage of the dual-power-supply system, and to control the differential voltage adding unit such that the output voltage is equal to the target output voltage.

2. A dual-power-supply system according to claim 1 , wherein the control unit controls the differential voltage adding unit to add a positive differential voltage to the voltage of the fuel cell when the output voltage is lower than the target output voltage, and to add a negative differential voltage to the voltage of the fuel cell when the output voltage is higher than the target output voltage.

3. A dual-power-supply system according to claim 2 , wherein the differential voltage adding unit adds the positive differential voltage to the voltage of the fuel cell by pulse discharging the secondary battery, and adds the negative differential voltage to the voltage of the fuel cell by pulse charging the secondary battery.

4. A dual-power-supply system according to claim 3 , further comprising an overcharge and overdischarge preventing unit configured to prevent overcharge and overdischarge of the secondary battery.