Patent ID: 7521898

Claim:
A charger for charging a secondary battery through a charging unit configured to control input power to be constant by using, as an input source, a fuel cell or a solar cell, having a relatively large impedance in a power-supply mode, comprising: a current-control circuit that is connected to the secondary battery, and a constant-power-reference-voltage control circuit that is connected between the current-control circuit and an input of the charger, wherein the constant-power-reference-voltage control circuit is configured so that, when an output of the charger is in a drooping state, by decreasing a reference voltage to increase supply power, constant power is obtained at a voltage determined by output-voltage-stabilizing control of the charger, and, when input power to the charger is in an excessive state, by raising the reference voltage, a reference value corresponding to the supply power is set, wherein the constant-power-reference-voltage control circuit includes two constant current circuits, an input-voltage detecting comparator, an output-voltage detecting comparator, and a reference-voltage capacitor, and the constant-power-reference-voltage control circuit is configured so that, when an output detected by the output-voltage detecting comparator is in a drooping state, by causing the reference-voltage capacitor to discharge through the constant current circuit to raise the output voltage, constant power is obtained at the voltage determined by controlling output voltage of the charger to be stabilized, and, when input power detected by the input-voltage detecting comparator is in an excessive state, by charging the reference-voltage capacitor through the constant current circuit, the reference value corresponding to the supply power is set.