The invention relates to a battery charger, comprising an input rectifier filter circuit, a high frequency inverse-excited converter circuit, an output rectifier filter circuit, an auxiliary power circuit, and a temperature detection-control circuit. It is characterized in that a constant-current stable-voltage control circuit which is formed by a current voltage detect-feedback circuit, an photoelectric coupled circuit, and a impulse-width modulation control circuit in the cascade connection; The rectifier filter circuit transfers the incoming alternating current into direct current, and via the high frequency inverse-excited converter circuit, to separate and output direct voltage to charge the battery; the constant-current stable-voltage control circuit and the impulse-width modulation control chip are combined to control the constant-current charging and the stable-voltage protection; the temperature detection-control circuit is used for temperature protection of battery to avoid overcharge. Said invention has the advantages of high property, lower cost, safety and reliability, while it can be used to charging variable nickel-cadmium battery, nickel-hydrogen battery and lithium battery.