A battery charger receives direct current power from an external source through a power connector. Switch means receives the power from the power connector and supplies it to a boost circuit which is removably and conductively connected to a battery. A control circuit is connected to receive a battery voltage signal from the battery and a battery temperature signal from a temperature sensor positioned to detect battery temperature. The control circuit determines the state of charge of the battery from the voltage and temperature signals. The control circuit supplies (1) switching signals to the switch means to cause the switch to be conductive, and (2) boosting signals to the boost circuit to cause the boost circuit to charge the battery in accordance with a preselected charging program related to the state of charge of the battery. A safety circuit may also be adapted to sense unsafe battery and battery charger parameters and cause the control circuit to generate a switching signal to cause the switch means to be nonconductive upon the occurrence thereof. The battery charger is particularly adaptable for use with vehicles.