Abstract: A switch-mode battery charger in which a pair of field effect transistors are operated at a relatively high switching rate. Protection against excessive overload is provided by directly monitoring the pulse trains produced by the transistors rather than the output charging current and reducing the duty cycle of the transistors when an excessive overload is detected. Protection against batteries which cannot take a charge is provided by monitoring the temperature in the vicinity of the transistors and first turning on a fan to cool the transistors and then disabling the transistors if the fan has failed to reduce the temperature an adequate amount.