Abstract: A circuit tests a back-up battery for a power supply. A power source is coupled to a load to deliver power at a normal rated voltage during normal operation. However, during a battery test interval, the power source voltage is lowered to a test mode voltage. This lowers the load voltage, and in response, the circuit attempts to supply the load from the battery. The load voltage is monitored during the test interval to determine if the battery is able to supply rated power. However, the main power source is still instantly available at the test mode voltage to drive the load if the battery fails during the battery test interval. This ensures uninterrupted operation during the test mode. Upon failure of the battery during the test mode, the power supply is quickly ramped up to the normal rated voltage (although the test mode voltage meets output specifications).