A circuit for controlling a power supply is disclosed. The circuit has a storage element for storing energy harvested from energy sources, a backup battery, and a comparator to compare a voltage of the storage element to a reference voltage. The circuit also contains a first switch for selectively connecting the storage element to an output. A second switch connects the backup battery to the output. A switch control circuit controls the first switch to disconnect the storage element from the output and the second switch to connect the backup battery to the output, if the voltage of the storage element is below the reference voltage. The backup battery is disconnected from the output while the voltage of the storage element is below the reference voltage. The control circuit controls the second switch to disconnect the battery from the output while the voltage of the storage element is below the reference.