Credit transfer within market-based resource allocation infrastructure

An improved a.c. voltage powered warning device for detecting the occurrence of predetermined phenomena, is characterized by battery backup for providing power to the device in the event of interruption of the a.c. voltage. In the disclosed embodiments the warning device is a fire detector having circuitry for supervising the energy level of the backup battery and for providing a warning upon the energy level decreasing to a predetermined value, and for testing a combustion detecting and alarm generating portion of the detector.