A detector assembly (10) powered by mains electricity and an elongate backup battery (76) is disclosed. The assembly includes: a ceiling mountable base (10) having a body defining a downwardly opening socket (24); a detector module (20) having a battery support portion (75) for holding the battery (76) in a vertical orientation, the module (20) receivable by the socket (24); a quick connection means for mechanically and electrically connecting the module (20) to the base (10); and shutter (50) mounted to the base (10). The shutter (50) prevents operation of the quick connection means when no vertical battery (76) is installed within the module (20). The quick connection means comprises co-operable pairs of conductors. The conductor pairs transmit power from the base (10) to the module (20). The pairs of conductors each comprise a stud conductor mounted to the base (10) and a forked conductor mounted to the module (20).