Optical mains smoke alarm cw 9v battery backup this optical smoke alarm that runs on 230v ac mains power and has a 9v alkaline battery back up capable of lasting up to 4 years in standby mode and 2 years without mains power the alarm gives a fire warning when smoke enters the alarm the alarm has a built in circuitry to aid suppression of voltage transients and rf interference to further reduce the chances of false alarms