Fire Angel Optical Smoke Alarm 5 year lifetime battery Designed to make protecting your home simple, with easy install and rigorous testing by independent parties, such as BSI Kitemark, to ensure quality and longevity. Optical sensing - a fast response to slow burning fires. Testing made simple with large self-test button. Sleep easy function silence low battery warnings for 8 hours Fire Angel Optical Smoke Alarm - FA6615-R Safety, SiteSafety, FireSafety, SmokeAlarms&Detectors.