FireAngel Toast Proof TST-625R Thermoptek Smoke Alarm with 5-year batteries Pack of 2.This FireAngel smoke alarm uses Thermoptek multi-sensor technology which pairs the the very latest in optical sensing with thermal enhancement and when compared to single sensor alarms reacts to fire in under half of the time. It comes with replaceable batteries which will need replacing once you hear the low battery alert. This alarm cannot be connected to other alarms in your home.