The Hager TG501AL is an optical smoke detector with a lithium back-up battery. Connect the smoke detector to an existing 230-volt current point in your ceiling. The lithium backup battery lasts up to 10 years. Even if the power goes out, you can be sure that the smoke detector detects smoke in time. Have the detector work independently or linked. If you have more Hager smoke detectors at home you can link them via a cable, so the audio signal of 85 decibels will sound in every room in which a smoke detector is installed. Always make sure you put a smoke detector along the escape route, like the hallway, landing and living room. Every other room in the house is an extra addition to your safety.