The Ei156TLH is an Optical Smoke Alarm that runs on 230V AC mains power, and has built in tamper proof rechargeable lithium cells that act as a battery back up in the event of mains failure. These rechargeable lithium cells are designed to have a ten year life and outlast the life of the smoke alarm itself, whilst providing up to six months of smoke alarm operation without mains power. The Ei156TLH has other advanced features such as high performance optical chambers fitted with integral insect screens to reduce the chances of false alarms, as well as the ability to interconnect up to twelve alarms to allow all alarms to sound if just one of the interconnected larms should be triggered. The Ei156TLH has built in circuitry to aid suppression of voltage transients and RF interference to further reduce the chances of false alarms under such conditions.