Optical mains smoke alarm 10yr lithium battery backup this optical smoke alarm runs on 230v ac mains power it has built in tamper proof lithium cell which acts as a battery backup in the event of mains failure the lithium cells have been designed to have a 10year life span the alarm gives a fire warning when smoke enters the alarm the easi fit base allows for quick and simple installation and it connects both mains power and battery as the alarm slides on