Combination Carbon Monoxide & Smoke Alarm provides two important safety devices in a single unit. This alarm includes a voice warning system that announces "Fire, Carbon Monoxide, Low Battery & Smart Hush Activation." The voice alarm eliminates any confusion & clearly warns you of a smoke or carbon monoxide danger, or if your battery is in need of replacement. This technologically advanced combination alarm is battery-operated, offering protection even during a power outage when many incidences occur. It is easy to install. This alarm uses ionization sensing technology. Battery-powered alarm provides protection even during power outages when many fire & carbon monoxide incidents occur. The battery door will not close unless the two AA batteries are properly installed.