The UL-listed Fire Sentry Battery-Operated Ionization Smoke Alarm is a single-station alarm with an early warning signal. This alarm utilizes ionization sensing technology to activate in the event of a smoke or fire incident and its compact, 4 in. design lends it a discreet profile in your home or business. Powered by a 9-volt battery, this smoke alarm will protect you, your family and home during a power outage. When the battery begins to lose power, a low-battery alert will begin to chirp to notify you it is time to replace.