The Battery Powered Smoke & Carbon Monoxide Voice Alarm (2 AA batteries included), is equipped with self-testing components that are always checking to make sure the alarm is on and working.* This smoke and CO combo alarm helps protect you and your family from 2 dangerous threats – fire and carbon monoxide – in one unit. What’s more, the included mounting bracket makes installation quick and easy. Battery-powered smoke alarms provide reliable detection even during power outages, for extra peace of mind. Leading authorities recommend that both ionization and photoelectric smoke alarms be installed to help ensure optimal detection of the various types of fires that can occur within the home. Ionization sensing alarms may detect invisible fire particles (associated with fast flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms can detect visible fire particles (associated with slow smoldering fires) sooner than ionization alarms.