First Alert BRK SC9120LBL Hardwire AC Smoke and Carbon Monoxide Combination Alarm with 10 Year Sealed Battery Backup 10Yr Lithium battery backup Locked battery drawer Electrochemical carbon Monoxide sensor - the most accurate sensor available as compared to other Sensing technologies Latching alarm indicator - remembers which unit initiated an alarm Intelligent Sensing technology helps reduce the Number of nuisance alarms