Interconnect Hardwire Smoke Alarm with Battery Backup. Interconnects with most First Alert and BRK hardwired alarms. When one alarm sounds, all compatible alarms sound (up to 12 smoke alarms and 6 other alarms). Smoke particles of varying number and size are produced in all fires. For maximum protection, use both ionization and photoelectric sensing technologies. Ionization alarms are generally more sensitive to fast flaming fires and photoelectric alarms are generally more sensitive to smoldering fires. End-of-Life Warning, Precision Detection, Battery Backup, Hardwire Interconnectivity, Hardwire Quick Connect Plug Included.