UL/CSFM Ref. Kidde Part i4618AC. Hardwired 120-volt power source with a 9-volt battery back-up. Red LED blinking lights indicate when: alarm is in standby mode, smoke or fire initiated the alarm, or is in hush mode.
Pro: fully compatible with older Firex model... Con: One of the four alarms was sensitive to the kiddle branded brand new battery. It was chirping all the time. Cleaned contact did not work. Replaced it with a new Duracell battery and the problem stopped. I had in the past battery poles heights/dimensions issues in other electronic devices. Some old tolerances issues with battery manufacturers.
Buying a few more to complete updating my smoke alarm system.