K7124C
In a proactive inspection it appears the smoke alarm in my hallway doesn’t work due to a dead battery, it seems like replacing batteries isn’t needed since it could be powered and recharged via a light socket right?. After a quick search, turns out there are ones that do just that.

“Never worry about changing batteries again. This unique smoke detector screws into any ceiling light fixture and operates off your home electricity. Reinstall the light bulb into the alarm, and now you’ve got a light source and smoke alarm in one! Recharges itself every time you turn the light on. Great protection for high-risk areas such as laundry rooms, garages, and workshops. Fully charged, it will operate for up to 30 days in the event of a power loss.” [via] – Link.