We've lived in our house for a long time and had many power outages over the years - heck my UPS says that it has kicked on for power dips/outages 70+ times since April of this year.

But new in the last year is that when the power goes out - all of our powered, battery backup fire alarms start going off! This is quite jolting when you are asleep (as what happened this morning - luckily we were about to get out of bed) - I can stop them by pulling the batteries - but one is in a vaulted ceiling and I have to grab a ladder.

My question is - why the heck is this happening now! I could understand if the behavior had always been power out -> alarms on - but it is new.

I did notice this time when racing around pulling batteries that if I just opened the battery cover the alarm stopped. Also when I pulled one of the 3rd to last batteries it stopped.

My googling hasn't come up with any reasons why this would happen, any insight appreciated. The kids, dogs, (and me) don't like it!

This is just wild speculation but perhaps the batteries are starting to get weak from age and that's what is causing it? Could test them with a multimeter and see if the voltage level still looks good.

Was my thought too, but I do replace them regularly. They do have a low battery "chirp" - it would seem odd to have if power is out and battery is low - MAKE LOUD NOISES :) I'm going to replace them all now then do a test of turning off the breaker for the house and see if it happens.

Maybe the smoke detectors think they are too old and want to be replaced. are they over 10 years old ?

..
idk why that would be tied to power outages though.

I had this exact problem at a condo I owned years ago. My set up was identical to yours - hard wired to the breaker box with a 9v battery back up in each. What I ultimately did was intentionally set off the alarms at the breaker and then I walked around to each one (I had 4 in the condo) inspecting them and one of them had a blinking red LED code. This unit was incorrectly sensing a fire from the power-off-at-the-box scenario and sending out the incorrect message to the others causing the problem. I replaced that unit and all was well until I sold the condo.

If your alarms dont have a viewable LED, my thought would be to disconnect one at a time from the loop and recreate the no power scenario until you find the culprit machine. Removing it from the loop will stop the errant fire signal which will be the defunct unit needing replacement.

Good luck.

reminds me of how to troubleshoot network problems in the 90's when we all had ethernet hubs and a printer was creating a storm.

and you had to find which one was doing it.