Several months ago, I had a chirping smoke detector. It was annoying to say the least because the hard-wired detector should not be chirping unless the sensor was gummed up or something. So I reached out to exam the SD and got out my extra power reading glasses to read the very fine print from the ladder.

I checked the model number on the web for a manual. Guess what (aside from the search taking days to find the correct information - even the manufacturer didn't have the manual on line or even list that particular model at all!) The SD has a battery backup in addition to the hard-wired circuit! I did not know that was even necessary but it does make sense - in event of a fire you may also lose your power supply and thus have an inactive SD. It's a bit more expensive than the typical hard-wried unit.

So after much fiddling around I replaced the 9-volt, tested the operations and voila... all's well and I got an education on SDs.

P.S. - also check on your carbon monoxide detector if so equipped near the garage or furnace.