My smoke detectors have been chirping for a while. I checked the 9v battery with the multimeter to see if they died. Nope, still good I checked the detector’s sticker on the back and saw it was made in 2003. They’re 20 years old now and I guess no longer good. So, I ordered a new pack of hardwired interconnect detectors.
It’s best to get the ones with built in batteries that last ten years.