At 4:40 AM, I awoke to this rather loud chirping sound, which also woke my wife. In my early morning fog, I wasn't sure what it was until I washed my eyes out a bit and realized it was the smoke alarm signaling the battery was dead. I stumbled downstairs trying to recall where I might have a 9V, found one, replaced it, and stopped the chirping, but now I'm up, she's not, but I made coffee.