Smoke Detectors and their Batteries!

Why is it that smoke detector batteries only die in the middle of the night? There I was, sleeping away, knowing I didn't have to get up super early when the smoke dectector started chirping! You know that dame little chirp goes off every minute or two to let you know that it needs a new battery. That little chirp that just loud enough so you cant ignore it. That smoke dectector started chirping at 5:15AM, a full hour and a half before I had to get up. So I get up, listen to the chirp for a few minutes to determine which smoke dectector it was and then get a chair to reach the damn thing. Once I got to it and opened it up, I had to go through 3-4 batteries to find one that suited it. I even disconnected the smoke detector from the main power at one point to shut it up but it appears to have an internal power source to power the chirp. Oh well, it's fixed now!