rant: Why do smoke detectors behave the way they do? 
I get that you want the battery to be good and tell everyone when it needs to be replaced, but... a beep every 15-20seconds forever is not a good way to do this. It's long enough so you can't figure out where it's coming from. It _always_ starts in the middle of the night. How about a bright light? Or even a constant beep so you can easily track it down? Or even a light and beeps if it's longer than a day or two? Anything.