You'd think I'd learn. When I had Mr W's room fashioned out of the back porch of this old house, they put in two smoke detectors that are directly wired into the electricity of the house. Except that they both take 9V batteries too, and those batteries need to be periodically replaced.
I'm all for home safety, but this system has an uncanny ability to start its alarm beeps in the middle of the night. Like last night, for instance. At 2:15 am, I hear something that wakes me up with a start, but then hear nothing, so think it was my dream. At 3:15, I hear three short beeps coming from Mr W's end of the house and know what's going on.
I get up and rummage through the box that has spare batteries. I can report I've got a bunch of size D and size C, but no 9V. I thought that I might have to go to Wal Mart in the middle of the night to get a battery until I had the idea to take the battery out of the regular smoke alarm in my room and see if that had enough juice to last until I could get new batteries for everything.
Thanks goodness it worked. I sort of drifted in and out until it was time to get up, and I think everyone else slept OK after that. I'm putting it on my Google calendar to do this next September 1st. I've been in this house (as of yesterday) seven years now. You'd think I'd learn... :-)