I've been pulling them in my stinking sister's house for the last year +. They are hardwired with 9v battery backup. When you replace the battery, they keep on chirping, so I just disconnect them, leave them up so there's not a hole in the ceiling* and put one of those mini detectors from Costco with the ten year battery next to them. As far as I'm concerned, all new construction should just have battery detectors, not hardwired ones. I'm sure there's some regulation against that though.
*She has like 12 detectors in her house. It's ridiculous. I don't know if it's a state of CA thing, but there seem to be way more than there need to be. In one case there's only maybe 12' between the one in her kitchen and her dining area, which is one big open space, then another 15' between the dining area and living room, which is also mostly open.