I have one on each floor with the one in the basement hot wired with a battery backup. Mine will scream at me when the battery is going out. What's funny is when one goes out I know for a fact the other one upstairs will be screaming at me within a day to a week from the other one.
When I bought this house it didn't have any which I thought was manditory these days to even sell your house. 1st thing I had installed though were smoke and carbon dioxide monitors as I have gas in this house and this house is older than the hils so a fire ready to happen. All my neighbors know to get the cat out if ever there's a fire when I'm not home. The rest can be replaced but not my little buddy Pepper.