Therefore I AVIDSEN, and it's really great since a machine rings all the others follow! And what's great is that if you put a normal detector in the basement, if there is fire, there is no ll'entendra in the room when we sleep for example! And that's the danger! While with these, being connected, if a fire broke out in the cellar, we were awakened directly! I put in every room and frankly, I'm reassured! For batteries is approximately once a year, it's not too dramatic compared to the security it gives us. And it rings every x minutes to warn that the batteries are flat. Just the configuration is a bit difficult at first, then when we realized the stuff in there. I have put a black dot on the detector "mother".