@mitch
Thanks for the reminder. Ours was overdue for its fortnightly test. It doesn't half make a racket!

You can get #COAlarms that last slightly longer. Ours claims to last ten years. It's managed more than nine so far. It shipped with a non-replaceable battery, so all I can do at the end of its life is to dump it in the WEEE pile.

#SmokeAlarms also have a finite life, and for the same reason: they use an alpha emitter that decays over time. Check the date on yours if you haven't done so recently.