Why does my Accenta alarm go off as soon as we get a power cut?

My Accenta alarm goes off immediately there is a power cut. I know they have a battery inside for this purpose if a burgular cut the electric - but I vaguely remember a 10 hour or so power cut a couple of years ago, where I think I was able to use they keypad to stop it ringing, until the back up battery ran out of power and it went off again. How do I get it to this state (if possible again) as scrabbling around at 5.30 this morning trying to shut it up is something I don't want to have to repeat, thanks