On Tuesday, 18 August 2015 18:13:47 UTC+1, David wrote:
Just had the mains powered alarm chirping every now and then.
Puzzling in itself because although this should be a sign that the backup
battery is at the end of its life, why just chirp now and then?

Because chirping uses up electricity which the detector is conserving for detecting and alarming purposes.

This allows it to remain operational and present an intermittent alarm over a longer period of time. No point in it warbling madly for an hour then dying completely if you happen to be out for that hour.

Owain