Safe And Sound Again.

I went out this morning after a good night's sleep, and bought a new cell.
The smoke alarm has now resumed normal service, actually being mains-powered despite having a battery cell.
I think it is for back-up, and the 'beep' is a warning of either power source failing.
Of course, it doesn't beep for long if you rip out both sources.
Irritating, belt-and-braces approach.
Anyway, the cell lasted for five years last time, so I should not have to touch it again for a while.