[MargaretR]

I have one that fits in a light fitting (see pic).
I recall that when fire alarms were discussed here in the past, firemen members didn't like this type.
Maybe it is because they are not ceiling mounted, so will not activate as soon as others do.
I have very low ceilings and short flexes on my light fittings so mine is only a few inches below what a ceiling mounted one would be, and is in a chandelier type shade which allows free flow of air/smoke.
I had a 10yr one which lasted 5, so wanted to avoid batteries altogether.

PS it does have a battery, but it charges itself from the mains - the light needs to be on for 1hr a week to ensure charging - it bleeps when its battery needs charge.