The AICO mains, ionisation, wired interlinked, "non-replaceable"
lithium battery backed up, smoke alarms are coming up to 10 years old
so approaching "end of life". One has started beeping and has been
removed (for the time being) and on investigation the nominal 3 V Li
batteries are down nearer 2.6 V each.

Looking around it seems that for landings/hallways, where these smoke
alarms are, the ionisation type is depreciated these days in favour
of optical.

CPC have the batteries at just over £4 each inc VAT. Or I can get a
new optical with user replaceable alkaline battery backup for around
£27.

Presumably the ionisation type age as the amount of active Americium
241 declines but with half like of 432 years is that real problem? If
I replaced the batteries would it be a problem at 20 years of age?

Presumably an optical sensor doesn't age?

Can't decide between replacing the Li batteries and thus keeping the
Americium 241 out of land fill or dumping them (they work according
to the "test" button) and consuming more resources in getting new
optical.

--
Cheers
Dave.