In article ,
"Christian McArdle" writes:
I have a pair of connected EI 151 smoke alarms, must have gone in when
the house went up 10 years ago.

Then they should be replaced entirely, not just the batteries. The sensors
degrade over time and should be replaced about every seven years. There is
often an expiry date on the casing. Batteries should be replaced at
manufacturer's recommended intervals, which are usually 1 year for alkaline
and 5 years for lithium cells.

Ionisation alarms are supposed to last at least 10 years in service.
I have some 20 year old ones which still seem to work fine but
aren't any longer in service (actually, the one on my garage is,
but it still seems as sensitive as new ones).

CO detector is another matter though. The detector elements are
quickly wrecked by standard household chemicals, and you shouldn't
assume they'll last more than a year or two. Some have a
plug-replaceable detector.

Quite high, actually. What is worrying is that you waited for the low
battery signal. The batteries should be replaced on a schedule and should
never get low.

Pick a day each year (like your birthday), and your annual present
to yourself is a replacement set of smoke detector batteries.

--
Andrew Gabriel