Our 15-year old external alarm box is still electrically OK but looks
very shabby. So I'm either going to have to attempt to clean and
repaint it or replace it with an entire new unit.

Researching the latter option, I see that many current products
have a built-in battery, typically NiMH rechargeable. But my main
alarm unit downstairs (15 years old) already has a 12V standby battery
(lead acid). Isn't that true of most such units? If so why is another
battery needed in the bellbox itself please? And why are they
apparently 7.2V or 8V, not 12V?

Also, are these batteries truly maintenance-free (not my experience of
many NiMH batteries)? Installing the bellbox under the eaves will be
hazardous enough, but I don't relish having to get up there again
regularly to attend to the battery.

--
Terry, East Grinstead, UK