wrote in message
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I work on various alarm systems that use mostly two 12.0 volt 7.0 or 8.0
AH rechargeable lead acid gel cell batteries in series. Although fifteen
or so years ago you could get up to ten years out of a typical set of
Japanese gel cell batteries, these days it seems that three to five is
about all you can expect from the Chinese knock offs that have flooded the
market. I used to keep replacements in my van but I found that if they
weren't used within a certain time frame they would sulfate.

AFAIK, some battery suppliers stock the batteries in a dry charged state -
the gel is supplied in ampules with a seal that is pierced when you press
the ampules into the top of the vent hole.

Last flooded cell motorcycle battery I bought was done like this, the dealer
has a bottle of acid to fill batteries as and when they are sold. It would
be reasonable to assume the same applies to car batteries.

No idea what the shelf life is - but certainly a lot longer than pre filled
ones.

Another thing to consider; lead acid batteries get lazy if they spend their
life on trickle charge and never have to do much work. An occasional heavy
discharge cycle can liven them up a bit if they get sleepy.