Alang wrote:
[...]
I have two car batteries. One is completely sealed from external
probing. At least I can't se a way of getting into it. The other is
one that still has fillers where you can get a hydrometer in. That's
the one I want to try and store.

The alternative is putting it on trickle charge and hoping it doesn't
die before I want to use it

Myself, given a choice, it's the modern 'maintenance free' one I'd be
keen to store. Charge it, store and forget. Typical English climate
and it'll sit there for a year and retain maybe 90% charge.
In stark contrast to those hi-falluting, hi-tek, hi-price Ni/Li types
which lose about 5% a day.
Gimme a drill fitted with an SLA any day