Our Westy batteries (Acid and marine) are subjected to all kind of
condition, the worse one. If your not driving your van daily the only way to
keep that battery alive for a long time is to unplug this one.  The only way
to measure the capacity of a battery is those big monitoring/tester that
shop have... and EVEN THEN! In general a battery will last about 5 years in
a daily working car, NOT in a westy.

Every time you drain that spare acid battery (Fridge, Propex, radio, light,
laptop...) remove 6 month (rough #) out of that 5 years. To your question,
an abuse battery will still show pretty high voltage but simply won't keep
it for long if used (Amp). How do I know, on top of having done 20 second
battery installation i have personally already replace BOTH of mine once in
3 years and you know what, they are almost ready to be replace AGAIN.

Why! Simple, i have an alarm system that drain .4 amp all the time, if i
forget the radio face plate, add another .5 amp, that is 1 amp. 1 amps will
drain it enough in one week that it won't start. 6 month off of it's life
each time, this happen to me 5 time last summer.

Someone suggested a simple trick, fully charge your battery, leave it unplug
for a week, check voltage after.

There is good intelligent charger now for pretty cheap (around 100$), when a
battery is getting tired those charger will never show 100% when charging,
they will stop at 80-90%.

Ben