"Dave Plowman (News)" gurgled happily, sounding
much like they were saying:

I have known good lead-acid batteries keep their charge for nearly a
year, but it's rare.

According to an expert on another group the average lead acid car
battery loses 60% of its charge per month. I'm only quoting this FWIW as
it's not my experience.

Nor mine.

I've left cars in lockups regularly for the winter, occasionally for 18
months or more, with either no drain (no clock, no stereo memory etc) or
with battery disconnected - and they've started happily come spring.

The battery in 'erselfs 2cv has been on for roughly a decade - it's used
occasional weekends through the summer, usually with at least one week+
trip per year. The battery earth is normally disconnected over the
winter, but left connected through the summer - with a stereo with memory
fitted. It's not had to be jumped or charged for years.

As far as the original question goes - just disconnect the battery and
reconnect when you want to use it again.