It's very bad to let a battery just sit for long periods
of time. The plates inside may be shorting from corrosion.
It's best to put a low amp trickle charger on them for
extended sitting or at least put a regular charger on
them every 2-3 weeks to keep them from discharging to the
point of causing damage. I've heard that the solar
trickle chargers work well for keeping batteries up that
aren't used very much if you want something that's
'hands off' for the winter.

Regards,
Indy