> One option, given that you can't plug in a charger when
> you're parked for
> long periods, would be to add an onboard desulphation unit
> that works while
> driving. I don't know how many manufacturers make them - the
> only one with
> which I'm familiar is the one from BatteryMinder.
There are also plans on the Internet for a DIY desulphator that would work
either parked or driving. It draws energy from the battery at 12v, then
makes a high voltage spike to feed back into the battery. One of those
combined with a solar panel should work well.
Google for "desulfator plans" (without the quotes).
Here's another commercial version I found:
Suggested annual donation $12.96