Martin,

Good luck next time around. Me, I use a couple of Battery Tender Pluses
(http://batterytender.com/product_info.php?products_id=2) when Mellow
Yellow is in hibernation. It may not be practical for you if AC power is
unavailable.

Using the sun is, to me, too iffy: where we live, coastal San Diego
county (lower left corner of the USA), we can go for weeks w/o sun. See
"May gray" and "June gloom"

(c.f. http://meteora.ucsd.edu/cap/gloom.html)

During that two-three month period, a battery has plenty of time to
self-discharge far enough to get itself into trouble.

...of course, if a fellow wanted to have some seriously geeky fun, he
could connect a data logger to his solar-powered battery and record
battery voltage while he was gone. Trick might be to find a data logger
that didn't use so much power that it affected the results.

--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR