as a semi non related word of warning.
if you have one of the hybrid gel batteries or a pure gel battery (appearing in some small tractors and motorbikes)
do not put a solar panel on them without a regulator..
these batteries do not like being charged over their nominal max charge voltage and will die in short order if the panel keeps upping the voltage on the terminals once the battery is fully charged. It doesnt take many days of good sunshine on a good panel to cook a gel battery.

