I second, third and fourth all of the above about keeping batteries charged and the benefits of using small solar panels. I use 5W panels without any regulator (max. current <300 mA) on individual machines parked outside (and electric fence energisers) and when we go away, all the vehicles in the shed are paralleled to a bruised and battered (fell off the roof of the Discovery and was run over by a truck - long story...) 32W Unisolar panel and 10A regulator.

I know the purists will say not to parallel batteries of different capacities or types (e.g. flooded, VRLA etc.) but this has worked for us. Just this month I have replaced the battery that came with our Grey Fergie (TE-D20) tractor in Dec 2005 (yes, just over ten years). It was a S29, the cheapest 'car' battery available from Supercheap Auto. Replaced it with the same and will continue to keep them all charged with solar.