My tractor is charging up right now - on SOLAR power.
Previously, I had done some solar charging from a 45watt 12V harbor Freight setup. However, that was with only 3 batteries in the tractor, and I had to manually move alligator clips, changing one battery at a time.
Right now, I have a 60watt/48V solar panel rigged up to a solar charge controller, 4 batteries (more to be added later), and a 48V power inverter.
I'm using that to make AC electricity that gets routed to the other electric outlets in my garage. The tractor is using individual, low-amp, battery chargers. Right now, the chargers are only pulling 65 watts total (the batteries are mostly full) so just sunlight should almost be able to keep up with that.
Once the sun is no longer on the panel, there's still 4800WH of juice in the UPS battery pack. That should run those suckers for a while!
I have 7 more of the 60watt/48V solar panels, but I don't have any extra cables with the MC3 connector on them (which is odd and expensive) so I really can't hook up any more panels right now.