Here’s an off-the-shelf option, if you want to go for a drive while generating:
Ground-drive PTO forecart (there was a discussion about them a while back, I&J is currently making one).
Find a small pto generator that can run at 540 rpm and put it on a trailer (with a charger and battery).
You could make a homemade rig that would be more compact, probably. Belt, chain, or gears from the wheels to the generator. For what you’re talking about, you wouldn’t need a bull wheel like a baler uses, just decent tread. Think mower or manure spreader wheels.
Otherwise, a treadmill or sweep power could run a generator in the barnyard while you’re doing other things.
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