> They don't normally go very far away ...

Depending on field traversal patterns, it might be possible to stage portions of the battery pack at field edges or corners so that only part of the battery pack needs to be carried with the tractor/combine/etc., thus reducing weight, drag, etc. and improving performance.

> There are places in Africa where: ...

The tsetse fly has been credited with protecting natural rangelands from human encroachment, and preserve them as food reserves needed by all those migrating herds we so enjoy to watch. Gaining access to such areas via solar-charged electric vehicles might provoke an application of the Jevons Paradox ( http://en.wikipedia.org/wiki/Jevons_paradox ) (i.e., facilitating access will enable us to exploit more of Nature and will ultimately be counter-productive).

-- Philip B. / Washington, DC

That is a point worth noting, but I suspect that overall, our reach into wilderness will be shrinking as the most powerful fuels subside. I would expect these E-Tractors, like Windmills and PV will serve to slightly slow our shrinkage, not cause more expansion overall.