"Who changes the batteries?" (or their equivalent) is an excellent question, and I haven't stumbled across a good solution in what I've read. A partial answer is to use gasoline engines, like the large walker linked above (I really do recommend watching the video, though it's an 11.5 Meg download). This option should give reasonable range, and would make refueling at a home base fast.

Fuel cells should give performance in the roughly the same range. Solar cells would work for something that moves very slowly, or that mostly stands still except for bursts of activity (think guard duty).

One option, for machines far from human aid, would be to have them defend a perimeter and air drop supplies inside it. Sucking up fuel should be easy compared to the problems already solved. What this option says is that telefighters could operate far from any at-risk soldiers. Of course, maintenance could be a major sticking point for this scenario, at least with today's technology. Sand is nasty, and many machines don't need any help to destroy their own parts.

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