There are backpacks available that have solar charging panels built in which can keep a fairly constant charge on batts in the unit which can then charge your gear - probably directly charge too.
I've been toying with the idea myself, it seems like they have enough juice to keep a cell phone or GPS or PDA topped off, but not much more than that. Plus you have to rely on the solar factor and that may be a non-starter.
They do make those instant charge (looks like a lighter) packs for cell phones, so perhaps one could be modified for use with your GPS as an emergency backup, but spare batteries are probably just as easy and far cheaper. Going with Lithium batteries or a Li-Ion rechargeable battery pack will help some with longevity. Running with a dim screen (if an option) helps a lot too.
If you are going with hand cranked, you should look into scientific supply companies that have equipment for physics experiments and try to get one that has no plastic parts. As mentioned, you probably want to charge a battery pack and use that to charge any electronic equipment. Either way, this is going to be a bulky solution. <img src="/images/graemlins/smile.gif" alt="" />
I can't wait for fuel cells, so I can carry a can of butane or ethanol to fill up my GPS, Cellphone, Flashlight, Lighter, 2-way, PDA, and my Laptop. The best part is that's really getting close to reality now....
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Experience is a hard teacher because she gives the test first, the lesson afterwards.