You are not going to get enough power from a small solar panel to run an inverter so some form of USB or maybe 12v charger is your only option. Can you get a car charger for the Nikon battery?
New camera batteries are all li-ion batteries and cannot be trickle charged in the same way as a car battery can. You either have enough power to charge them or you don't.
Can you get a AA adapter for a Nikon? (Canon allows you to use AA's in the hand grip but not in the main camera). The AA metal hydride batteries can be charged from a solar panel without too much problem. I'm not sure how many shots you get from a pack of AA's, for Canon you need 6 AA's and you don't get many shots.
Rob
You don't inherit the world from your parents, you borrow it from your children.
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1979 Unimog 416 Expedition Camper
1974 Unimog 421
2004 Dodge Ram 2500, 4x4, Double Cab, Cummins Turbo Diesel
2006 25' Airstream International CCD
2009 Harley Davidson
Sugarloaf, Boulder, CO