I happen to own a messenger bag that has a solar power setup built in. This setup consisting of three solar panels, a power routing element, a lithium ion battery pack, selectable dc-dc convertor, and mega-assortment of possible output jack options, was reversibly harvested to power the camera and triggering circuit.
The camera usually accepts two AA cells, which of course imply a 3V operating voltage. The dc-dc convertor can only go as low as 3.5V, though this is perfectly okay for powering the camera. ‘Perfection’ does have a price though, and the camera does get pretty hot when powered on. Combined with the 100 degree heat, and the back-of-the-car greenhouse, it’s probably not good for the camera in the long run. But hey, I’m probably only doing this once.
If the lithium battery ever runs low, I can also recharge it using a wall outlet, or the cigarette lighter plug in the car.