I use a folding 10W solar charger. It looks like a small notebook that opens. Here is a similar model:
So far I have used it only at camp but next trip it will be mounted to charge while in motion when all that good sun shines. I use it to charge a power pack then plug the cell phone into that later as well as charge the lights. As Samad says, on short trips its no lighter than an extra power pack but on those long trips it's great.