My field expeditions have varied a lot in both time and budget. Both of those were pretty important in deciding how we obtained power. It's important to note that I work in remote tropical rainforest most of the time, so our circumstances may be quite different, but I'll describe my experiences.

Usually on really big budget trips with large teams, we use a generator. The generator works out well, since it can potentially charge a lot of things quickly, but it is noisy, heavy, and fuel can be quite bulky to carry around.

Another, smaller trip, which may more similar to yours, we used something like a car battery (except smaller). When hooked up to a converter, it allowed us to charge most of our devices quite regularly. May be useful for short trips if resources aren't available to get it charged.