Portable USB chargers drain power quickly if you're charging multiple devices, and the charger itself always needs to be charged up to work. So, Instructables user 3zuli decided to make his own universal charger that can draw power from just about anything.
The charger can take power from any source from around 7V to 30v, then deliver a 3A USB charge. That means you can power it with anything from a battery pack to a car battery or a solar panel. Then, that power can be distributed to almost any device that charges via USB. The end result is a pretty handy emergency kit-style charger that makes it possible to charge your devices from just about anywhere.
Universal Portable USB Charger [Instructables]