The Goal Zero Switch 8 adds charge to your iPhone, GPS or satellite communicator and takes up no more space than a full size Tootsie Roll.
If you’re going on a two day trip to the boonies and don’t want to carry extra batteries or a full solar charger kit but you can’t afford to run out of juice this might be a good solution. I have been using one of these for a couple of weeks now and like it. I charge it from the truck, Goal Zero Nomad 7 solar panel or wall socket then put it in my pack for later. I can top off the charge on a device or use it as an emergency reserve.
Get the specs and more information from Goal Zero.
UPDATE – Just to give you an example, the Switch 8 took my iPhone 4S from 49% to 100% charge in 79 minutes and it still had 2 of 4 LEDs lit which is about half a tank.