I am interested in building a portable charger for the Nexus 7-2 that uses a battery pack, like a 6-cell NiCd car pack. I have seen some YouTube videos that involve a 9V, a 5V regulator, and a couple resistors for the data lines? I figured that you only need the battery and regulator? eBay has some imported solar chargers that are not very expensive, and might work if you are outdoors with good weather. I have also seen a reference to a hand-crank charger. I think I would rather go with the battery pack, as I can quickly charge (in 20 minutes) that in the car, but I would presume the extra battery would be enough for a day of walking, with the GPS tracking. I think even a couple 9V batteries would suffice, but I'd still need the adapter. Suggestions? I'd appreciated a link to a web page with a circuit diagram, and I have a Radio Shack handy, plus a Dollar-store.