That works but the way I did it was like this.

1) Get a cheap car USB adapter - 1 port. Cost me a whole $4
2) Rewire the connector for deans (or whatever your favorite plug is)
3) Get a 12v power source. It could be a 3s Lipo, Lithium Ion. It could be 8 rechargable aa cells - they can hold a decent charge. Also you can use ordinary aa batteries as a bail out if you run out and really need them. Attach the usb adapter to your phone with the cord and you have... a charger. The batteries will probably cost around $20-$40 but, it will run your phone for 6-8 hours.

Now if you overkill with a large 12v battery like a 5ooomah lipo (be sure to get one with the discharge preventer built in!) You can run yuor phone for a loooonnnngggggg time.