Some of the extended trips we go on battery life on my phone becomes a problem. I checked some of the store bought chargers and none had very good reviews, so I built one. Everything came from Radio Shack and total cost was $13.
Drill an 11/8" hole in one end.
Push outlet in.
Solder wires on to the battery holder.
This will completely charge a phone multiple times. Most phones only require 3.7v but I decided to make it 12V (8 AA batteries) in case I want to charge something that requires 12v. Overall dimensions are 5x2.5x2.