via instructables

Ever been on a long hike on a warm day just to have it ruined by your phone going dead? Well, now your problem is solved with a solar powered phone charging backpack!
The instructable shows you how to install solar panels into your backpack in a seamless way.

Materials
1 (1) backpack. There are 2 requirements when choosing a good backpack for this project: room to install your solar panels and a pocket to store your cell phone. The Drifter square backpack is perfect for this project.
2 (1) portable solar panel with usb output.
3 Plenty of felt. It should be enough to provide backing for your solar panel and
4 (1) role of Velcro
5 Plenty of thread. Be bold and have it accent your solar panels or have it match the color of your backpack. It is up to you!
Tools
1 Scissors. Fabric or brand new scissor work best.
2 Stitching Needle.
3 Utility knife.
4 Fabric pencil or crayon.
5 Hot glue gun.
6 Freshly made fruit smoothie. This is to keep you hydrated and energized while you work.
7 1 extra hand (optional). This will definitely help and you can also give it high fives after each step!

Full tutorial