For hikers and campers a small power bank apparently is not enough, you can prepare a large-capacity backup battery or take a look at the folding USB solar cell with backup battery, it may be a better solution.
This is a practical, eco-friendly charging solution designed for hikers, campers and cyclists. As we can see from the images, the charging kit consists of two parts including a folding cell with integrated solar panels and a backup battery, and Using the built-in USB port the folding cell can easily connect with the included power bank, while the custom designed holes on the folding cell allow you to fix it on your backpack in order to conveniently charge the connected power bank in daylight during hacking, camping or cycling. When needed you can use the backup battery to charge your USB-enabled mobile devices.
At present the team of the folding USB solar cell is raising fund at Kickstarter. Pledging $40 will let you own a 5W folding USB solar cell (without backup battery). If you’re interested, jump to Kickstarter official site for more details.