Make sure the rechargeable batteries are low on power so you can charge them. If you want you can put the charger in an altoids tin or a Tupperware. ...
This is quick and easy to make. I have a portable USB charger that is run by batteries. Sometimes I charge my phone while I'm at school because battery is low. The batteries last for about 15 hours of charging. The big idea is that I wanted to not use so many batteries and then throwing them away. Then i started using rechargeable ones and made the solar battery charger.
Step 1: Purchase Materials
6v solar panel (RadioShack)
2 Blocking diodes (Amazon)
AA battery holder (Amazon)
2 AA rechargeable batteries (Amazon
Electrical tape (Home Depot)
Soldering iron (Amazon)
Solder (Local Hardware Store, Amazon)