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.
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)
Step 2: Wiring
Take the positive wire from the solar panel and one end of the wire of the blocking diode and twist them together. Take the positive wire from the battery holder and twist it to the other end of the blocking diode. Do the same steps with the negative wires.
Step 3: Solder
Solder the twisted wires and then put electric tape over the solder.
Step 4: Finishing Touches
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. I'm not sure how much time it will take for you to charge the batteries. It will depend on how many mah your solar panel and batteries are. Enjoy