Did you know that you can buy 9V rechargeable batteries? You can, but they're rare in stores. I recently bought some off ebay to use on projects and loved them. Only downside is that I don't have a 9V battery charger and all the ones I found on ebay are quite expensive.
Luckily I'm resourceful and decided to build my own. The entire project only cost me $10 and didn't take that long.
Time: 1/2 Hour
Step 1: What You Need
All the supplies for this project can be found on my website, http://www.browndoggadgets.com. I even have a 9V kit of everything you need. 90% of all sales go towards more fun projects. The other 10% goes to doggy toys.
Step 2: What kind of solar cells?
We have to be careful when picking solar cells, much more when designing trickle charging circuits for AA batteries.
When trickle charging NiMh Rechargeable batteries, the highest amount of power you can throw at them is 10% of their capacity. For instance most AA NiMh batteries have between 2,000-3,000 mah of charge, which means we can throw between 200-300 (depending on the battery) of current at them.
Now my 9V NiMh batteries have only 300 mah of charge, meaning the max I can throw at them is 30 ma. As most cells you find online are at least 50 ma of current we're going to have to find a better solution.
Enter in some cheap solar garden lights. I found these at a local store for $3 each, on sale for $2. Their cells put out 3.5V @ 20 ma. To make our 9V charger we're going to combine 4 of them together to get 14V @ 20 ma.
You can probably find similar cells elsewhere on the internet, but where would the fun be in that?