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?
Step 3: Gutting the Solar Light
Most solar garden lights have very similar setups to the ones I have, probably because they all come from the same factory in China.
First remove the base of the body.
Then remove screws.
Inside you'll find all kinds of goodies. Remove the battery and the circuit, then snip the wires as close to top as you can.
Bonus: Save the circuit and the battery. You can always use the light detecting circuit for other projects. If nothing else you have a free battery and LED.
Step 4: Remove the Solar Cell
This part is tricky. The best solution I found was to use big tin snips to cut away the plastic until the solar cell was free. Be careful as the cell is cased in glass which does chip.
You also need to be careful because you don't want to rip out the two soldering points from the cell. Trying to get new solder on can be a big pain.
Step 5: Solder them together
Once you have your four cells, you need to solder.
We'll be connecting these in a series, going positive to negative all the way around. This way our voltage goes up while our current stays the same.
I placed mine in a little square.
Then put some tape down to hold them together.
Using small bits of wire I connected the cells together.
If you have a multimeter you can always give them a test when you're done.
Step 6: Add Battery Holder and Diode
Lastly we need to add the diode and battery connector.
The reason we use a blocking diode is to prevent the solar panels from trying to suck power back out of the batteries at night. Without the blacking diode we'd end up with damaged cells and dead batteries.
First wrap the positive (red) wire from the battery connecter to the black stripped side of the diode. It's important that you connect the battery connector to the BLACK STRIP side of the diode otherwise you'll end up blocking power coming from he solar cells.
Now all you have to do is solder the other end of the diode to the positive point on the solar cell and the black wire from the battery connector to the negative point on the solar cell.
For good measure, tape things up. You can never use enough tape.
Now you've got a mini solar panel that puts out around 14V @ 20 ma.
Step 7: Housing
You can use anything to house this charger in. Shoot, you could just leave it like it is if you're going to be keeping it inside.
I like using some plastic food containers from my local $1 store. The ones I find have a rubber o-ring in them to keep moisture out.
That being said I wouldn't want to leave these out in a rainstorm.
And yes, I did just use a bunch of tape to stick the cells on. Be fancy and use caulk if you wish, I'm going cheap with tape.
Step 8: Enjoy
We'll you're done. Never again will a smoke alarm in your home, or guitar amp in your basement be without a 9V battery.
I hope you've found this instructable helpful. If you're in need of parts for this project, or for a premade charger, I have all the necessary parts on my website http://www.browndoggadgets.com. At a user request, I put together a 9V charger kit at a bit of a discount. 100% of all sales go right back to other projects, which means more instructables.
It also means I can keep annoying my 4th grade science club with silly inventions and gadgets. Children are so easily amused.