So in this guide I'll give you a bit of info on solar power and battery charging, as well as show you how to make a solar battery charger for all of $4.
If you'd like some solar panels or solar kits I have quite a few on my gadget site, browndoggadgets.com or you can also buy them off ebay or various other websites.
Step 1: What You Need
A clear, water-proof container. (Dollar Store tupperware with built in O-Ring)
AA Battery Holder (Radio Shack, also fits AAAs if you're careful)
One or Two Solar Panels rated 4 Volts or above
Blocking Diode (Radio Shack, or buy 100 for $1 off eBay.)
Tools you need
Time: 20-30 minutes
Step 2: Things You Should Know
When making a battery charger there are things you should keep in mind.
First, know your batteries. NiMh batteries are the most common these days, and you can find them at any store. Your typical AA NiMh battery probably is 1.2 Volts and has anywhere between 2000- 3000 mah worth of charge in it. (Check you batteries, they probably have the capacity written on them. That or check the maker's webpage.)
Secondly you need to know your solar panels. For instance the ones I'm using in this project put out a max of 4.5 volts and 80 ma of charge.
With only 4.5 volts coming in I really shouldn't try charging up any more than two batteries (hooked up in a series giving me 2.4 volts). Also, because one of my solar panels only puts out 80 ma at a max it's going to take a long time to charge up all 3000 mah hours my batteries hold. In this guide I hooked up two panels in parallel to give me around 160 mahs worth of power coming in. If I had a bigger case I could hook up another one or two to give me even more power.
Your'e probably asking yourself, "hey, why doesn't he hook up a whole lot of panels to thrown down a massive amount of amps and fast charge those batteries!" Good point, but if I did that I'd kill the batteries. Your standard wall charger has brains that let it fast charge a battery without blowing it up. We're going about our charging using the "trickle" method. As a general rule of thumb you don't want to throw more than 10% of the capacity of the battery (C/10) at the battery when charging. As our batteries are 3000 mah capacity, and we're throwing 160 mah of charge at it, we're ok. (AAA batteries hold between 800 -1800 mah, so we're probabl ok for them as well as we're never going to actually get the full 160 ma from the cells.)
If you really want to charge up your batteries fast you could try and hit the C/10 power supply. Though this being solar it would still take awhile.
So there you have it. Now you've got a basic idea of how to add solar power to your projects. Now go out and buy some Solar Panels and NiMh batteries.
Step 3: Panel Power
Because my solar panels have little tabs on them I'm going to need to do some extra soldering and taping that you might not need to do with your solar panels.
That being said, no matter what kind of solar panel setup you're using, you'll want to be wiring them up the same way.
(For instance you could use a smaller panel to charge up a single AA battery, or a bunch of panels in a series to charge up a whole bunch of batteries at once.)
Step 4: Blocking Diode
(If you're only using one solar panel, you've actually done with soldering.)
Why do we need this? Well solar panels are great at creating power when it's sunny out. When it get's dark they try and suck power back into themselves, which then destroys them. To stop this we use a blocking diode so that power only flows in one direction.
Also, see that black bar on the diode. Always make sure you know which way it's going. You want the black bar pointing in the direction you want power to flow.
Step 5: Strength in Numbers
To hook them up in parallel we're going to use two more wires to connect both positive tabs and both negative tabs.
Cut two wires at about the right length for a bridge and solder.
Remember, we're hooking positive to positive, and negative to negative.
Step 6: Lots of Tape
Then just tape the panels into the lid of your enclosure. That was easy.
For my enclosure I used a little tupperware thing I got from a local Dollar Store. It has a freshness o-ring in it that keeps moisture in, which also means it'll keep moisture out. Handy for projects you want to leave outside for long periods of time.
Step 7: Enjoy
So the breakdown is this.
$2 Solar Panels
$1 Battery holder
$0.02 Blocking Diode
Time: 20 minutes.
You can use this EXACT same setup to power little light up projects. Throw in a few transistors and resistors and you can make a dark detecting circuit for all of $0.20 more.
If you're looking for solar panels or little solar kits I have several available on my website browndoggadgets.com.