Step 1: The Big Idea
Well, we're going to make a solar panel from scratch that can charge up a simple phone.
See, cheap phones like mine are real simple to charge--they just need 5V and they'll take care of the rest. They'll draw as much power as they can (my little nokia will draw up to an amp while charging), but they're smart enough to limit their current draw to whatever the source can provide.
So, that means we need to make a 5V solar panel. Solar panels are made by wiring up a set of solar cells--big silicon P-N junctions. A solar cell puts out a very low voltage--0.65V open-circuit, or around .55V under an ideal load. To get the 5V that we need to charge a phone, we have to wire up a bunch of solar cells in series.
But how many solar cells? Well, a solar cell without anything connected to it will produce .65V in bright sunlight, but as I draw power from the cell, the voltage will drop. This relationship is called an I-V curve. The max current the cell can output is based on the area of the cell, the intensity of the sunlight and the temperature of the cell, but the max voltage is always the same, regardless of the size of the cell.
Every solar cell has a peak power point, a current-voltage combination that gives me the most possible power for that cell. For my panel, I want to run my cells at their peak power point. I happen to know that the peak power point of my cells is .5V at 130mA. So, if I want to put out 5V, I need to stack up at ten solar cells in series, giving me an output of 5V. If I'm drawing less than 130mA, my output voltage will "float" up, and if I draw more, my voltage will get pulled down. Now, it so happens that phones are pretty smart and robust, so I can actually give them a bit more than 5V, and they'll be fine. I'm actually going to add an extra cell, giving me a total of 5.5V coming off my cell. This makes my panel a little more robust--the higher voltage means that, in lower light, it'll still put out enough voltage to charge up my phone, and it'll be less sensitive to shading or pointing it away from the sun.
So, I'm going to make a solar panel with eleven solar cells in series, I'm going to wire the output into a phone, and it's going to charge. Cool, right? It's pretty quick and easy to do, too. Here's a video of the whole process:
Step 2: What you need
Solettes--cut up pieces of solar cells. You can get them as part of a DIY solar kit that a friend and I put together, or pick some up on ebay by searching for "solar cell pieces", like these: http://goo.gl/C25he.
a cheapo phone charger that fits your phone. Mine was a nokia, and I got a knockoff charger for $2.
Copper tape--available at michael's, Sparkfun (http://www.sparkfun.com/products/10561), and plenty of other places online, for cheap
and wire strippers, scissors and a soldering iron. Ready, set, go!
Step 3: Tune out, Build up
Kill me now Pete, I'm in heaven
Step 4: The ol' strip-n-cut
Step 5: Coppertape
It's like you made an adorable little heart resuscitation device for tiny little mice having cute heart attacks
Step 6: Stick that where the sun don't shine
Smush the clothespin together to stick the tape on well. It's like you made a little electrical alligator!
Step 7: Pick a cell, any cell
Fan them out pretty wide--the better they're spaced out, the more each solette will show, and the more current you'll be able to pull off the panel.
Now fan yourself and talk about mint juleps in that adohrable suthehn gentlmuhn accehn yuhv been a-wohkin on
Step 8: I pinch!
The positive (usually red) wire going onto the bottom of the stack of solettes (the side that's not blue), and the negative wire goes to the top.