How to build your own solar powered charger

Just before it started snowing this weekend, Saturday was shaping up pretty damn clement. Clement enough for the wife and I have a go at titivating the deck and smartening up the crop in time for spring.

But, oh no! Clumsy oaf that I am, one of those cheap solar powered garden lamp things rolled off the deck and cracked the plastic dome. Catastrophe.

But the wife says, “Those things are rubbish anyway. I was thinking of chucking ‘em.” Immediately my inherent Yorkshire sense of thriftyness kicked in, and I salvaged those four rubbish garden lamps to see what other uses China’s finest engineering could be put to.

Here’s the result of that afternoon’s tinkering - a portable emergency mobile solar charger that fits inside Pocket Gamer’s favourite see-through container, the CD slip case.

This does take a bit of soldering, and if you get it really wrong (and I mean really wrong) you could do your phone harm, so be warned. Most likely, though, getting this wrong will only mean a waste of time, so don't be too worried.

Anyway, here’s one we made earlier.

This is our solar powered garden light before we start to butcher it. You might need a bit of extra willpower to get into it - ours had one or two screws and a lot of glue holding it together.
But it couldn’t stop us in the end. Cut off the wires connecting the small solar collector to the circuit - one at a time in case there’s any power in the battery. Note that even if there is power in the battery, it’s not enough for you to feel it, but you don’t want to harm the solar collector. Our solar cells were stuck down with sticky pads, so be careful not to snap it when taking it out.
Holding it up to the light shows an output of 1.8v from a single cell, and this is indoors and not even in particularly bright daylight. Gather up all four of your solar collectors, and test each one just to be sure it’s working and putting out around 2V - hopefully nearer 2.5v in direct, bright sunlight.
We’re now going to wire two of them up in series. This means connecting the positive terminal of one cell to the negative terminal of another. A serial configuration adds the two voltages together, giving us an output between 4 and 5v. Wire up the other pair of solar cells the same way, so you’ve got two pairs of cells wired in series - each putting out around 4 - 5v.
The only other component this charger needs is a diode. This prevents current flowing from the phone's battery back into the solar panels. We robbed the diode from the small circuit inside the garden light. The diode will look like a small black cylinder with a white band around one end (kind of like a Duracell battery). If you need to buy one, ideally you want something like an N5817, which is a Schottky diode. These are better because they have a lower forward bias. A standard diode (N4001) drops the voltage by 0.6v, while the Schottky diode only drops it by 0.2v - and that can be the difference between charging and not charging on such low current applications as this.
Next we need a connector. We’ve gone for a USB extension cable, as most devices these days will charge from a USB connection. Cut the plug off, leaving around 10cm of cable on it (more on this in a few minutes). The socket end will be longer, so strip it back to expose the wires inside. The colour of the data wires often varies, but you’ll pretty much be guaranteed red and black for the power connections.
Solder your diode to the positive terminal on one of your pairs of series-wired solar collectors. The white band on the diode should be AWAY from the solar panel’s terminal. Solder the red wire from your USB socket’s lead to the other leg of the diode, and the black wire to the negative terminal on the OTHER pair of solar collectors.
The next thing we need to do is wire up the two pairs of solar collectors in parallel (as opposed to series). Wiring them in parallel doubles up the current, but leaves the voltage of 4 - 5v the same. Solder a wire from the ‘empty’ positive terminal of one pair of solar panels to the positive terminal on the other pair. Then take a wire from the negative terminal of one pair of solar panels to the negative terminal of the other pair. Our wires cross over here because we didn’t want to strip back too much of the USB cable to reach between the positive and negative terminals of the two solar collectors. Sorry if it looks a little untidy and/or confusing.
Next we put some double sided sticky tape on the back of each collector and stuck them inside a clear CD slip case for a bit of extra weatherproofing. We also stuck a bit of extra tape where the USB cable leaves the CD slip case, to prevent us from accidentally pulling on the soldered connections.
Now to test it. Take the USB plug you cut off the lead (the one with 10cm of cable left on it), and strip it back so you can get to the red and black wires. Plug it into the USB socket attached to your solar collectors, put it into direct sunlight and check you’re getting 4 - 5v output. As you can see, we were getting 4.23v without even going outside. Nice.
And you’re finished. We tested it out on a Bluetooth headset and an MP3 player - both of which happily charged with sunlight through the window. We even gave it a shot with our G1, which set off charging straight away (see the orange light and small power symbol at the top right of the screen). Don’t expect this to charge your gear quickly (it’d never give a G1 a full charge, for instance), but as a very portable and light emergency backup to give you a little extra juice when you really need it, or to extend your standby times, this could be a life saver. One final quick tip: to get some juice into your phone a little quicker, switch it off when you put it on charge.