Solar USB charger

Many devices today, especially the MP3 players, video cameras and PDAs support USB charging.
Those devices, being designed to be very portable and small, can often run out of power if you are away for an extended period, or if you forgot to charge them before leaving your home or office.

There are multiple solutions for this problem. Most of them involve expensive additional batteries, which can be used to charge our devices.
There is however a relatively inexpensive solution: A solar panel charger.
If you look on eBay, you can find some suitable solar panels (6V, 225 mAh) for about 10 USD, sometimes cheaper if you buy more. I bought mine from here, but feel free to look around and see if you can find better deals.

A solar panel charger is nice, but not all the days are sunny. And what do you do if you want to charge your device at night?
The answer is simple: Use some rechargeable batteries with your solar charger.
With a 6V panel, you can recharge 4 AA NiMh or NiCd batteries. Some NiMh batteries are rated 1.2V @ 2.5 AH, which is enough to charge even the most energy hungry portable device. They need to be placed in series with eachother, and in parallel with the solar panel. They will generate a little over 4.8V when charged, which is within the USB specifications.

So here is what you will need:
A. 1 or more solar panels. My design includes 2, but you can use as little or as many as you want. Of course, the more you use, the less portable your charger becomes.
B. A battery holder for 4AA batteries.
C. A blocking diode (RadioShack, old electronics, etc.)
D. Suction cups (optional)
E. Soldering iron/gun and solder.
F. An USB female connector (cannibalized from some broken device, or from an USB extension cable)
G. Copper tape (optional, you can use wires if you want).
H. Some strong glue.


The construction is very easy, and it takes only about 1 hour. I will assume you are going to use a two panel charger, like mine.

The first thing is to glue the battery holder between the solar panels, to hold them together.

Then cut the wires from the solar panels just above where they come out (you can't solder on panel directly, it has some plastic to protect it from water).

Place the copper tape as shown in the picture (you want to connect the panels in parallel).

Place the blocking diode as shown in the picture (a multimeter can be very useful, to determine the diode placement). The idea is for the electricity to flow from the panel to the battery, and not the other way around.

Glue the USB female connector somewhere on the panel, and connect the red wire to the plus of the panels, and the black wire to the diode, just as it comes out of the battery (on the - terminal). Again, look at the picture.

If you want, you can glue two suction cups on the surface of the panels, to be able to place them on a window or windshield.

To test it, place the least expensive rechargeable USB device into the USB female connector, and see if it works fine.
Ideally, you should measure the voltage and polarity, to make sure everything is OK. Otherwise you might burn your device.

That should be it. The total cost should be around 30 USD.
You can build it for less if you skip the whole battery thing (no need for a diode, batery holder and 4 batteries.

Some warnings:
1. This is a very simple construction, and as such, there is no special charging circuit for the batteries. It might shorten their life if charged excessively. So don't let that thing sit in the sun all the time.
2. Remove the batteries while not in use, because if a wire comes lose it can shortcut the whole thing and set your place on fire.
3. Ideally, you should be near it while you use it.

If you have questions, comments, suggestions, please leave a comment.