Here I am going to show you how to set up a solar panel, buck convertors and batteries to create a solar power solution that will keep your pi powered 24/7. A great idea for your personal server or timelapse projects. The solar panel is sufficiently powerful to provide power in cloudy conditions. In this case, I am using the Pi as a Lighttpd web server to host my website which contains a live feed from my IP camera and many interesting pictures.
http://shaundobbie.no-ip.biz For those of you who may be interested.
This instructable was not intended to be terribly detailed it only outlines the setup which works effectively.
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Step 1: What You'll Need
You will need:
1 40w solar panel
1 22Ah lead acid battery http://www.ebay.co.uk/itm/190855719649?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
1 UBEC (this takes high voltages down to 5v for the Pi. I use Turnigy UBECs because they work very well. You can find these on Ebay or Hobbyking
1 Variable buck convertor to take in the solar panel voltage and output 13.8v http://www.ebay.co.uk/itm/130901684801?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
You can use a voltmeter module and keep it on the battery constantly or just check up on it with your multimeter. I have an extra 2 7Ah lead acid batteries in parallel with the 22 Ah one but that really isn't necessary. I just do it to keep them in good condition.
Step 2: Solder the All the Parts
Soldering the UBEC
I very much hate the idea of using a micro USB port for power. Really, where's the need? You need to check that the UBEC is set to 5v. It will have a jumper for this. You need to solder the wires onto the capacitor next to the USB power port. Negative to the black mark and positive to the marked side. You can put glue on it to secure the wires.
Setting up the buck convertor
Put the solar panel in the sun and attach it to the input on the buck convertor. Put your multimeter on the output and adjust the multiturn potentiometer until it reads 13.8v. The reason I am recommending a buck convertor is that you can keep the battery at a float voltage. I did try a solar charge controller but it had the battery at 14.8v and it was gassing. The charge controller did not drop to 13.8v and there was no way to adjust it.
Once you have it set at the right voltage, attach the output of the buck convertor to the battery.
Finally, attach the input of the UBEC to the battery, put the panel in the sun and your Pi should power on. The battery will keep itself charged. Just keep the battery out of the sun because they are better when kept cool.