This project has a web server running on Raspberry Pi powered 100% by solar power. via pi.qcontinuum
Before I purchased my solar panels, I had to calculate the power requirements. I bought solar panels rated at 45 Watts based on these calculations.
45 Watts at 12 volts would be 3.72 Amps. But this would be only the best case scenario, so for this math I used 3.5 amps. To charge an 80AH battery using a 3.5 Amp charger, it would take 22.86 hours. Of course there are losses, so my thoughts were that it would take 24 hours to charge the battery from dead to full. That would be 24 hours of sun light.
The Raspberry Pi Model “B” requires a 700ma 5 Volt (3.5 Watts) power supply. At 12 volts it would use 292ma. Of course, no voltage converter is perfect, so for calculations I used 350ma. An 80AH 12 Volt battery should run the Raspberry Pi 228.5 hours before its dead. Of course, the battery’s voltage would drop off before it was totally dead, so it is hard to say just how long it would run it. In a 24 hour period the Raspberry Pi would use 8.4AH of the 80AH battery. So, the solar charger at 3.5 amps would need to change the battery at least 2.4 hours each day to stay even with the power usage of the Raspberry Pi.
I ended up picking the 45 Watt Solar Panel Kit from Harbor Freight. The reason I chose it is because of the price. I did my homework, and knew the kit wasn’t the best quality. Since I am only using it for this project, I did not want to spend a ton of money on a real solar kit. The kit is also complete with everything I needed except the battery. Lucky for me, I had a battery laying around that I could use for the project. If I had to, I could replace any part of this system without any downtime, as long as I have good monitoring in place. If the battery starts to die off and not hold a charge, I can hook one in parallel and then remove the bad battery. If the panels or charging unit dies then I will have a day or so to come up with something while the server runs on battery power. There are single points of failures, but the same is true of the Raspberry and its one SD card.
The panels needed to be outside in the sun. I have a nice house and a wife I love, so I did not want to start drilling holes in the roof or do something to cause her (or the neighbors) to freak out. Also, this was a cheap “for the fun of it” project and I did not want scope creep to take over. The back of my house faces west, and we have a big deck. I decided set up the panels on the deck facing west. I cable tied it in place so I did not have to drill any holes in the deck or railings. I then cable tied the wiring along the bottom of the rails, down a gutter down spout, and along the bottom of the stucco siding. I lucked out because the stucco siding had a tin rail running under it with stamped holes in it. Every so many feet I put in a screw and cable tied the cord to it. The twelve volt cord that came with the panels (between panels and regulator) was only so long so I put a regular female extension cord plug on it and for the rest of the length. I used a standard extension cord I already owned. I wanted all the electronics in my datacenter in the house (I also have a LAN center in my house but that is another story). I used an old, but good quality extension cord that had already been cut for another reason years ago. Extensions cords work great as dirt cheap durable wiring. Once inside the house, I terminated the connection on the regulator.
I decided to buy an Arduino Uno to go along with the Raspberry so I could use it’s inputs and serial output to monitor the voltages. I used RRD tools to graph the voltages and Xymon to monitor and send alerts, if something goes wrong. The Arduino is connected directly to the Raspberry so it can collect the data. There is just so much you squeeze out of the limited resources of the Raspberry Pi, so I’m running RRD tools on a different server. In addition, I did not want to run an external HD connected to the Raspberry. I wanted the Raspberry run 100% off solar power and I thought the HD would use too much. I wasn’t sure if the Raspberry’s SD card would handle the constant rewrites of RRD tools. Both the Raspberry Pi and the Arduino run off the solar power.
Which kind of circuit are you using to switch between solar power and battery power?
While it is convenient, please do NOT use regular AC connectors for anything other than their rated voltage. The risk is too great of someone plugging your 12V stuff into 120V, causing a fire or shock hazard. It is also against the NEC.