[Links to previous posts: Part1, Part 2, Part 3, Part 4, and how to run the Pi from a battery]
Today, my 12V battery and my 12v regulator arrived. I have a fully functional Raspberry Pi system running completely off the grid. So I will list what I got and how I connected them.
First: Solar panels.
I got a broken solar panel from eBay [original auction link]. Some hail rock fell on it, and shattered it. It means the solar panel is not waterproof anymore, and the shattered glass greatly reduces its efficiency. As I am running it from my balcony, rain is not a problem, and for seven euros, it’s the best ratio watt/euro possible.
Turns out the seller didn’t deliver, so I had to drive to some village 200km from here. As the seller also sold new panels, I bought a new one, for 100 euros, of the same size. It gives me 180W over 36V, from a company called Trina Solar. The only place I found a similar one on the web is at Solar Town (USA), for an absurd price. It looks like this is not produced anymore.
Connectors, connectors, connectors.
The standard connector on solar panels is called MC4 [wikipedia]. They allow you to connect several panels together, they are easy to plug, UV-resistant, water-sealed, and people sell them at ridiculous prices. I managed to find them for 2 euros a pair on eBay, which is quite a good price for it. The panels themselves come with them attached, but I needed the counterpart to connect to them.
Because I have two panels, I had to connect them together. I put them in parallel, so I keep the tension constant. For that, I got a MC4 splitter for 8.50€ on ebay. This allows me to have these two panels with only one output (so I only needed one pair of the aforementioned MC4 connectors). This is how it looks like:
As I plug and unplug everything all the time, instead of soldering everything together, I used 2.5mm connectors everywhere. Those allow me to unplug everything easily, which I do a lot. Besides, the 2.5mm is pretty much the same used on most notebook rechargers, USB hubs and on my bitcoin miners. To keep it standard, everything that provides energy has a male connector, everything that receives energy is a female connector. I got some with screws on one side, in order to make it easy to remove if I needed. I regret this now. These are the male and female [eBay] ones:
I mentioned I regret it not because of how I decided to connect things, but because the quality of these is appalling. The green thing on the back almost falls apart when you screw it, and the hole is too small, the cables keep falling out of it. I plan to replace them with soldered ones [eBay], now that I have a stable rig:
Solar panel controller/regulator.
Initially, I didn’t want to go with these, instead powering the Raspberry PI and the miners directly from the solar panels. It doesn’t work. Germany is just too dark. You need to store the energy somewhere, and to do so, you need batteries, and a regulator.
The regulator receives the power input from the solar panels, and recharges the battery (and importantly, it STOPS recharging the battery when it’s full), and provides a stable output for whatever you connect at it.
Because my panels are 36V, I looked for a 36V regulator. However, these can cost A LOT. I’m talking 700 euros, or even more. Instead, I went cheap, and got a 12V one, for 10 euros on eBay (and I will explain later how). You can buy these for as low as 6 euros, but they are delivered from China and take a month to get here. I paid more for next-day delivery. This is how it looks like:
It’s working can’t be simpler: You attach the solar panels, the battery, and your “load”, which is whatever device you want to use the system’s power. There’s no set up, nothing. It’s plug and forget. This is how it looks like, with everything connected (I just got this picture from eBay):
First voltage regulator
Because the solar controller needs to receive either 12V or 24V (and my battery is 12V), I had to lower the panels’ 36V to something it accepts. So I use a Buck voltage regulator that I got for 13 euros on Amazon:
I put it in this electricity box so I can isolate it and make it waterproof later. On the lower right corner of the image, there is a screw. Rotating it you can choose the output tension (which I set to 12V). It works around 95% efficiency, so I don’t really lose much energy on the conversion.
Attached to it, I put a simple LED voltmeter, which I got from 2 euros on eBay. It’s interesting that even on the input side, the power lowers from 36V to 12V after everything is connected:
The second connector of the solar regulador is a battery. No surprises here: I got a 12V, 7amp battery from amazon for 15 euros. I connect it directly to the regulator (with a plug, of course, so I can disconnect it at any time).
From 12V to USB: second regulator
So far, the whole system is 12V. However, USB runs with 5V, so I need to step the voltage down again. In order to do so, I found a very practical device, that receives anything from 4V to 40V and converts it to 5V, with a LED multimeter attached AND a usb port. Got it in Amazon, for 10 euros. With this, running the Pi is a matter of plugging it on this controller.
In order to measure power consumption, I got this thing from Amazon, for 15 euros. I don’t keep it connected to the system, of course, as it consumes power itself, but it’s useful to see how much energy you need:
Finally, the raspberry Pi itself I am using a Pi 2 model B, which I got on Amazon for 39 euros. The best micro-usb cable I have is from Nokia, on Amazon for 2.50 euros, and the only wi-fi that works reliably with the Pi is the Edimax,EW-7811UN, bought on Amazon for 7 euros.
To show everything working together, I made a small video:
2x 180w 36V panels -> MC4 Splitter -> MC4 (one cable) -> Buck 36V-to12V -> Solar Power Regulator -> 12V Battery -> 12V-to5V USB regulator -> Raspberry Pi.
I will test this for some days, and then I will post about the bitcoin miners.
Update: this is how the system runs when there’s no light: