Don’t miss Part 1 Right Here:
Last week, I started talking to you about how to build your own solar panels. We got as far as connecting the individual cells together to make strings. If you have been able to order your solar cells, tab them and then connect the tabbed cells together to make strings, you’ve had a busy week. But, let’s go on. You can always save this letter, so that you can know what to do next, when you get to that point.
I told you before that we’re going to need four strings of 9 cells. So, we’ve got to connect those strings together. But if we do that before mounting them, there’s a really good chance that we’re going to end up breaking something. So the next step is to mount the cells to some sort of backing board.
There are a number of ways you can do this and a number of different types of materials that can be used for a backing board. I’m going to vote for simplicity and use simple underlayment, otherwise known as 5mm thick lauan plywood. You don’t need a lot of strength, but you do need something thin. You also need it to be weather resistant, so give it a couple of coats of good quality latex paint on both sides, before attaching the solar cells. (If you paint it only on one side, it will probably warp).
Once the paint is dry, draw a grid on one side, to define the positions of all the cells. Basically, you’re outlining each of them and where they will go. They are already about 1/8 of an inch apart, so put your strings about 1/8 of an inch apart too. In addition to this space, you’ll need about 1-1/4″ all the way around. This means that if your cells are exactly 3″x 6″ and you kept all your spacing correct, you should need a backing board that’s 30.5″ x 26.825″. But don’t count on my dimensions, measure it yourself, because your cells or your spacing may not match what I’m saying.
I’ve added a space between the second and third strings on my panels, to allow me to put a spacer there. This helps ensure that the glass doesn’t flex enough to cause it to make contact with the solar cells and break them. For a spacer, I used the same double-sided foam adhesive mounting tape that I’m using to install the glass to the panel.
The purpose of the grid is so that you know where to put your adhesive. I use regular silicone caulking for this. It’s designed for a long life, makes a good shock absorber and is a great adhesive as well. You’re going to work on one string at a time, so put a small dollop, about the size of a dime, in the middle of each square on your grid, for the first string.
Center the string over the grid on your backing board, holding it by the tabbing wires at each end. Align it with the grind and slowly lower it into place. Then gently press down in the center of each cell, one at a time, to flatten out the silicone at stick the cells in place. You don’t need to push them all the way to the backing board, allow them to float about 1/16″ inch above it.
Repeat this process with each of the other strings. But here is the important part. You need to make sure that you alternate the direction of each string, so that the positive end of the first string (tabbing wires coming from underneath the cells) is by the negative end of the next string (tabbing wires coming from on top of the cells).
Double and triple check the polarity of your strings to ensure that you have alternated the ends. If you miss this, it will cause large problems for you. With all four strings mounted to the backing board, allow the silicone to dry overnight.
The next step is to connect the ends of the strings together with buss wire. This is similar to the tabbing wire you’ve been using, but it is 5mm wide, instead of 2mm wide. Like the tabbing wire, it is pre-tinned, so it is extremely easy to solder. Simply slip it under the tabbing wire, heat the two with your soldering iron to melt the solder and then remove the soldering iron to allow it to solidify.
In the diagram above, you can see the alternating ends of the strings, as well as how the buss wires (shown in purple) are installed. At one end, the top two strings are connected together and the bottom two strings are connected together. At the other end, the tow middle strings are connected together, and the two outer strings are not connected to any other strings. But the two rows of tabbing wire in these strings are connected together.
All that’s needed now to make the panel functional, electrically speaking, is to solder a connector to the two ends of the string. Those are the two strings on the right end of the diagram, which aren’t connected to anything else. Use a polarized, weatherproof connector, something like an automotive connector, so that your contacts don’t become corroded by exposure to the elements.
At this point, it’s a good idea to test your panel. Place it in the sun and connect a multi-meter to the contacts of the connector. You should be getting 18 volts, or close to it, from your panel, in direct sunlight. Cloud cover will reduce that somewhat, dropping the available voltage down rather rapidly.
The next stage is to encapsulate the solar panel in a frame, with glass or Plexiglas, to protect it from damage. I’ll discuss that step with you next time we meet. Until then, keep your powder dry and your survival gear close at hand.
Stay tuned for Part 3!