Mike Davis | |

....Mike builds telescopes too....
....Mike builds telescopes too....

Here’s the second half  of a feature by our fearless DIY renewables enthusiast Mike Davis, on how to build your own solar panel. Part One is lower down the page, and is a free service provided by Off-Grid 101 – the technical and informational part of this web site. Many sites attempt to charge hundreds of dollars for this kind of information:

Now that the structure of the solar panel is finished, it’s time to get the solar cells ready. Once separated from their brick form, they are amazingly fragile and difficult to handle and store. I would recommend leaving them as bricks until you are ready to install them in your solar panel. That way you won’t wreck them before you get to use them. So build the panel first.

Then it’s time to start installing your solar cells. I started by drawing a grid pattern lightly in pencil on each of the two pieces of pegboard, Then I laid out the cells on that grid pattern upside-down so I could solder them together.

wire the cells in series
Wire the cells in series

All 18 cells on each half solar panel need to be soldered together in series, then both half panels need to be connected in series to get the desired voltage.

Solder on

Soldering the cells together is tricky at first, but you’ll get the hang of it fairly quickly. Start out with just two cells upside-down. Lay the solder tabs of one cell across the solder points on the back of the other cell. Make sure the spacing between the cells matched the grid pattern.

I used a low-Wattage soldering iron and fine rosen-core solder. I also used a rosen pen on the solder points on the back of the cells before soldering. Use a real light touch with the soldering iron. The cells are thin and delicate. If you
push too hard, you will break the cells. I got careless a couple of times and scrapped a couple of cells.

Now repeat the above steps and solder the cells together until you have a string of six. I soldered tabs from scrapped cells to the solder points on the back of the last cell in the string of six. Then I repeated the whole process two more times to get three strings of six cells for a total of 18 for this half of the panel.

The three strings of solar cells need to be wired in series. So the middle string needs to be rotated 180 degrees with respect to the other two. I got the strings oriented the way I wanted them (still upside-down) on top of the pegboard panel before
the next step of gluing the cells in place.

Careful with that caulk Eugene
Gluing the cells in place can prove tricky. Place a small blob of clear silicone caulk in the center of each cell in a six cell string. Then flip the string over and set it in place on the pencil line grid laid out earlier. Press lightly in the center of each cell to get it to stick to the pegboard panel. Flipping the floppy string of cells is also tricky. Another set of hands may be useful in during this step.

Don’t use too much glue, and don’t glue the solar cells anywhere but at their centers. The cells and the solar panel they are mounted on will expand, contract, flex and warp with changes in temperature and humidity. If you glue the cells too tightly to the
substrate, they will crack in time. Gluing them at only one point in the center allows the cells to float freely on top of the substrate. Both can expand and flex more or less independently, and the delicate solar cells won’t crack.

Next time I will do it differently. I will solder tabs onto the backs of all the solar cells. Then I will glue all the cells down in their proper places. Then I will solder the tabs together. It seems like the obvious way to go to me now, but I had to do it the hard way once to figure it out.

Solar systems are go

A finished half panel
A finished half panel

Here is one half panel, finally finished.  I used copper braid to interconnect first and second strings of cells. You could use solar cell tabbing material or even regular wire. I just happened to have the braid on hand. There is another similar interconnection between the second and third strings at the opposite end of the board. I used blobs of silicone caulk to anchor the braid and prevent it from flopping around.

9.3 volts. It works!
9.3 volts. It works!

YAHOO! It works! In weak sun through clouds the half panel is producing 9.31 Volts. Now all I had to do is build another one just like it.

Once I had two half panels complete, I could install them in their places in the main panel frame and wire them together.

Each of the half panels dropped right into their places in the main panel frame. I used four small screws to anchor each of the half panels in place.

Run connecting wires through vent holes
Run connecting wires through vent holes

Run wires to connect the two half panels together through the vent holes in the central divider. Again, use blobs of silicone caulk to anchor the wire in place and prevent it from flopping around.

Diodes required

Each solar panel in a solar power system needs a blocking diode in series with it to prevent the panel from discharging your batteries at night or during cloudy weather. I used a Schottky diode with a 3.3 Amp current rating.

Schottky diode in situ
Schottky diode in situ

Schottky diodes have a much lower forward voltage drop than ordinary rectifier diodes, so less power is wasted. Every Watt counts. I got a package of 25 31DQ03 Schottky diodes on Ebay  for only a few bucks. So I have enough left-overs for lots more solar panels

My original plan was to mount the diode inline with the positive wire outside the panel. After looking at the spec-sheet for the diode though, I decided to mount it inside since the forward voltage drop gets lower as the temperature rises. It
will be warmer inside the panel and the diode will work more efficiently. More silicone caulk was used to anchor the diode and wires.

Knot the wires for strain relief
Knot the wires for strain relief

I drilled a hole in the back of the panel near the top for the wires to exit. I put a knot in the wires for strain relief, and anchored them in place with yet more of the silicone caulk.

It is important to let all the silicone caulk cure well before screwing the plexiglass covers in place. I have found through past experience that the fumes from the caulk may leave a film on the inside of the plexiglass and the cells if it
isn’t allowed to thoroughly cure in the open air before screwing on the cover.
Use still more silicone caulk to seal the outside of the panel where the wires exit.

Male or female plug?

I added a polarized two-pin jones plug to the end of the panel wires. A mating female plug will be wired into the charge controller I use with my home-built wind turbine so the solar panel can supplement it’s power production and battery charging capacity.

A note of warning here. Power sources should always have female pugs on them to prevent short circuits.

Unsealed but completed panel
Unsealed but completed panel

Here is the completed panel with the plexiglass covers screwed into place. It isn’t sealed shut yet at this point. I wanted to wait until after testing it because I was worried that I might have to get back inside it if there were problems.
Sure enough, a tab popped off one of the cells. Maybe it was due to thermal stresses or shock from handling. Who knows?

I opened up the panel and replaced that one cell. I haven’t had any more trouble since. I will probably seal the panel with either a bead of silicone caulk, or aluminum AC duct tape wrapped around the edges.

Bang on target -18.88 volts.
Bang on target -18.88 volts.

Here I am testing the Voltage output of the completed panel in bright winter sunlight. My meter says 18.88 Volts with no load. That’s exactly what I was aiming for.

Here I am again  testing the current capacity of the panel, again in bright winter Q sunlight.  My meter says 3.05 Amps short circuit current. That is right about what the cells are rated for. It means the panel is working very well.

3.05 Amps short circuit current
3.05 Amps short circuit current

So how much did all this cost to build? I have a well stocked work shop so many of the materials were available free. My biggest costs were $74 for the solar cells from e-bay, $20 for lumber and $6 for the Jones Plug. You probably wont find commercial panels for much under $400, so that’s a fraction of what a commercial solar panel would cost. And then of course there is all that free power.

It’s about more than money

But its not just about saving money. I got deep satisfaction from designing and building the panel, my beautiful Arizona hideaway is just that little bit more comfortable. All I have to do now is find time to get down to the real point of the
exercise which was watching the stars.

PS I get dozens of requests for help each day. Sadly I simply don’t have time to answer the majority of them. I have created a FAQ to handle these repetitive questions.

But there is no way I can help you out with complex issues, teach you electronics theory, help you locate parts, build a charge controller for you, or custom design a system for you. There just aren’t enough hours in the day. Sorry.  Good luck, Mike Davis.

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