How I Built a Folding 15 Watt Solar Panel

Several years ago I built a 60 Watt solar panel. It's a great panel. I still use it a lot. However, it is big, and heavy, and I don't always need 60 Watts of power. Sometimes I just need a little power for a portable device or two. I'd like to be able to charge up my cell phone, or MP3 player without having to lug around the big solar panel. I'd also like the panel to be more compact and easier to pack when I am traveling light. So I decided to build a 15 Watt Solar panel that folds up to be more compact. Some people may complain that it is too much effort, and doesn't save enough money to make it worth the trouble of building their own 15 Watt panel. For me though, it is all about the joy of building. Anybody can buy a solar panel. How many people build their own? Plus it folds up, which most commercially made panels don't do.

I experimented with putting plexiglass covers over both halves of the panel, but in the end decided not to use them, so those steps are omitted from this Instructable. You can see the mistakes I made, and the omitted steps, and lots more about constructing this solar panel on my web site. See also my home-built wind turbine and home-built biomass gasifier alternative energy projects while you are at my web site.

Here is a video walk-around of the finished solar panel. This Instructable will explain how I built it.

Step 1: Getting the Solar Cells

The main stumbling block to building solar panels is acquiring solar cells at a reasonable price. New solar cells are very expensive, and can even sometimes be hard to find in quantity at any price. Blemished and damaged solar cells are available on Ebay and other places at a fraction of the cost of new perfect cells. These second rate solar cells can be used to make a solar panel that will work just fine.

My first solar panel used 3 X 6 inch monocrystalline silicon solar cells. They are highly efficient and produce a lot of current. But they are also big and delicate and difficult to work with. For this project I decided to use a different kind of solar cells. I used 40 thin-film Copper Indium di Selenide (CIS) solar cells. Each cell is actually a miniature solar panel 60mm x 60mm x 2mm thick, that will produce a little over 4.5 Volts and 80 mA in bright sunlight, that only comes out to about 0.375 Watts per cell, but 40 of them together produce the 15 Watts I was targeting. I would wire them together in groups of 4 cells in series to get about 18 Volts, and then wire the groups of 4 cells in parallel. I would divide the 40 cells into two groups of 20 and build a folding box to mount them in. I bought 40 of the (CIS) solar cells off of Ebay and got to work.

Step 2: Building the Box

I drew up a layout for 20 of the solar cells and rough dimensions of half of the folding solar panel. The other half would just be a mirror image of this. I did things a little differently when I actually wired the cells together, but I followed this initial plan fairly closely.

Now it was time to start building the panel. I used similar construction technique to what I used on my first solar panel, just scaled down to a smaller size and hinged in the middle. I built two shallow wooden boxes 15 7/8 by 13 inches, using 1/4 inch thick plywood and 3/4 x 3/4 wood side rails.

I hinged the the two halves together. I didn't bother mortising out for the hinges. I just mounted them flat against the sides of the boxes.

The last photo shows the finished hinged box opened up. I painted the box my usual white color that seems to be becoming the signature color for all my alternative energy projects.

Step 3: Gluing Down the Solar Cells

I marked out the position for each of the 20 solar cells in each half of the panel in pencil.

I then cut the tape holding down the tabs on the back of each of the cells, and folded the tabs up and over to the top side of the cell. I also marked the negative side of the cell better so I could see it from the front of the cell. Then I began gluing them down on the spots I had marked out earlier. I used a small blob of silicone caulk in the center of each cell and pressed it in place.

After all the cells on one half were glued down, I waited a few hours for the silicone to set up before starting to wire the cells together. I wanted to complete and debug one half, just in case I ran into problems, before doing the other half.

Step 4: Wiring the Solar Cells Together

Once the silicone had set up and the cells were firmly stuck down, I started soldering the tabs together. The tabs on adjoining cells were sticking up like is seen in the first photo.

I used my fine needle-nose pliers to grab the tabs that needed to be joined, and rolled them together on the tips of the pliers. Then I flattened the resulting little roll of foil down to prevent it from unrolling. The tabs are made of thin metal foil. So be careful since it isn't hard to damage them or rip them off the cells.

Finally I used my soldering iron and a little solder to finish each joint. Be quick. Don't let your soldering iron linger on the tabs, or they may unsolder from the cells.

Once I was done soldering all the cells together, I had 5 strings of 4 cells wired in series. Now I had to connect the 5 strings together in parallel. I used some copper braid I had on hand to wire together all the positive sides of the cell strings. I used small blobs of silicone to hold the braid in place.

Step 5: Adding the Blocking Diodes

I then soldered the anode of a 31DQ03 Schottky diode to the positive rail. This diode acts as a blocking diode, preventing the solar panel from discharging my battery bank at night or when shaded. Each half of the panel gets a blocking diode since the two halves will be wired in parallel. I bought a bunch of these diodes cheap on Ebay, and have been using them for a while now in my solar panels.

Next I installed the wires that leave the panel. I ran them out through a hole drilled in the hinge side of the panel. I soldered the red wire to the cathode of the diode and the black wire to the negative rail. I tied a knot in the wires for strain relief, and used blobs of silicone caulk to secure everything in place.

The other half of the panel is a mirror image of the first. I joined the wires for the two halves together outside the panel.

Once you have all the solar cells glued down and wired together, the solar panel is essentially finished. I experimented with adding plexiglass covers to both halves of the panel to protect the solar cells. However, I found that heat built up inside the panel, and the heat reduced the performance of the solar cells. Since these solar cells are much more robust than the ones I used to make my 60 Watt solar panel, I decided to omit the plexiglass. You can see the mistakes I made, and the omitted steps, and lots more about constructing this solar panel on my web site.

Step 6: Wiring the Two Halves Together

The wires from both halves of the panel were joined together using butt splices and connected to a quick-disconnect plug. The connector I used on the panel. It is a polarized quick-disconnect plug available at Radio Shack and other places.

I actually bought several of these plugs. The other end of the plugs connect to various different things. I can switch what the panel is connected to just by changing plugs. One of the more useful things I connect to this solar panel is a car cigarette lighter type plug (not shown) that I can plug small chargers like for my cell phone or MP3 player into.

I also have a long cable made from an extension cord that allows me to have the solar panel in a sunny spot, but use the power somewhere far away in the shade.

Step 7:

I completed the solar panel just in time to take it with me on vacation to my remote off-grid property in Arizona. My property has previously served as a testing ground for my home-built wind turbine, and my 60 Watt home-built solar panel. A photo below shows the solar panel unfolded and laid out on the hood of my truck to catch the sun.

I wanted to test the solar panel extensively. I wired it into my charge controller to supplement the power being produced by the wind turbine and larger solar panel. I also wired the panel to a 12V cigarette lighter type plug and tested it powering various small appliances like my car cell phone charger and the charger for my MP3 player. The panel worked very well.

So how much did all this cost to build? Well, I saved all the receipts for everything I bought related to this project. Also, my workshop is well stocked with all sorts of building supplies and hardware. I also have a lot of useful scrap pieces of wood, wire and all sorts of miscellaneous stuff (some would say junk) laying around the shop. So I had a lot of stuff on hand already. Your mileage may vary.

Part                                       Origin                            Cost

Solar Cells                          Ebay                              $30.00*
Misc. Lumber                      Already on hand          $0.00
Hinges                                 Homecenter Store      $3.49
Silicone Caulk                    Homecenter Store      $3.95
Wire                                      Already on hand          $0.00
Diodes                                 Ebay                              $0.40±
Quick disconnect plug      Radio Shack                $5.50
Paint                                     Already on hand          $0.00

Total                                                                             $43.34

* The price of solar cells on Ebay has gone up quite a bit since I bought these cells.
± This price represents 2 out of a lot of 25 diodes I bought on Ebay for $5.00.

Not too bad. That's a bit cheaper than what a commercially made 15 Watt solar panel from someplace like Harbor Freight would cost, ($70 as of this writing). However, my panel folds up to save space. The Harbor Freight panels don't, and they are nearly as big as my 60 Watt solar panel. I have plans to further experiment with these CIS cells, and build more panels to add to the capacity of my system. I'll post more here as the project evolves. Stay tuned.

You can see the mistakes I made, and the omitted steps, and lots more about constructing this solar panel on my web site. See also my home-built wind turbine and home-built biomass gasifier alternative energy projects while you are at my web site.

Here is a video walk-around of the finished solar panel.