Cheap Solar Panel for Under $30

Right now, it seems that someday we will all be charging our cars with solar power. Solar power is both awesome and sustainable, and is a hot topic in social media. I was given the opportunity to lead a tech workshop, so I decided it would be a good idea to teach people about solar power.

This instructable teaches readers how to make a weather proof portable solar panels for under $30. It's a rudimentary panel, not including all the downstream gizmos of the panel's output such as regulators, inverters, energy storage methods, etc. The panel is constructed by chaining 10 solar cells in series (with bypass diodes - more on this later). The solar cells I purchased were listed at a 0.5 Vdc output with an ideal max 8 amp output (this is an ebay listing). So, for 10 cells in series, the panel's output is ideally 5 Vdc with a 40 W power output.

This is all well and good, but in practice, the panel I made produced about 5 Vdc at 72 mW on an overcast day. So I question the ebay listing's specifications. However, if you want to learn about basic solar panel design, this instructable is worth a glance.

Step 1: Solar Cell Theory - Elevator Pitch Version

Solar cells are made of photovoltaic (PV) materials. This long name is intentional: when PV materials are exposed to solar radiation (photons --> photo), the material outputs a voltage (voltaic). What voltage means, in layman terms, is the electrons in the material are excited. If you funnel these excited electrons through a wire, you have electrical current. One can harness this current to do work downstream of the PV material.

PV materials are made of P-N junction semiconductors (not explained in this instructable). A classic example of a P-N junction is a diode. A P-N junction optimized to be light sensitive is more or less a PV material. Thus, a solar cell is essentially a light sensitive diode, or a photodiode. Thus, when thinking about solar cells, one can think of them as photodiodes.

One interesting fact about solar cells is that their PV material works even when shattered. In the above images, you can find the front and back view of a shattered solar cell. When one electrically connects the fragments of the cell, they collectively produce the same amount of power as the solar cell originally did, while still in one piece.

Step 2: Solar Panel Circuit and Theory

The circuit for a solar panel is rather simple. It helped me to imagine the solar cells as voltage sources, because that's essentially what they become under sunlight. A simple solar panel schematic places the solar cells in series.

Another interesting fact about solar panels is that when shade drapes across one cell, the cell actually acts as an open circuit. This is a problem, because if even just one cell becomes shaded, it cuts power from the entire solar panel. This problem is solved by adding 'bypass' diodes. Bypass diodes are diodes oriented in parallel with the solar cells that enable current to flow around a particular chain of cells when it becomes shaded. In this panel, I have incorporated four bypass diodes, thus enabling the panel to function if multiple quadrants of the panel become shaded. This is shown in the video for this step, which shows how shadows reduce output voltage, but never reduce it to zero.

One last thing to note is that when it's dark outside, solar cells can actually sink current, acting as a resistor. Thus, in front of the circuit, I have included a blocking diode to ensure that solar panel never acts as a power drain.

Step 3: BOM

Here is a list of the materials you will need. The prices listed are on a per panel basis. I bought in bulk because it was a workshop:

• 10 Solar Cells - $ 13
  • I used 10/panel, feel free to add more
  • Note: you will probably break about 5, so make sure to buy plenty extra
  • Ebay Link
• Solar Tab + Bus Wire - $3
  • Tab wire - used connect panels to panels
  • Bus wire - used to connect rows together
    • Wider and thicker than tab wire
  • Ebay Link
• Test Lead Clips - $2
  • Don't buy the ones I did, they were overkill
  • Ebay Link
• 14 gauge wire - less than $1
  • I picked 14 gauge wire based on this chart: Wire Gauge Current Handling Chart
• Diodes - less than $1
  • I chose 10A10 because I knew they could handle the ideal current and were very cheap on ebay
  • Ebay Link
• Plastic Film - $2
  • This will weather proof the panel, more on this later
• Zip Ties (optional) - $1
  • Used to secure the solar panel to the backing material

Soldering materials:

• Solder core wire (I used one pre-impregnated with flux)
• Soldering iron
• Wire cutters + strippers
• Helping hands

Circuit debugging tools:

• Multimeter
• Alligator clips

The remainder of the materials are common arts and crafts materials

• Scissors
• Cardboard
  • This will be the backing material
• Tape
• Hot glue gun (optional)
• Plastic gloves

Step 4: Solder Order

On my solar cells, the current flowed from the front of the cell to the back (conventional current). Thus, the front can be regarded as "-" and the back can be "+".

The tab wires will be soldered from the back of one cell onto the front of the next cell, in order for current to flow through them. It's important to stick to this pattern, because remember solar cells are like photodiodes, and if you orient one incorrectly it will mess up the panel.

Any mistakes made before soldering are easy to fix, but mistakes after soldering are hard to fix. My advice to you is to plan out everything and check your work using alligator clips and a multimeter. Ignore the diodes for this step.

Step 5: Cut Tab Wire

My solar cells were 6" x 6", so I cut the tab wire to fit these cells. It helps to make a hand sketch to get an idea of where each length of wire will be used.

Tab Wire

• Meant to join two cells: 12.5" lengths
  • 18 segments
• Meant to connect a cell to a bus: 6.5" lengths.
  
  • 24 segments

Bus Wire

• Ends of panel: 13" lengths
  • 2 segments
• Join series rows together: 7" lengths
  • 4 segments

Step 6: Soldering the Backs

Before starting soldering, you need to know a few things:

• The solar cells are very brittle, so it will take some getting used to
  • I broke many cells before getting procedure down for repeatable success
• Do not solder the cells over a bumpy surface. Either solder over a metal sheet, glass plate, or cardboard
  • If you solder over a bumpy surface you will probably break your cells
  • I used cardboard because it has a little bit of 'give' to it
• The tab and bus wire are pre-tinned (usually), which means you don't have to add any solder. In practice, I found this tabbing to be insufficient, so I would add some solder.
  • The less you use the better, because solder on the backing acts like bumps underneath the panel when soldering the front, which increases your chances of cracking the solar cell

Note: the soldering was a very hard thing and came with a learning curve. Here is what I can say to do:

• Place a small amount of flux on tab wire before soldering
  • Some people use flux pens, I did not have access to one
• Use a chisel tip soldering iron
  
  • I had my iron on 380°C
  • Keep a clean tip
• Use a just dab of solder to initiate wicking but not too much more beyond that
• Try to drag the iron with light pressure on the panel and at a constant rate
• Use a weight to hold the tab wire in place initially

Step 7: Solder the Fronts

Soldering the front of the cells is the hardest part. I have included a couple pictures of my failures to show you that this step can be frustrating. It took some practice to get the soldering methodology down. Here are some tips:

• Wear gloves, because your finger prints stay on the cells
• Use little pressure, because using more will crack the cells
• Try to drag the iron faster here, but test throughout the process that the tab wire is actually properly soldered onto the panel
  • This creates a smoother finish, and the front solder job is what will be seen on the final panel
• Use little flux
  • The flux spills off the sides and leaves dark marks on the solar cell. I am not sure if this residual decreases the cell's power output
• Try to solder cell pairs before combining them into chains

Step 8: Solder the Bus Wire

Congratulations, you have made it past the hardest part! Up next is to combine the ends of the tab wire onto the bus wire, forming several voltage nodes. I would advise to solder the shorter bus wires first, and leave the long wires for last. Soldering the bus wire can be dangerous because it is so close to the fragile solar cells. Be careful on this step.

After the bus wires are soldered, congratulations, you should have a fully functioning solar panel! Next step is to add in the diodes and test lead clips for the finishing touches.

Step 9: Solder Test Lead Clips Onto the Panel's Terminals

This is an easy step. Cut two segments of wire, each with whatever length you desire. I chose 2'. Strip about 0.5" of insulation of both ends of the wire, and tin the exposed copper.

Next is to attach the test lead clips. I pulled one of the handle covers off, and underneath it were three thin tabs. The wire should be placed between the tabs and then have the tabs bent onto the wire, holding it in place. Solder the test lead clips to the wire, permanently securing the connection. End by sliding the handle cover back on, this time having to feed it along the wire you just attached.

Step 10: Solder the Diodes

Now the next step is to solder the diodes. Helping hands or a weight are particularly useful in this step. In my case, I soldered the diode directly to the bus wire, and then used the 14 gauge wire to connect other end to the next bus. Any time there is potential for a short, be sure to use electrical tape to cover any exposed leads.

Note: each time you solder a bypass diode, be sure to test it with a multimeter. One time, I soldered a diode in the wrong direction and then broke a cell (and had to replace it's whole chain) while trying to de-solder of the diode.

Step 11: Test Your Panel

Now that the panel should be electrically connected, you are all done! Hopefully you have been testing your panel throughout the process. Before mounting any of the cells firmly on your backing material, I would test all of the cells to ensure they are producing a reasonable voltage.

If you are testing it indoors, I found the cells produced very little current and also did not quite reach their specified 0.5 Vdc production. Keep this in mind.

I used a multimeter and idealized the solar panel as a thevenin equivalent circuit (even though this probably isn't a correct practice because the panel behaves nonlinearly). I found the ideal resistance to get a max power draw to be about 36 Ω.

Step 12: Finishing Touch: Secure Cells to Backing Material and Add Plastic Covering

Now that you are confident your panel works, go ahead and hard mount and weather proof it. The mounting is simple: use electrical tape, zip ties, or hot glue to firmly attach the panels to the backing material. Note, any glue used may melt on hot days or if the solar cells heat up.

Weather proofing the panel entails cutting out two equal sized plastic sheets, and placing them above and below the panel. Now get your soldering iron, and follow a procedure similar to this one: Fusing Plastic Sheets. I can no longer find it, but another instructable had suggested sandwiching wax paper between the soldering iron and the two plastic sheets to prevent yourself from totally caking plastic onto the iron.

Note: if you are concerned about ruining your solder tip, this is a very valid concern, as it will probably bake the plastic onto the tip. If you are smart, clean the tip often throughout the process and use a fan to blow away all smoke (as plastic fumes are bad to breathe).

You can't perfectly seal the wires because you can't easily melt the plastic film onto the wires. If you want an airtight seal, use clear packing tape to seal any openings. In my case, I left some openings for air to escape. However, in moist climates, this might have been a bad call.

The corners are hard to perfectly seal. My advice would be to start in one spot and make a clockwise lap around the panel. Otherwise, wrinkles may form which ruin and air tightness you had.

The plastic film slightly interferes with the solar cell's efficiency, as it's reflective. However, compared with the trade off of weatherproofing, which enables one to take the panel into the great outdoors (to endure wind/rain/dust), I thought it was worthy. The video shows the slight detrimental effect of the plastic film on the open circuit voltage of the panel.

Step 13: Conclusion

In conclusion, one can gain a respect for solar panel manufacturers when constructing a solar panel. The panel I made produced only about 72 mW of power in overcast weather. While this is not that much power, it would make a half decent trickle charger for under $30.

Advantages:

• Low cost
• Requires only simple tools to make
• Can be made in about 5 hours
• Highly customizable
• Works even if partially shaded

Disadvantages:

• Very fragile
• Low power production
• Not perfectly weather proof

Areas for improvement:

• Spend the money on a hard backing material such as MDF
• Potentially make a 'cookie cutter' for the solar cells to hold them in place
  • Also could restrain loose wires away from front of cells to maximize power production
• Spend more $$$ on higher quality solar cells
• Standardize a method to melt plastic film into a protective seal around panels
  • Potentially use different housing material that is more 'optically clear' to let in more sunlight
  • Figure out how to prevent condensation, which again would block sunlight and also can introduce shorts
• Don't touch the solar cells without gloves as it's easy to leave finger prints
• Add more cells in series to get a higher voltage output
  • 5 V is only useful for small electronics, whereas 12 V can be used to charge a car battery
• Find some mirrors to reflect sunlight onto the cells, increasing the intensity of light on the cells --> more power!
  • Make sure you think about heat dissipation in this case
• Become better at soldering

I am always looking for recommendations, please post your comments if you see any gaps in the electronics/theory section, or have general advice. Thanks for reading!