Repairing Polymer Degradation on Solar Cells

I love my neighbor’s garbage; I call their garbage my parts department. My neighbors buy solar garden lights, put them in the garden or along their walkways, and forget about them. Then every spring like clockwork, while they replant their garden or fix up the yard, they discover the lights don’t work, and toss the lights in the waste bin, this is where I come in.

On garbage day I would go around to my neighbor’s garbage and pick the things I might find useful like garden lights, and electronics. Many of the things I get are in need of repair or only salvageable for parts.

One of the repairs needed to some of the solar lights is polymer degradation of the solar cells. This can be caused by UV light from the sun, exposure to chemicals, or oxidation from the oxygen in the air. To fix exposure or oxidation that leaves the surface of the solar cell covered in white residue you need to clean the residue off the cell and clear coat it. Test the solar cell and make sure they work to some useful degree.

https://en.wikipedia.org/wiki/Polymer_degradation

As an added green project, all the components in this Instructable are salvaged and repurposed.

Multi Meter

Screwdrivers

Pincers

Paintbrush

Soldering Iron

51Ω Resistor

Newspaper or Drop Cloth

Solder

Clear Coat

To start, dissemble all the unrepairable solar lights.

Many of the solar lights I get just need to be cleaned or a new battery, others need a new switch or a wire replaced. Then there are the ones that are only salvageable for parts.

Start by dissembling the solar light, this is a good time to reverse engineer the circuit board and make a schematic. This is important because later when you are looking for a charging circuit you will already have one for that solar cell and a battery. A charging circuit for that solar cell and a battery is the solar light circuit with the LED removed.

https://www.instructables.com/id/Reverse-Engineering-a-Small-Amplifier/

Strip out all the useful components and the solar cells.

Keep all the solar cells, many damaged cells can be repaired or refurbished for use in other solar lights and projects.

After gathering up a number of solar cells separate them into the repairs needed, for this Instructable you want the ones with Polymer Degradation.

This is one you can add to your 101 uses for toothpaste; dab a little toothpaste on the solar cell and add a little water, then buff. The abrasives in the toothpaste acts like the abrasive in polishing compound, this scuffs the surface so the clear coat will stick to the solar cells surface and it buffs out the scratches high spots.

Rinse and repeat as necessary to remove the poly oxide and high spots from scratches.

Here is one you can add to my Instructable Plastic Coat, repairing solar cells.

https://www.instructables.com/id/Plastic-Coat/

When the solar cell is completely dry after buffing and rinsing, lay down some newspaper or some other drop cloth.

Place the solar cell on something to raise the cell off the drop cloth, and using a small paint brush, coat the cell with the clear plastic coat.

In a couple minutes the plastic coat is dry and you are ready to test the solar cell.

Now that you have refurbished the solar cells it is time to find out what you can do with them. You do this by checking the voltage and the amperage produced by the solar cell. A good sunny day is best for testing the solar cell, just a small cloud across the sun or the cell not being 90⁰ angle to the sun can affect the output.

Never check the voltage of the solar cell unloaded, that means do not just attach meter leads to the solar cells leads. Unloaded the meter misinterprets the current going through it as voltage and gives you a much higher voltage than the solar cell is producing, as in this case 10.55 volts.

Start by connecting the solar cell to a resistor, the resistor can be any size. I chose a 51Ω resistor because I wanted to use the same resistor for checking the current. Then measure the voltage across the resistor, now you get a much more accurate output voltage of 6.174 volts.

It is always good practice to never test a power source’s current without a load, dead shorts tend to be detrimental to electronics. With the 51Ω resistor attached to my circuit I got a current of 121.9 mA.

Solar cells are less affected by dead shorts; most solar cells convert less than 8% of the suns energy to electricity. If you dead short a battery the current will climb until something blows, if you dead short a power supply the current will climb until something blows.

With a solar cell if you connect the amp meter to the cell without a load, the current will climb like a battery or a power supply but the current will stop climbing once it reaches 8% of the energy of the sun. That doesn’t mean this is safe to do in all cases, just some solar cells will not be damaged by it. Without a load the current only went up by 2.1 mA to 124.0 mA.

Now you have a solar cell, a charging circuit for batteries and the basic specks of the solar cell:

6 volts

120 mA

720 mW

To charge the high capacity of a NiCad battery or battery pack it is recommended to charge the battery at the rate listed on the battery label.

To achieve a complete charge of a NiCad battery it must be charged at a rate equal to or greater than C/10. Where C = cell capacity in mAh. For example: A 1000 mAh cell requires 1000/10 or a minim 100 mA charge rate or greater. Charging at a lower rate than C/10 will not result in a completely charged battery.

http://www.powerstream.com/NiCd.htm

So this solar cell should be able to charge four, 1.2 volt, 1000 mAh batteries in series.

Powering an electric motor with a solar cell can be challenging; the startup surge current of a motor can be twice the running current, so this 720 mW solar cell may not start a 720 mW motor when the motor is connected directly to the solar cell. You would need a charged capacitor or battery between the solar cell and the motor for the added startup current.

However smaller motors like this 175 mW motor can be powered directly from the solar cell since it only needs 35 mA to run, it is less than 1/3 of the of the solar cells output.