Saving Solar

Arduino-based Solar Panel Tester

Dennis Stanley

Issue 39, October 2020

An Arduino Pro Mini based tester that estimates a solar panel’s performance quickly in the field to see if it can be saved from the rubbish dump.

When Dennis noticed discarded solar panels being sold out at his local council’s waste & recycling centre, he used his maker skills to come up with an easy solution to see if the panels still worked.

We caught up with Dennis to find out what motivated him to make his handy tester and how it works.

Thank you for sharing your solar panel tester project with us Dennis. Firstly, tell us a little about yourself?

I’m an electrical engineer. I currently run Australia’s Under Water Glider program http://anfog.ecm.uwa.edu.au/. Ocean gliders are long endurance autonomous underwater vehicles we use to acquire research data on Australia’s oceans.

Previous to that though I had a career in renewable energy working at Westwind turbines, a West Australian wind turbine manufacturer and AUST CRC for renewable energy (ACRE) where I designed wind turbine generators, wind turbine controllers and renewable energy power systems for indigenous communities.

That’s an interesting career change from wind gens to AUVs. What got you interested in electronics in the first place?

My interest in electronics started as a child. My father was an electronics enthusiast and technician. He started his early career in valve radios and worked for the motion picture branch. But he got into computers and worked his whole career in large scale computer installations. My interest started from there, pulling bits of high tech gear apart to build other things and got an engineering degree and the rest is history.

It’s a common trait for makers to pull gadgets apart to understand how things work. What inspired you to make your solar panel tester?

A lot of solar PV is going to waste in Australia. There has been a perverse outcome with the way we encourage uptake of solar through the issuing of renewable energy certificates. The long and the short of it is that if you have an existing installation you wish to expand, the only commercial solution available to you is to remove your existing system and replace it with a larger one. This is feeding our recycling industry with thousands and thousands of perfectly serviceable solar panels. While there is not a technical reason for these panels to be reused in grid connect installations there are various regulatory hurdles. So a fantastic resource is available for those who have non-grid connect applications.

The ability to test a solar panel in the field quickly and easily would help this industry greatly. People are led to believe these panels are being scrapped because they are faulty and in my experience, this is not the case. Apart from physical damage or moisture damage, both of which you can evaluate from a physical inspection, I have never seen a faulty panel. I have seen short circuit bypass diodes which makes a panel appear faulty but these can be removed (replace if desired but the panel works without them). Ok, I haven’t tested hundreds of panels and I’m sure faulty panels exist but the majority of panels being scrapped are perfectly serviceable.

A portable panel tester would enable someone buying a panel to test it prior to buying it. It would also enable a waste recycle business to quickly test each panel and label them with their current performance to give buyers confidence.

There’s a great opportunity for our readers to make their own solar powered solutions using these perfectly usable discarded panels. What types of panels does your tester work with?

The tester is designed to work with standard household mono or crystalline panels in the 150-300W range, with an open circuit voltage (Voc) up to about 70V and a short circuit current (Isc) up to about 10A. These limits essentially get down to component selection in the Darlington preregulator and current shunt and shorting MOSFET. So, it can be more or less managed by component selection. The circuit itself is pretty tolerant.

My personal view is the 60 cell panel is my preferred option. This is for a real technical reason though and that is that in a 12V charging application, a DC DC converter for a 60 cell panel would be operating at almost exactly 50% duty cycle. A two phase synchronous buck converter 180 degrees out of phase with each other would cancel out all ripple current, producing a low loss, high reliability, compact design (very low ripple current in input and output caps) but yes that’s another project.

Great! Can you go into a little more detail about testing a solar panel’s performance?

Panel performance can be evaluated by a short circuit current test and an open circuit voltage test. This essentially quantifies the end points of the panels IV performance curve. So you can evaluate a panel’s short circuit current and open circuit voltage and compare it to the factory specified value on the panels label. In this way, you can determine the current performance of the panel.

There are a few small issues though. The open circuit voltage is temperature dependent and the short circuit current is dependent on the level of illumination. In short, the factory specified performance is obtained under reference conditions that may be hard to replicate at the dump.

Dealing with the temperature issue is easy. The level of voltage change is pretty small and an ambient temperature correction could be applied easily.

The level of illumination problem is solved by having a “reference cell”. In my case, I used a salvaged panel from a discarded solar garden light which is used to measure the ambient illumination and normalise the measured short circuit current back to reference conditions. The corrections are explained in a paper called "Effect of Illumination Intensity on Solar Cells Parameters" (M.Chegaar et al 2013).

Does it matter if the panel you are testing has a solar charge regulator onboard?

The tester does not involve the charge regulator and is designed to be connected to the panel wires directly and powered by the solar panel under test.

Early prototype.

Can you please explain the circuitry involved in your project?

I’ve designed and built a prototype of this project so it would be very cheap and able to be assembled easily by hobbyists.

Technically, there is a Zener Darlington series preregular that supplies 20V to a 1000µf electrolytic from a 25-100V input. This allows a 78L05 to be safe and acts as an energy store to keep the micro and display going while we short out the panel to test the short circuit current.

The panel sensing conditioning electronics consist of a resistive voltage divider to measure open circuit voltage (not strictly open circuit as the tester is powered by the panel).

A logic level switching MOSFET momentarily short circuits the panel. I’ve used an integrated hall effect current sensor, but a resistive shunt would be fine to convert the current into a voltage for the micro to sense. We only short the panel for about 10ms every second or two so a shunt needn’t be very high power.

The rest of the prototype uses an Arduino Pro Mini and a 16*2 LCD. For a hobbyist version, I’d suggest an Arduino Uno. That way, a standard 16*2 LCD pushbutton shield could be used and the solar panel tester bit just reduces to a piggyback shield between the Arduino and the display shield. What I like about this approach is that hobbyists are often skint and in this way, the expensive resources of an Arduino Uno and display shield are only utilised when you want to test solar panels. After that they can be repurposed in another project if one wishes.

Being able to repurpose the Arduino when the tester isn’t required is a great idea. Why did you choose the Pro Mini for your tester?

An Arduino pro mini was used because it’s cheap and you get to all the pins with most versions. I’m a bit torn in a hobbyist version to use a pro mini or make it an Arduino Uno shield. I think the design will have a longer life choosing the Arduino Uno as these are so standard now. I believe this Uno design will be around for a long time where the pro mini is already being superseded with the Arduino Nano (which has native USB support). While the Arduino Uno is more expensive it is reusable for the hobbyist to use for other things when not testing solar panels.

Did you code it yourself, and how does it work?

I have provided the code for your readers to download from your website. The code is fairly well commented if it’s not self explanatory, but I can explain some more.

Essentially, it starts up, sets up all the ports, displays a welcome message on the screen, and outputs some text to the serial port for debugging.

We get into the main loop.

We read the voltage from the reference panel current shunt.

Then, we read the open circuit voltage from the resistive voltage divider. I use a kind of averaging here when I actually sum 16 successive ADC readings (the sixteen readings are coded as one initial one then a for loop of 15 more). Instead of then dividing by 16 to get an average ADC reading, the divide by 16 value is built into the calibration value to go from (ADC x 16) to volts.

The current value is read. The hall effect current sensor is bidirectional giving out full scale /2 (i.e. 2.5 volt) volts for a 0 current reading. Negative currents read down from 2.5 and positive currents read up from 2.5. I wired the current sensor in what you may consider reverse polarity so that the short circuit current measured lowered the measured value from the 2.5V of zero current down toward 0V. This is to maximise the resolution of the ADC rather than having the first 2.5V of resolution wasted on negative values I was never going to generate.

So measuring the output of the current sense device is a two stage process. First, without the panel shorted (so no current flowing) sum 16 measurements to get the offset (or zero value), then short the panel (do a few bogus reads to waste some time while the reading settles) then sum 16 measurements to get the short circuit current ADC reading.

Then display raw values, apply the corrections for illumination, and display estimated specification parameters.

Loop back, do it again.

Great. Thanks for the detailed explanation. What were the main build challenges you needed to overcome?

The biggest challenge was the power supply because of the required high input voltage range, and the fact that the device had to short its own power supply but stay running and accurately sense the short circuit current.

That power challenge isn’t something that most makers face. If you were to start over, would you do anything differently?

I have started over in refining the design, designed a PCB, had it fabricated.

The simplifications include using a resistive shunt rather than the current sense IC and using an integrated Darlington transistor rather than two discrete transistors wired as a Darlington pair. For a hobbyist version in your mag I’d suggest using a resistive shunt but leave the discrete transistor arrangement from the prototype. The discrete arrangement has a higher component count but uses transistors you can easily find locally.

Good advice. We’ll include the schematic in the mag for anyone wanting to build something similar. Is there anything our readers should know that we haven't covered already?

Yes. A further note on my experience using this tester, and it has to do with the presence of bypass diodes.

Not all panels have bypass diodes but those that do can cause issues with the testing by making a good panel look faulty or a bad panel look good.

The very function of bypass diodes is to allow the rest of a functioning panel to still deliver power if part of the panel is suffering from low output. In normal operation, this low output is because of partial shading of a panel but as far as the bypass diodes are concerned a corroded or damaged cell looks just the same as a shaded one.

If the panel has bypass diodes installed, the short circuit test may still read like a good panel if the unbypassed section of the panel is still functioning ok.

Bypass diodes can fail short circuit which will make a good panel look faulty. Shorted diodes cause the panel’s open circuit voltage to read low even though the panel itself is undamaged.

So in summary to be 100% sure disconnect the bypass diodes before testing. As a note this advice isn’t just specific to this tester even a manual open circuit – short circuit test will be effected in this way.

Also, a visual inspection of the panel is still important. A common failure is the delamination of the plastic backing material which lets moisture in.

Great! We hope we’ve inspired our readers to make their own tester and to start repurposing solar panels as you have. Finally, what are you working on now?

Currently, I’m trying to find a way to prevent coral bleaching on valuable sites on the Great Barrier Reef. Data I’ve collected through my work shows that the thermal stress seen by coral during bleaching events is confined to the upper water column and is brought about by the failure of normal upper ocean mixing associated with extended periods of low winds, and as such may be relatively easy to resolve by temporarily supplementing those natural mixing processes. This site summarises some of my findings. https://www.rrrc.org.au/reef-havens/. Check out the Myth Busters Style sea trial video to see model aircraft parts, a quadcopter battery, 2 oil drums and 20m of ventilation duct in action delivering cool water to the surface.

Amazing! Your electronics knowledge getting put to great use obviously. We admire what you are doing Dennis, and wish you all the best with it.

Dennis Stanley

Dennis Stanley

Electrical Engineer, Maker, and Ocean Glider Operator.