Author Topic: Solar Panel Voc and Isc  (Read 8873 times)

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Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #25 on: November 03, 2023, 12:21:28 am »
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If that reduction in current were due to a bad connection, that connection would quite quickly be glowing red hot as it would be dissipating ~150W.  Also, where could there be a bad connection external to the panel if you have it shorted with the meter?
Isc was measured with a clamp-on meter (not inline).
 

Offline bdunham7

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Re: Solar Panel Voc and Isc
« Reply #26 on: November 03, 2023, 02:16:53 am »
OK, that's fine, but they just shorted the two panel connectors together, right?  So there's no other connectors worry about?  If a panel was capable of 9.9A into a real short but was only putting out 6A due to a bad connection, that connection would get very hot in short order.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline ejeffrey

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Re: Solar Panel Voc and Isc
« Reply #27 on: November 03, 2023, 02:42:13 am »
That's really weird.  It definitely looks like the panels with 6 and 7.2 amps are not working properly but the other two with 9-10 amps seem OK yet all of your panels are showing abnormally low power? 
 

Offline bdunham7

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Re: Solar Panel Voc and Isc
« Reply #28 on: November 03, 2023, 03:00:14 am »
That's really weird.  It definitely looks like the panels with 6 and 7.2 amps are not working properly but the other two with 9-10 amps seem OK yet all of your panels are showing abnormally low power?

Actually it isn't so weird if you think about it.  Short-circuit current may not be all that useful in finding partial panel failures.  Say you had a the panel that has 3 bypass diodes (typical) for 3 subsections and two of those subsections had one cell with bad busbars that limited its current output to 5 amps.  What do you get when you short the panel?  I think you get the full rated short circuit current or very close to it since you only need enough additional voltage drop to forward bias the 2 bypass diodes, so perhaps 2 volts. All you need is one good subsection to drive full current through that. If you look at a typical MPPT curve, you'll see that the current is fairly constant over a wide voltage range from zero to MPPT.   

« Last Edit: November 03, 2023, 03:47:25 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline floobydust

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Re: Solar Panel Voc and Isc
« Reply #29 on: November 03, 2023, 05:22:45 am »
Again, I think a thermal imaging camera can solve the puzzle.
Usually I see cracked cells (1st reason) or corroded interconnects (2nd reason, crappy not even well soldered inside) causing weak output.
At the panel level, any bad connection quickly melts and burns up due to the higher current and dissipation.
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #30 on: November 06, 2023, 03:16:19 pm »
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I suggest to borrow/rent an IR thermal imaging camera and inspect the array when it's operating.
Are either of these cameras acceptable?

https://www.harborfreight.com/thermal-camerainfrared-digital-imaging-thermometer-58111.html

https://www.homedepot.com/p/VEVOR-Thermal-Imaging-Camera-240x180-IR-Resolution-43200-Pixel-Infrared-Camera-with-16G-SD-Card-LED-Light-Minus-4-F-to-662-F-FYHWRXYWWWIFINPTKV0/325835409

Should the inspection be performed several inches from the panels or several feet?
 

Offline f4eru

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Re: Solar Panel Voc and Isc
« Reply #31 on: November 08, 2023, 09:28:12 pm »
Do thermal cameras even see anything on panels ?
I inspected thermal solar panels once, and all I could see was the reflection of the sky in the glass.

Offline ejeffrey

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Re: Solar Panel Voc and Isc
« Reply #32 on: November 08, 2023, 09:42:54 pm »
They probably won't see much from the silicon itself but if an interconnect is heating up locally due to bad contact that can be apparent.
 

Online mzzj

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Re: Solar Panel Voc and Isc
« Reply #33 on: November 09, 2023, 08:35:24 pm »
Do thermal cameras even see anything on panels ?
I inspected thermal solar panels once, and all I could see was the reflection of the sky in the glass.
Haven't tested myself but supposedly it works based on examples I have seen.
https://westerninfrared.com/problems/solar-panels/
https://www.irpod.net/en/news/fault-diagnosis-of-photovoltaic-systems-using-infrared-thermal-imaging-cameras/

If the panels don't have any faulty spots then you won't see any differences and what you see is mostly "low quality" mirror surface.
 

Online nctnico

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Re: Solar Panel Voc and Isc
« Reply #34 on: November 09, 2023, 09:24:22 pm »
Do thermal cameras even see anything on panels ?
I inspected thermal solar panels once, and all I could see was the reflection of the sky in the glass.
AFAIK it will greatly depend on the angle and how reflective the panels are. In theory solar panels shouldn't be super reflective so a thermal camera should pickup the hot spots.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #35 on: November 23, 2023, 12:20:05 am »
I used a thermal imaging camera to scan 2 of the panels (the best and worst performing panels).I divided each panel into 9 areas so there a 9 scans per panel.

After looking at the images I did not find anything remarkable. There weren't any hotspots on the cell borders (or anywhere else, for that matter). But then, I have no experience looking at thermal images so others may see something that I missed. Others may also recommend that I make additional scans (closer, different angle, or something else).

Attached are images from the worst performing panel (the best performer will be in my next post – limited to 10 images per post).

« Last Edit: November 23, 2023, 12:23:02 am by ee753 »
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #36 on: November 23, 2023, 12:20:53 am »
Here are the images for the best performing panel.

 

Offline floobydust

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Re: Solar Panel Voc and Isc
« Reply #37 on: November 28, 2023, 06:51:33 pm »
I think looking for small temperature differences, oddball spots is most important.

Interconnects get hotter if their resistance goes up. Shorted blocking diodes heat up, so that is easier to find.

But solar cells that are putting out more current... run cooler. It's counter intuitive.
About 2-5°C cooler as the incoming energy gets put out into electricity- instead of dissipating in the cell that is doing nothing.
So look for the slightly hotter objects and see if that is a pattern. Cells along the panel outside perimeter get better cooling and are naturally cooler.

pic '708 best' has one cell hot top left? One cell would make that string of say the three, weaker.

The only other way to troubleshoot arrays is treating them like LED's and inject say 3A in to a panel at night to get them to emit infra-red light and again look for inconsistencies.
That one guy is injecting 400VDC 3A to do that!
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #38 on: November 28, 2023, 07:28:25 pm »
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pic '708 best' has one cell hot top left? One cell would make that string of say the three, weaker.
It looks like 708 (1) mid-left, (2) mid-right, (3) upper-left, (4) upper-mid all have cells that are running hot. From what you are saying these 4 hot cells could be reducing the output. That makes sense.

But I don't see any similar hot cells on panel 809 which had the worst current output of all my panels. That is puzzling.

I am still suspicious of the MC4 connectors, also. I found that the connectors were installed at the panel and inverter factories (not by the installers). That implies that the proper crimping tools were used and the connections should be good. If they were attached by the installers using pliers rather than a crimper the joint would be highly suspect. But that is not the case. I will inspect the connectors if the installer lifts the panels. I should be able to determine whether a crimper or pliers was used.

If my calculations are correct, a 1 ohm resistance in the connector would drop the current from 8.5A to 6.5A. The power dissipated in the connector would be about 40W. That would make the connector quite warm, but not blisteringly hot. When the installers lift the panels I will measure the temperature of the connectors with the thermal imager. With a near-zero resistance I expect their temps to be about the same as the shaded roof temperature. If the temperature is significantly above that the connections may be an issue.

The microinverters have been eliminated as the culprit because their efficiencies are all above 95% and unchanged from Day 1. Currently waiting for the panel manufacturer (LG) to get back to the installer with further instructions.
« Last Edit: November 28, 2023, 07:34:56 pm by ee753 »
 

Online nctnico

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Re: Solar Panel Voc and Isc
« Reply #39 on: November 28, 2023, 11:03:25 pm »
The microinverters have been eliminated as the culprit because their efficiencies are all above 95% and unchanged from Day 1. Currently waiting for the panel manufacturer (LG) to get back to the installer with further instructions.
Have you tried to swap the microinverters between the panels? Microinverter efficiency says nothing if they are somehow broken and fail to do MPPT properly.

About the thermal images: you'll be better off making an image from the entire array and look for hot spots. From your images it looks like you also caught a lot of reflection causing too high/too low temperatures. Using a thermal camera on a glossy surface is not a good idea, especially if you want to make comparisons between images.
« Last Edit: November 28, 2023, 11:07:10 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #40 on: January 30, 2024, 05:55:17 pm »
Good news!

After Enphase agreed that the microinverters were functioning properly and after providing data to LG they will be replacing all of my panels. Before receiving LG's response I checked the inverter-to-panel connectors and found them to be at ambient temperature proving that they were not the problem.

LG will not reclaim my original panels. So what do I do with them? Buy a few more inverters and put them on the roof? (Already over-producing so I don't need them unless I purchase an EV.) Sell them at a VERY REDUCED price on eBay as "functioning, but not at 100%"?
 
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Online nctnico

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Re: Solar Panel Voc and Isc
« Reply #41 on: January 30, 2024, 08:22:23 pm »
Nice that LG respects the warranty even though they pulled out of the solar panel business. Maybe you'll get panels from a different brand.

FYI: It looks like more people have problems with LG panels:
https://www.classaction.org/lg-solar-panel-problems-lawsuit
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #42 on: January 31, 2024, 05:42:14 pm »
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FYI: It looks like more people have problems with LG panels:
https://www.classaction.org/lg-solar-panel-problems-lawsuit
Yes, I am aware of that. Clearly I am not the only one. But most people are not geeks like me who analyze the data monthly. That's the only way that I found the problem. There are probably hundreds or thousands of homeowners who have the same problem but don't know it.

Unfortunately lawyers will get involved through classaction.org which will cost everyone more and the owners will get pennies from the class action suit. Better to provide LG with hard data as I did. They have already shipped a complete new set of panels to me and are making me good. They told me to keep the original panels. Not sure what I will do with them. Maybe a recycler will pay me for them. Otherwise, the installer will cart them away.
« Last Edit: January 31, 2024, 05:45:14 pm by ee753 »
 

Offline boB

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Re: Solar Panel Voc and Isc
« Reply #43 on: January 31, 2024, 08:44:16 pm »

Since power output goes down with cell temperature,  maybe it is progressively warmer now than it has been in past years ?

K7IQ
 

Online mzzj

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Re: Solar Panel Voc and Isc
« Reply #44 on: February 10, 2024, 06:38:24 pm »
They told me to keep the original panels. Not sure what I will do with them. Maybe a recycler will pay me for them. Otherwise, the installer will cart them away.
Based on diysolarforum.com there is  active market in US even for partially broken & used solar panels. SanTan solar is even doing business with them but they are probably not interested in small quantities.
https://www.santansolar.com/product-category/solar-panels/used-solar-panels/

List at 100usd/kWh to Graigslist or something like that?
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #45 on: March 03, 2024, 04:39:09 pm »
Good news!

LG replaced all of my 280W panels with 315W panels under warranty. After the originals were removed I placed each in full sunlight (no clouds or other obstructions), pointed them directly at the Sun and measured the open circuit voltage (Voc) and short circuit current (Isc). After correcting for solar irradiance for my location and date I found that all of the panels were within 5% of the nominal specification. Obviously that was a big surprise. I didn’t want to interfere with the installers so I did not measure the new panels.

However, when the new panels were in full sunlight (no clouds or other obstructions) the maximum 15-minute output for the day was 878 W-Hr. Several days earlier under the very similar conditions the original panels output was 616 W-Hr. That is a 42.5% increase in production. Changing from 280W to 315 W panels accounts for a 12.5% increase. That leaves an additional 30% increase. That just happens to be the degradation that I measured on the original panels. So clearly the original panels have significantly degraded
But if they degraded, why is Voc and Isc still within specifications? What other factors affect output?
 

Online nctnico

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Re: Solar Panel Voc and Isc
« Reply #46 on: March 03, 2024, 04:48:08 pm »
That just happens to be the degradation that I measured on the original panels. So clearly the original panels have significantly degraded
But if they degraded, why is Voc and Isc still within specifications? What other factors affect output?
As noted before, measuring a panel at short-circuit is not an operating condition so the result tells you very little. You should have used a DC load and find the maximum power point.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline bdunham7

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Re: Solar Panel Voc and Isc
« Reply #47 on: March 03, 2024, 04:54:10 pm »
If you have one section (typically a third) that has a cell that is cracked or something and it therefore has reduced current carrying capacity, the panel will still produce full voltage when open circuit.  And if you short it, the bypass diodes will allow the full current from the other two sections to flow through unimpeded.  You'll only notice the problem when you try to extract power from the panel and when you do, you'll find that the MPPT voltage will be about 2/3 of what it should be because one section is bypassed.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline ee753Topic starter

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Re: Solar Panel Voc and Isc
« Reply #48 on: March 03, 2024, 07:13:42 pm »
Now it is starting to make sense. bdunham7's posted a typical MPPT curve on Nov 3, 2023. When Isc is measured V = 0 so there is no power generated. What is important is the MPP (not Isc). In my case the MPPT had probably decreased. Why did both Enphase and LG ask for Voc and Isc? That was worthless information and mislead me. It seems that every installer should have an MPPT tester.

How can I measure MPP without an MPPT tester? I would need a variable load of about 0-5 ohms @ 500+ watts. It looks like an MPPT tester might be the only practical way to measure it.

BTW - My output degraded on every single panel between 8-15% during year 6. They all decreased another 8-15% during year 7. I wonder if they would have continued to degrade at that rate and eventually reach 0 output? Why would they all start losing cells and continue to do so?
« Last Edit: March 03, 2024, 07:16:02 pm by ee753 »
 

Offline uer166

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Re: Solar Panel Voc and Isc
« Reply #49 on: March 03, 2024, 09:10:20 pm »
If you have an E-load with constant voltage mode, you can sweep Vin and measure power at each setpoint. That's how MPPT controllers work more or less.

If course a dedicated MPPT tester would likely be cheaper, so this only makes sense of you have or can borrow an E-load.
 


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