Solar panel diode tests

SolarShed

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Jul 20, 2021
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Hi, I wanted to do a solar panel diode test, not because I have a problem, but just because I wanted to see the health of my panels.

So I have 4x100 watt panels mounted on on a roof. The panels have the same specs, and are the same

Vmp = 18V
Imp = 5.5A
Voc = 21.85V
Short Circuit = 6.03A

but they vary in age from 1 to 4 years old.

I know the proper way to do this is to measure across each diode and then sum them up in the junction box, but its a bit of pain to remove the panels from the roof (they are not on a rack), so I basically just disconnected my panel from the fuse box and measured the two wire leads positive and negative of each of the 4 panels.

I put cardboard on the panels to block the sun, or most of it.

The good news is, when I do the test, my meter registers only on way traffic (OL) when positive on positive lead, and negative on negative leads. So when I reversed the leads, I did get some numbers, but they were slightly different,

2 were reading ~1.0 V, one was! 0.9V and one was 0.65V. At this point, I noticed that the two panels that were ~1.0 V, they were in the shade, whereas the 2 other panels, were in the sun, and the carboard over it was mostly blocking the light, but not all of it. The wires to the panels vary in length and gauge.

So, I was expecting the numbers all to be similar, but they weren't.

Just wondering if this is a sign that there could be some issue with the diodes or is it because I wasn't blocking the sun completely, or the wires leading to panels being different.

Thanks
 

Bud Martin

Photon Sorcerer
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Aug 27, 2020
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1. Did you set the meter in diode mode test?
2. The Cathode of bypass diode is connected to the Positive terminal of the PV panel, the Anode of bypass diode is connected to the Negative terminal of the PV panel, so to test the bypass diode, if it only has one diode, you will cover up the panel completely then you put Red probe on the Negative (Anode of the bypass diode) terminal, Black probe on the Positive terminal of the panel to read the Forward Voltage of the bypass diode, if the panel has more than 1 diode in series your meter will not likely to be able to forward bias two diode for you to read the Forward Voltage of the two diode in series. Notes, if you do not cover the panel completely, you may damage your meter during diode test because the PV panel Voltage will be present to the meter input.
3. Did you do the shorts current test?

 

SolarShed

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Jul 20, 2021
Messages
120
Hi thanks for your reply. I'll answer your questions as best as I can and I'll dig into that link that you provided.

1. Yes.
2. It has two diodes. Here's a photo of the junction box before I mounted it. Oops it was covered mostly, perhaps 98%. Hopefully the meter, UNI-T pro clamp meter is OK.
3. This I did not do? How do I measure the short current?
 

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Bud Martin

Photon Sorcerer
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If you do have clamp-on DC Amp meter then all you have to do is to cover the panel first then connect two connectors (MC4) of the panel together,put the clamp-on to the wire and remove the cover, you should then see the current reading on your meter when panel is exposed to the sun light, when you are done, cover the panel before disconnecting the connectors.
 

SolarShed

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Jul 20, 2021
Messages
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thanks, this is no problem, except, if the sun is not ideal would it not give me a value much less than the rated value? It not easy unmount my panels.

My original question is, it seems the diodes are doing there job in the sense that they are only allowing voltage to flow one way, but my why are the values different, and could some partial sun leaking onto the panel, have an affect on the values?
 

RCinFLA

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Jun 21, 2020
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PV cells have leakage. Some can be quite low, more than your DVM will put out in current during diode testing. It could be as low as couple hundred ohms for PV cells leakage across one bypass diode. Typical DVM pushes 1 mA for diode test.

You have either to disconnect one end of diode, or use a current limited power supply pushing about 100 mA through diode with power supply CV set to 5vdc. One way should be the normal forward biased diode of about 0.65v, Reversing power supply you should see close to the 5v.

Another way is to use a 18650 LiPo cell with 33 ohm resistor in series. Measure voltage across bypass diode with battery and series 33 ohm resistor put across diode, both directions. If diode is good, one way close to LiPo voltage, other way close to 0.65v diode drop.

This all assumes you don't have light on panel.
 
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SolarShed

Solar Enthusiast
Joined
Jul 20, 2021
Messages
120
thanks all. I guess there is no easy way to do this unless I unmount my panels, or, wait until the summer sun and measure the Isc on dead-center sun.

Ok, thank you again all I've bookmarked this thread.
 
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