Very low open circuit voltage on some panels compared to others (4-9v instead of 30-40v)

[Bo295]

Hope everybody is doing well.

I have recently picked up some used abound solar ab1-67b panels and two out of four of them have a really low open circuit voltage (4-9v instead of 30-40v). The weather here is currently quite bad but even with this limited sunshine I wouldn't expect such big difference in voltage.

There are no visible cracks, discolorations / disconnected bondwires as far as I can tell, but they are CdTe based panels instead of the more standard silicon versions.

Luckely the seller is a good guy and I can trade them in for tested working ones next week.

But this has me curious what could cause this kind of problem. My google searches always pointed back to shading / visible damage but this does not seem to be the case.

If anyone knows what could be going on, please enlighten me Video underneath for reference.
And please excuse me for any errors, English is not my native language

 

[FilterGuy]

It takes very little light to generate a large Voc on a solar panel. If I had to guess, I would say the two panels with low Voc have an internal short.

 

[Vigo]

Glad you are able to swap them out! Im not sure of the mechanism causing the symptom but i do agree that its more likely ‘loading down’ to 4v due to a short, than simply not making trhe voltage at all. ?‍

 

[Quattrohead]

Open circuit diodes

 

[Bo295]

It takes very little light to generate a large Voc on a solar panel. If I had to guess, I would say the two panels with low Voc have an internal short.

Okay, learned something new, thanks!

 

[Bo295]

Glad you are able to swap them out! Im not sure of the mechanism causing the symptom but i do agree that its more likely ‘loading down’ to 4v due to a short, than simply not making trhe voltage at all. ?‍

Yes that would indeed make sense! Forgot to mention this but the short circuit current remained about the same, wich I think might confirm the theorie of series strings being shorted.

 

[Bo295]

Open circuit diodes

Thanks but I doubt it since the panel only has bypass diode(s) and no blocking diode. If the bypass diodes (in parallel with each series string) are open circuit they shouldn't have any influence on the voltage as far as I know. open circuit blocking diode would be problematic.

The junction box is completely potted with resin so unfortunately I can't do some measurements. But I don't think series blocking diodes are really implemented in panels themselves, only bypass diodes