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Used/aged solar panels calculation

smalenfant

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Feb 27, 2022
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I bought some used cheap panels just to get myself introduced to solar.

- Panels were rated 140W, they produce merely 90W in full sun (on a cool day)
- Panels VoC was 78.8V, but measures about 72V now

Can use the lower voltage in my calculation to put them in series or not?

They don't look so good and cheap anymore, but still want to put them to good use.
 
I bought some used cheap panels just to get myself introduced to solar.

- Panels were rated 140W, they produce merely 90W in full sun (on a cool day)

Tested how?

If you were getting less than 1000W/m^2 on the panel (high noon, summer solstice, panel perfectly perpendicular to sun) or cell temp was higher than 25°C, you'll never get rated.

If battery was in absorption, then it wouldn't take anymore than 90W even if available. Add loads > rated power to force more performance if it's available.

What did the Isc measure vs. rated?

- Panels VoC was 78.8V, but measures about 72V now

Can use the lower voltage in my calculation to put them in series or not?

So, Voc measured is 72V vs. rated? If so, yes, but you still want to allow yourself sufficient cold temp margin
 
Thanks for replying. And of course, testing to the best of my knowledge at this time.

Here's the spec which I believe is what is important:
  • Imp 2.34A
  • Vmp 59.8V
  • Isc 2.65A (Not measured)
  • Voc 78.8V (measured 72.2v)
Specs: http://www.solardesigntool.com/comp...on/2364/STO-140/specification-data-sheet.html

I measured VoC with a multimeter. But I'm not sure how accurate that is.

I've tested the panels on 2 MPPT (Victron 150/35) and the output was set to 20A (12V and also 24V battery). The bulk mode was displayed and with half-empty battery.
At first, I was wondering why they would not produce more ~63w (12v, ~126w with 24v) but figured out the output charging Amp was set to 5A (default).

I'm asking those questions because I can't find this information anywhere else. It's important since I would like to connect 2 of those panels in series which the rated Voc is above the 150V allowed on the Victron. I don't think I want to connect 8 of them in parallel. Now we are talking about combiners or fuses. It that's getting a little expensive (more than what the panels are worth).

It seems like I can hit the Vmp number (according to Victron) but not Isc.

I would love to connect 2 of the panels in series (which I did yesterday but I'm not supposed to). V Max was 128.83v and P max was 170w.
 
Thanks for replying. And of course, testing to the best of my knowledge at this time.

I measured VoC with a multimeter. But I'm not sure how accurate that is.

If you're not confident in the meter's accuracy, I would only use it for reference.

I've tested the panels on 2 MPPT (Victron 150/35) and the output was set to 20A (12V and also 24V battery). The bulk mode was displayed and with half-empty battery.

In bulk, it should have delivered whatever it can supply, so good on that.

I'm asking those questions because I can't find this information anywhere else. It's important since I would like to connect 2 of those panels in series which the rated Voc is above the 150V allowed on the Victron. I don't think I want to connect 8 of them in parallel. Now we are talking about combiners or fuses. It that's getting a little expensive (more than what the panels are worth).

Don't risk it. if you're charging a 12 or 24V system, there's no reason for 2S unless you're making a very long PV cable run. MPPT actually get less efficient the greater the difference between PV and battery voltage... not a lot, and this is usually offset by reduced wiring losses.

It seems like I can hit the Vmp number (according to Victron) but not Isc.

I would love to connect 2 of the panels in series (which I did yesterday but I'm not supposed to). V Max was 128.83v and P max was 170w.

128.83V max Voc or max observed in operation?

If your multimeter has a 10A ammeter mode, you can test Isc.

I would test Voc and Isc on both panels individually on a perfectly clear day at high noon with the panels perfectly perpendicular to the sun.
 
Use the stock PV parameters. You can't measure the exact parameters outside a laboratory and you can't get the exact aging impact.

Old panels IIRC have a drop in both the voltage and the current. So if you go theoretically you can estimate some values. But don't measure them in field conditions.
 
If you're not confident in the meter's accuracy, I would only use it for reference.



In bulk, it should have delivered whatever it can supply, so good on that.



Don't risk it. if you're charging a 12 or 24V system, there's no reason for 2S unless you're making a very long PV cable run. MPPT actually get less efficient the greater the difference between PV and battery voltage... not a lot, and this is usually offset by reduced wiring losses.



128.83V max Voc or max observed in operation?
No, that was the P Max seen by Victron. 2 panels in series. The measured Voc (with multimeter) in full sun was 140v.
If your multimeter has a 10A ammeter mode, you can test Isc.
My multimeter fuse is blown. I guess that's why all my amp measurement are not working.
I would test Voc and Isc on both panels individually on a perfectly clear day at high noon with the panels perfectly perpendicular to the sun.
I've heard how to do that. Need a beer can.

Thanks!
 
Use the stock PV parameters. You can't measure the exact parameters outside a laboratory and you can't get the exact aging impact.

Old panels IIRC have a drop in both the voltage and the current. So if you go theoretically you can estimate some values. But don't measure them in field conditions.
Part of me didn't want to risk my solar charger. At the same time, just wanted to know.
 
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