buying second hand pv, what to look out for?


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Redpacket

Active member
Joined
Feb 28, 2018
Messages
1,416
I am doing something wrong, or they are beyond 20 amps at 41 volt in direct sunlight.
Sun was going down with the v numbers also...sun is setting to night View attachment 27626View attachment 27627View attachment 27628View attachment 27629View attachment 27630
where to put my leads to get the A, clearly i did it right??but i forgot how to?
End of this week they will be on my roof fially, but what are they cable off?
And how to measure properly, again: now i am doing something clearly wrong.

With best regards Igor K
Yes that is correct.
Leads connect to same place on panel for Isc & Voc measurement.
Voc = 43V like it shows in pic.
Your panel is 100W so panel will make Isc maybe approx 3A (leads in 20A, DC20A setting) - no way it will make more than 20A. Even a new 400W panel only makes about 11-12A @36V.
Panel must be in full sun, no shade even on edges. Must be 90 deg to sun, not angled much.
Suggest wiping panel clean too, those spots might take 10-20% off power.

Re cracked panel, it's no good, throw in rubbish. Air & moisture will destroy the silicon & corrode the wires.
If you are lucky, maybe a few cells are not damaged, you could silicone (neutral cure!) to new glass & use for small project.
 
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italianuser

Member
Joined
Feb 25, 2020
Messages
505
To measure Amp you must move the red cable on the multimeter to the 20A port (the hole on the far left of your DMM), and leave the black cable where it is. And set the measurement to DCA 20.

DMM_ampere_measurement.jpg


To measure Volts again you put the red cable back into the V port (the hole on the far right of your DMM, where it is in the photo).

The writing on your DMM says 20A unfused. Maybe I'm a bit paranoid but I'd quickly solder a 15A fuse on two pieces of wire to do the Amp test, just to avoid the DMM breaking for any reason.

Panel+ --> Fuse --> DMM red cable
and
Panel- --> DMM black cable

If you invert red and black cables of the DMM it's the same, because the current goes round in a circle.

[ I never used the method @Korishan suggests to measure "dead short" amperage of a panel but I suppose it works ]
 

Redpacket

Active member
Joined
Feb 28, 2018
Messages
1,416
To measure Amp you must move the red cable on the multimeter to the 20A port (the hole on the far left of your DMM), and leave the black cable where it is. And set the measurement to DCA 20.


To measure Volts again you put the red cable back into the V port (the hole on the far right of your DMM, where it is in the photo).

The writing on your DMM says 20A unfused. Maybe I'm a bit paranoid but I'd quickly solder a 15A fuse on two pieces of wire to do the Amp test, just to avoid the DMM breaking for any reason.

Panel+ --> Fuse --> DMM red cable
and
Panel- --> DMM black cable

If you invert red and black cables of the DMM it's the same, because the current goes round in a circle.

[ I never used the method @Korishan suggests to measure "dead short" amperage of a panel but I suppose it works ]
If you are not getting a reading on DC current 20A when leads are correct, could it be that the meter does have a fuse for DC current but not for AC current/voltage (see red vs black outlines around sections DC A, AC A, etc)??
I am doing something wrong, or they are beyond 20 amps at 41 volt in direct sunlight.
Sun was going down with the v numbers also...sun is setting to night
where to put my leads to get the A, clearly i did it right??but i forgot how to?
End of this week they will be on my roof fially, but what are they cable off?
And how to measure properly, again: now i am doing something clearly wrong.

With best regards Igor K
This is the correct place for the leads but red & black other way.

leads.jpg
 

ajw22

Member
Joined
Nov 16, 2018
Messages
681
Just a quick 101 on the basic construction of a solar cell/panel. Hope it helps.

Below is a closeup of one solar cell.
Voc (Voltage Open Circuit, ie no load connected) is generally around 0.6Volt, affected somewhat by temperature, a little by light intensity, but definitely not size. Current output depends mostly on the size, and light intensity.
The top(front) side is negative, bottom(back) side positive. The width and number of conductors are a compromise between maximizing current carrying capability and minimizing the blocking of light that generates said electrical current.
1655953401728.png


Looking closely at the interconnecting conductors going above / under each cell, one can see that all the cells are connected in series.
There are 24 cells in series between terminals (6) and (5), so the expected Voc is 24 cells * 0.6V = 14.4V. Same between (5) and (1), and also between (1) and (0). So between (6) and (0), the total voltage should be 72 cells * 0.6V = 43.2V.
Each of the 3 diodes allow the panel to bypass sections of the shaded/damaged panel. Hypothetically, if the Red loop were shaded or somehow cut, that diode would bypass the red loop, and the overall panel voltage would be reduced to around 48 * 0.6V = 28.8V.

1655952722511.png


1655952570092.png
 
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slimf

Member
Joined
Sep 26, 2017
Messages
219
To test the short circuit current (Isc) of a panel, You literally short circuit the positive and negative leads - using your digital multimeter in current mode - just connect one lead to positive and one to negative. This is a dead short (minus the tiny shunt resistor in the dmm)

Point panel at sun and view the current. It should get close to the Isc rating of panel.

If you have a DC capable clamp meter, the test is even easier. Connect the positive and negative leads on the panel together, put said clamp meter around them and point the panel at the sun.

When solar systems are installed - even in large strings, a short circuit current test as above is performed before the systems are provisioned.

Second - Insulation test.

Testing the negative or positive lead to the panels frame ( to test its insulation to the frame) with a regular DMM is an almost useless test.

The resistance to the frame will measure open circuit. However a panel with faulty insulation will fail when the panel is in a high voltage string. To test insulation resistance you need an insulation tester - this will send 1000v + through ONE of the leads on the panel and it’s frame. If there is a fault in the insulation this test should help detect the leak.

I put a bit of info about this here: https://powerwall.fergo.net/2021/09/12/adding-more-dc-solar-part-3-testing-second-hand-panels/
 
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100kwh-hunter

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Joined
Mar 2, 2019
Messages
805
@ajw22 and @slimf very useful info, thank you very much it is really appreciated, thank you again.

Will apply (and did) will let you know the outcome.

with best regards Igor K
 
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