Used solar panels

Made this video on getting used solar panels. I'd love any feed back. Thank you.

You demonstrated testing Isc by mating MC4 connectors with full sun on panel. That would arc. Suggest covering panel or tilting it away from sun while making/breaking connection.

Make (or buy) a non-latching pigtail to banana cable set before going to do the test with standard ammeter (of sufficient amperage), or shorting cable for clamp meter.

Voc & Isc do NOT always detect bad panels. In my testing of 5 bad panels, 2 of them were in family for those tests but delivered lower Imp or rather I(load) which I measured with a resistor.

For a small lot of used panels, you might take that risk, but for large quantity you might want to buy a PV panel tester.

@rloveless Please keep in mind that your post can be considered self-promotion and is against the rules. I'll leave it up this time, but keep that rule in mind for the future.

Hedges said:

You demonstrated testing Isc by mating MC4 connectors with full sun on panel. That would arc. Suggest covering panel or tilting it away from sun while making/breaking connection.

Make (or buy) a non-latching pigtail to banana cable set before going to do the test with standard ammeter (of sufficient amperage), or shorting cable for clamp meter.

Voc & Isc do NOT always detect bad panels. In my testing of 5 bad panels, 2 of them were in family for those tests but delivered lower Imp or rather I(load) which I measured with a resistor.

For a small lot of used panels, you might take that risk, but for large quantity you might want to buy a PV panel tester.

Click to expand...

Thank you for the reply. So help me understand your btesting. You simply just put a resistor between the positive and negative connections and then tested the current? Is that right?
Thanks for your tip on shading. I'll include that, I didn't think of that with this being just one panel.

Basically, yes. Current and voltage (which could be calculated knowing resistance.)
The resistor value and wattage were selected to load PV panel approximately to Vmp/Imp according to its specs. Two electric radiator heaters in parallel, each with 600/900W elements, some of them enabled. That was for a 165W "24V" panel. A 400W panel might need several, if not higher voltage.
This was my DIY approach at home, but a hand-held tester could be worthwhile to take on a buying trip.


I think what Voc/Isc could miss is a cracked or otherwise failed cell. If one cell only delivered 50% of Isc, the panel makes full Voc and when output shorted, good sections push current past bypass diode of bad section.

Hedges said:

Basically, yes. Current and voltage (which could be calculated knowing resistance.)
The resistor value and wattage were selected to load PV panel approximately to Vmp/Imp according to its specs. Two electric radiator heaters in parallel, each with 600/900W elements, some of them enabled. That was for a 165W "24V" panel. A 400W panel might need several, if not higher voltage.
This was my DIY approach at home, but a hand-held tester could be worthwhile to take on a buying trip.

Measuring PV panel performance

I suppose I could buy a solar panel tester like Will reviewed, but that doesn't feel DIY enough to me. Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way. I'm presently working on an underperforming string of Sharp 165W...

diysolarforum.com

I think what Voc/Isc could miss is a cracked or otherwise failed cell. If one cell only delivered 50% of Isc, the panel makes full Voc and when output shorted, good sections push current past bypass diode of bad section.

Click to expand...

I would think that measuring the Short circuit current(ISC) would detect a bad cell... at least on a sunny day, because the short circuit current would read lower. I know the bypass diodes would still send current but a full row of cells would still be bad the output reading would still be low.

A cell, yes. But not a panel that had a bad cell (for most panels with bypass diodes.)



Consider a 150W "24V" panel, which might be about 36Voc, 30Vmp, and has three diode-bypassed sections.
If Isc = 5.5A and Imp 5.0A, each section is 12Voc, 5.5A Isc.

At maximum power point it puts out 30V, 5.0A. Voc = 36V, Isc = 5.5A

In the event one section had a cell broken such that it only delivered 2.5A:

At 30V it delivers 2.5A, 75W
Open circuit, Voc = 36V
Short circuit, the two good sections push current past bypass diode of bad section, Isc = 5.5A

This panel would deliver maximum power at 20V, 5.0A, 100W
Only some SCC would move past the 75W peak to find higher wattage at lower voltage.
Same issue with partial shading, shading of parallel strings, shading of half-cut panels.

So only measuring under load is that detected. If you look through tables of measurements in my thread, I highlighted bad ones with colors. Two panels delivered less than half the power that good ones did. Their Isc was reduced, but not by that much. Possibly, Isc was only reduced due to bad (but not as bad) cells in other sections. If just one cell bad, possibly Isc would not have given any indication.

I've thought the degradation could have been due to bias voltage away from ground. Unfortunately I didn't keep track of series connection order while disconnecting for testing (wiring was interleaved to minimize use of extension wires.) The panels spent most of their lives positive biased in a negative-ground array, but the last year before these tests in an ungrounded array with transformerless inverter. That meant half were biased negative, and may have suffered PID. This could have affected all cells to some degree, although those closer to the frames maybe more so.

It is not unlikely that some batches of panels are on the used market because they suffered degradation. Others we read are because part of a solar farm was damaged, and the surviving panels were offered for sale. Caveat Emptor, which is what testing is about.