How Do I check the Faulty Panels?

[paulcheung]

Hi guys,

I have two Identical strings of panels, 4 in series. they are all 225watts panels. This morning I notice one controller give less current than other who usually be more. I have wired these panels with different incoming breakers, so I turn off the others feed and left these two strings. when I check these two strings, one give me 7.1 amps in the 8:15am, and the other only give me 1.7amps. so there is obvious some thing is wrong with the string only give me 1.7 amps. they are identical panels. same direction and place.

I once read about the panels can't be damage by short circuit by itself? is that true? Can I use DC amperes meter to check which panel(s) is faulty by short each panels? The roof is flat concrete so I can easily access to all the panels. Any suggestions?

Thanks.

[Sunking]

With Voc and Isc test.

[paulcheung]

Ok thanks

[bcroe]

Ok thanks

If the above method doesn't find the problem, I have had panels where only one section was
bad. The panels had 3 groups of cells, each with a bypass diode. In operation one group
couldn't keep up, so its voltage dropped to zero as all good cells conducted through the bypass
diode.

Isc won't find this one, because the good cells will still conduct properly (through one bypass
diode). Measure the voltage across every panel in the string in operation. Expect a panel to
read 1/3 less voltage than the others (if it has 3 bypass diodes). I use 3 way splitters inserted
to access the circuit. good luck, Bruce Roe

[Sunking]

Isc won't find this one, because the good cells will still conduct properly (through one bypass
diode).

VOC test should find it.

[bcroe]

VOC test should find it.

Yes it usually does. But I have dozens of "surplus", "cosmetic reject" panels from Ejunk, and a
few had marginal problems. I tested the one in this pic for Voc and Isc, and decided to give it
chance. But that broken cell section (blocking an apparently good cell) wouldn't deliver the
current at full sun, so the operating voltage indicated 1/3 of the cells were bypassed. I have
found others that weren't so conspicuous.

Another check is the temp of the panel surface at power. This cheap thermal leak detector
easily found the above, might work on other problems.

When all my reject panels are pulled, there ought to be a way to put them to work (reduced)
in their own setup. A single string, or maybe all in parallel? Bruce

[russ]

When all my reject panels are pulled, there ought to be a way to put them to work (reduced) in their own setup. A single string, or maybe all in parallel? Bruce

Bruce - You and Green are two peas in a pod - you guys love/live to tinker! Well done to both of you!

[paulcheung]

I use the ISC short circuit to find the problem panel. one panel only put out 2 amps ISC. So call the supplier and he take it back and lend me one (as he don't have no more those panels)that is about 0.5 amps short. but it is ok, the string back to work and everything is good.

Thanks guys

[inetdog]

VOC test should find it.

Voc test will find a section of the panel which is completely open, either from a crack in a cell or damage to the tab or bus wires.
That seems to be the most common failure
But we also have had reports from people who have damaged cell which just causes a greatly reduced cell current.
Unfortunately those will read just fine on the Voc test because the meter is drawing a current below the maximum for the damaged section.

Certainly Voc and Isc tests are the simplest way to start, but if they come out OK and you still see an unexplained reduction in power, then you need to go deeper.

[Sunking]

Certainly Voc and Isc tests are the simplest way to start, but if they come out OK and you still see an unexplained reduction in power, then you need to go deeper.

IF Voc and Isc pass, then it is not likely a panel problem. Voc and Isc are the test the factory will run on a warranty claim.

[bcroe]

But we also have had reports from people who have damaged cell which just causes a greatly reduced cell current.
Unfortunately those will read just fine on the Voc test because the meter is drawing a current below the maximum for the damaged section.

Certainly Voc and Isc tests are the simplest way to start, but if they come out OK and you still see an unexplained reduction in power, then you need to go deeper.

Isc won't find it, it will just operate thru a bypass diode. Running that test for warrantee, better
disconnect the bypass diodes first. Running it in a string with mpp load will find it. The other
series panels will set the current where it should be. Then the bad section will bypass, giving
a voltage across the bad panel 1/3 lower than other panels, or whatever fraction is bypassed.

I also found, the bad section may not show up until the sun is pretty strong. Bruce Roe