Testing Panel Output (Used Panels)

[bcroe]

As discussed, you are going to need to operate the panel at around its peak power for the sun its getting then. Too
low a voltage won't show up bypassed sections, and too little current misses the point.

I managed to debug some marginal panels in a string arrangement as described in another thread. The system
was applying an MPPT load, and I made a few assumptions
1. the Vmpp doesn't vary much;
2. several identical series panels all receiving the same sun should all perform about the same.

I added 3 way MC4 taps to get a voltmeter where needed. When one string didn't keep up with another
(clamp on DC meter), I started checking voltages across individual panels. Sure enough, I found a panel
running at 1/3 voltage, because 2/3s of its cells were being bypassed. Although it did OK at lower sun, it
dropped out at peak sun.

Replacing this panel wasn't the end; now ANOTHER panel became the weak link, running 2/3s voltage.

So the the weak panel gave itself away under full sun, compared to other identical panels, by bypassed level
voltages in a string ap. I may still find a place for it, maintaining some starter batteries? Bruce Roe

[Sunking]

That is probably the cheapest way to do it too, unless you are willing to play around with various light bulb arrays.
You would still need the light bulbs to discharge the battery.

Car head lights. Cheap and high wattage. I use to make load boxes out of them when I made DC power supplies in the 70/80's for ham radio operators. Still works today.

[inetdog]

Well wait a minute, I just figured out how to do it. Well sort of. Buy a 30 amp MPPT controller and a 12 volt 100 AH battery that is discharged.

That is probably the cheapest way to do it too, unless you are willing to play around with various light bulb arrays.
You would still need the light bulbs to discharge the battery.

[Sunking]

I really have to disagree with you on this one, Dereck.

How dare you

OK I agree and get your point. Point I am trying to make is what can be done in the field? All you got is Voc and Isc. You could build a device but it would have to be capable of sinking 9 amps of current, dissipate up to 400 watts, and vary its resistance from 2 ohm's to Infinity to mimic a MPPT Tracker. Even if you did that and found Vmp, the readings would be meaningless because you have no idea what irradiance is striking the panel.

Well wait a minute, I just figured out how to do it. Well sort of. Buy a 30 amp MPPT controller and a 12 volt 100 AH battery that is discharged.

[posplayr]

That is incorrect. The Voc test confirms the bypass diodes are working, and the Isc confirms th ecells are producing maximum rated current.

I asked this before. Why does the Voc/Isc test not work for TFP? I assume you can't short circuit the output.

THIS DOES NOT APPLY TO THIN FILM PANELS.

I assume a rated load would work?

[posplayr]

PS: loved the reference to "route-cause analysis."

One of my last carreers involved working on theoretical aspects of failure detection (a part of CBM+); I guess it shows. It was entirely inadvertent: geek: .

[inetdog]

I don't know much about solar panel failure modes, but here is one at least that a loaded tests would have detected whereas the Voc/Isc would have failed to detect. The load tests would need to be done with panel comparison to identify the culprit. Voc/Isc tests would not detect any series resistance correct?

http://www.pv-tech.org/news/fire_ris...ten_solar_modu

Voc testing will not detect resistance in the range involved.
And with Isc testing the difference in current of a string might not be noticeable without a calibrated light source or simultaneous measurements on individual panels.

PS: loved the reference to "route-cause analysis."

[posplayr]

That is incorrect. The Voc test confirms the bypass diodes are working, and the Isc confirms th ecells are producing maximum rated current.

If you send a panel for replacement, or if the installer does it, the only check they make is Voc and Isc. If those check out the panel is good. Well assuming you test at noon on a bright sunny clear day.

A fixed resistor is not going to tell you much unless you just happen to be inputting 1000 watts/ meter2 which is not really possible. You can calculate what the fixed resistance from Vmp and Imp at 100% Solar Irradiance, but you would never see it happen in the field. Example a standard 100 watt 36 cell battery panel will have a Voc = 22 to 24 volts, Vmp = 18 volts, Imp = 5.55 amps and Isc = 6.25 amps. One would think the Resistance to test at is 3.24 Ohms. Connect that up and you woul dbe falsely led to believe the panel is bad because you will never get 18 volts out of it because 100% or 1000 watt/meter2 input is only a lab condition. Say the which is a real Irradiance is only 500 watts/meter square which is a real life value and that Resistance is up to 6.5 Ohm's at MPPT

A solar panel is a current source and all current sources are tested the same way. Open circuit voltage, and short circuit current. If it passes, the current source is good.

OK It seems we are discussing this characteristic which within the voltage limits of the cell is a current source proportional to solar power.



From the curve, it is pretty plain to see that the current is essentially independent of the outside load being only a function of solar irradiation (as you described a "current source"). Perhaps better described as a solar irradiation current source (i.e. the panel acts like a current source proportional to incident solar power).


If you need a test that is easiest to interpret then the Voc/Isc tests are pretty good as long as you understand that

a.) Voc can vary but is generally close to the rated output voltage and
b.) Isc is proportional to incident solar power.

In effect these two measurements are independent of the load although the Isc is still proportional solar power.

I can see why the Insitu measurements might be problematic as you have no reference and you are comparing one to the other and without a pair it just gets more confusing. And even the known load will have variability, but that variability is no worse than the solar current source. You can't look for rated Vmpp or Impp at less than rated solar power and expect to get the same answer. As mentioned above the Voc at least does not vary much.

I don't know much about solar panel failure modes, but here is one at least that a loaded tests would have detected whereas the Voc/Isc would have failed to detect. The load tests would need to be done with panel comparison to identify the culprit. Voc/Isc tests would not detect any series resistance correct?

http://www.pv-tech.org/news/fire_risk_fiasco_as_dutch_government_warns_about_650000_scheuten_solar_modu

[inetdog]

That is incorrect. The Voc test confirms the bypass diodes are working, and the Isc confirms th ecells are producing maximum rated current.

I really have to disagree with you on this one, Dereck.

The Voc test shows you that the bypass diodes are not shorted, and that all of the cells are contributing voltage. It cannot tell you that all cells are capable of delivering any current beyond what the meter draws.
The Isc test only shows you that at least one segment of the panel is working and that the bypass diodes on the other segments are capable of carrying current. The few volt total drop across the bypass diodes is not going to have a noticeable effect on the Isc of the remaining good segment of the panel.

The number of times that you have one or more weak cells may be small compared to panels which just do not meet their Isc at all, but it does happen and the combination of Voc and Isc testing will NOT find it. It is particularly likely to happen in panels that have suffered shipping damage but no visible cracks or panels that fell out of the QC limits on the production line. The production tests specifically test Vmp and Imp along with the other parameters.
On an MPPT input, you will see that with one weak cell the Imp is close to normal while the Vmp is much lower (depending on the number of bypass segments in the panel.)

We have had members troubleshoot their systems and find exactly this problem. You can also apply the same reasoning to strings of panels.
If Voc of the string and Isc of the string is good, you could still have one or more defective panels in the string, including panels that are almost totally open except for their bypass diodes.

[Sunking]

I can see that the Isc test is inconclusive given the internal structure of the panel (e.g. by-pass diodes). The panel has to achieve it's rated values to be operational but achieving that value is no guarantee that the panel is good. In other words it is confirmatory but not conclusive test.

That is incorrect. The Voc test confirms the bypass diodes are working, and the Isc confirms th ecells are producing maximum rated current.

If you send a panel for replacement, or if the installer does it, the only check they make is Voc and Isc. If those check out the panel is good. Well assuming you test at noon on a bright sunny clear day.

A fixed resistor is not going to tell you much unless you just happen to be inputting 1000 watts/ meter2 which is not really possible. You can calculate what the fixed resistance from Vmp and Imp at 100% Solar Irradiance, but you would never see it happen in the field. Example a standard 100 watt 36 cell battery panel will have a Voc = 22 to 24 volts, Vmp = 18 volts, Imp = 5.55 amps and Isc = 6.25 amps. One would think the Resistance to test at is 3.24 Ohms. Connect that up and you woul dbe falsely led to believe the panel is bad because you will never get 18 volts out of it because 100% or 1000 watt/meter2 input is only a lab condition. Say the which is a real Irradiance is only 500 watts/meter square which is a real life value and that Resistance is up to 6.5 Ohm's at MPPT

A solar panel is a current source and all current sources are tested the same way. Open circuit voltage, and short circuit current. If it passes, the current source is good.

[posplayr]

Yup. The problem is that the resistance needed to hit Vmp/Imp exactly is a function of the amount of light as well as the temperature and health of the panel.
If you can come within 50% of the ideal resistance, on the low resistance side, not the high resistance side, it should be good enough for a test. It will not catch marginal cells, but it will be more comprehensive than the simple Isc test. However the Isc test is so much easier that it is a good place to start.

But if you are testing several panels (individuals) of an array, then it would seem worth the extra little effort to just use a voltmeter attached to a dummy load to compare outputs as you go down the line. In my mind inconclusive tests are pretty much worthless especially if there are multiple potential faults that you only detect back at the full up level. You are back to square one.

Had you done the loaded tests and got similar output from each panel, what latent (panel) defects would you have not detected? The fact that all the voltages are the same is enough for you to know they (the voltages) are alright for whatever light and resistor you happen to have.

I'm not familiar with the MC4 plugs, but could you not just (make a test plug) and measure the panel voltage insitu without any dummy load and look for consistency of voltage?
Measure total and all the individuals should tell you plenty and be hard to get fooled.

[inetdog]

I can see that the Isc test is inconclusive given the internal structure of the panel (e.g. by-pass diodes). The panel has to achieve it's rated values to be operational but achieving that value is no guarantee that the panel is good. In other words it is confirmatory but not conclusive test.

So with the resistive load, you only have to measure voltage and power is V^2R? (assuming you have measured R). I guess R can change due to heating and to be precise you could use an amp meter and volt meter but with uncalibrated solar incidence does it really matter?

It has been my experience that you are less likely to get tricked when testing at full load. Seems to apply to solar panel functional tests as well.

Yup. The problem is that the resistance needed to hit Vmp/Imp exactly is a function of the amount of light as well as the temperature and health of the panel.
If you can come within 50% of the ideal resistance, on the low resistance side, not the high resistance side, it should be good enough for a test. It will not catch marginal cells, but it will be more comprehensive than the simple Isc test. However the Isc test is so much easier that it is a good place to start.

[posplayr]

A properly sized resistive load is one which will draw approximately Imp when driven by a voltage of Vmp.
If the light conditions are low, the current will go down and the voltage will drop below Vmp.
The problem with testing only Isc is that it gives you no idea how the panels will perform under load.
Take, for example, a panel whose cells by design have an Imp of 5A. Now take one cell in that panel and break it or otherwise damage it so that it can produce only 2A under the same conditions.
When you measure Voc it will still be correct.
When you measure Isc, the section of the panel which includes the bad cell will not be able to produce 5A and so the bypass diode will conduct, allowing the full Isc to be supplied.
Assume three bypass diodes and three subsections in the panel. Now if you load it with a proper resistive load or an MPPT input you will get a current of 5A (Imp) and a voltage of just ~2/3 of Vmp. That tells you the panel is damaged even though both Voc and Isc are normal.

I can see that the Isc test is inconclusive given the internal structure of the panel (e.g. by-pass diodes). The panel has to achieve it's rated values to be operational but achieving that value is no guarantee that the panel is good. In other words it is confirmatory but not conclusive test.

So with the resistive load, you only have to measure voltage and power is V^2R? (assuming you have measured R). I guess R can change due to heating and to be precise you could use an amp meter and volt meter but with uncalibrated solar incidence does it really matter?

It has been my experience that you are less likely to get tricked when testing at full load. Seems to apply to solar panel functional tests as well.

[inetdog]

EDIT: I guess I answered part of my own question.
I looked at a spec sheet for a Astonergy panel and Impp (or rated ) and Ioc is essentially very close; so a short is as good as a 7.5ohm power resistor?

http://pdf.wholesalesolar.com/module...771.1434675899

A properly sized resistive load is one which will draw approximately Imp when driven by a voltage of Vmp.
If the light conditions are low, the current will go down and the voltage will drop below Vmp.
The problem with testing only Isc is that it gives you no idea how the panels will perform under load.
Take, for example, a panel whose cells by design have an Imp of 5A. Now take one cell in that panel and break it or otherwise damage it so that it can produce only 2A under the same conditions.
When you measure Voc it will still be correct.
When you measure Isc, the section of the panel which includes the bad cell will not be able to produce 5A and so the bypass diode will conduct, allowing the full Isc to be supplied.
Assume three bypass diodes and three subsections in the panel. Now if you load it with a proper resistive load or an MPPT input you will get a current of 5A (Imp) and a voltage of just ~2/3 of Vmp. That tells you the panel is damaged even though both Voc and Isc are normal.

[posplayr]

Using a environmentally controlled lab using a xenon strobe light with an intensity of 1000 watts per square meter with controlled at 25 degree Celsius, and 20% humidity. You cannot test them. All you an do is test Voc and Isc.

Panels are current sources and current varies with direct proportion of Solar irradiance input. You cannot use a fixed resistive load, it is a moving target. Only way you could use a fixed resistance is to control the Solar irradiance input of a calibrated source. That can only be done in a lab.

I should have clarified "standard", I meant for field testing or DIY. That would rule out any calibrated source of irradiance. So as the OP has rationally attempted to do, his test light source is to use the sun as an uncalibrated source. I think that is what we are talking about and what I meant. At zenith sunny day, what is he going to be able to quantitatively determine at the output. The results of any measurement whether Open Circuit, Short Circuit or a loaded test is going to be a function of the incident light ? Correct?

You say the panel acts as if it is a current source, producing current in proportion to light. Does it act like an ideal current source for a given light input or does it have a Norton equivalent. By spec they sure seem to act like they have series resistance of a Thevenin equivalent. In fact are there not equivalent models and can they not be used to "test" output power at close to rated load with a resistive load?

I have asked Inetdog what a "properly sized" resistive load is. He is the first to mention this. It also seems safer.


EDIT: I guess I answered part of my own question.
I looked at a spec sheet for a Astonergy panel and Impp (or rated ) and Ioc is essentially very close; so a short is as good as a 7.5ohm power resistor?

http://pdf.wholesalesolar.com/module%20pdf%20folder/CHSM6612P-305_Specs.pdf?_ga=1.59718958.269488771.1434675899