Checking out newly bought panels before installing on a camper

[Lee L]

Newbie to solar and this forum... hello all.

I've been slowly assembling the bits and pieces to solarize my truck camper so as to be able to be off cord for longer before plugging in.

This week I bought 2 new 36 cell poly panels each rated at 160 watts, 22.7 volts Voc, 9.14 amps Isc.
So before drilling the holes in my Bigfoot fiberglass roof and mounting these panels, I want to check that they appear to be working so I could take them back to the local supplier if I have a problem.

The sky has been a bit hazy with forest fire smoke so a totally clear day has not presented itself. Nonetheless using my multimeter, I was able to read a Voc of 21.1 volts around noon. The multimeter on a 10 amp scale at the same time reads around 5. amps and went as high as 7.0 amps. I assume this is the way to measure Isc.
This is all with no battery or charge controller connected, only a multimeter on the output cables from the panel connected directly to my meter.

Next, I cut a piece of opaque cardboard to the same dimensions as 1 cell on the panel, and covered one of the cells. I was expecting a strong decrease in the current reading but that isn't what happened. The current dropped maybe .05 amps out of 5 when the cardboard was used to cover a cell.
More than one cell produced the same drop in current and one produced NO DROP in current when shadowed with the cardboard.

Is this normal for a panel connected only to a multimeter on the 10 amp scale?
Is there something wrong with the panel or the diodes?

I had read that shading one or two cells would really drop the current output of a panel a lot, but it didn't.
( Yes, I know that on my meter you have to plug the probe in to the high amp port to get proper reading. ).
e
Is this panel working normally ? Any knowledgeable help with this would be greatly appreciated. I am stumped.

[littleharbor]

What happens to the voltage when you cover the cell? Bypass diodes work by takin g a section of cells out of the loop, so to speak. This allows the current to stay the same from all other cells. All the cells in a panel should have the same current rating and approx. .5 volts. When assembled in a string of cells the voltage adds up but the current doesn't thus a 36 cell panel puts out about 18 Vmp. Bypass diodes do this so the affected panel in a series string of panels keeps the current the same and doesn't take out the whole string. The voltage would be cut in third, or possibly half, depending on the construction of the panel.

[Lee L]

Thanks for the response LittleHarbor.

The sun has come out again and I did do some more playing around with these panels and have learned the following:
1. The cells in my module iare wired as two 18 cell series segments, each with a bypass diode.
2. The Voc I can measure with my multimeter will drop a little ( .04 volts ) if I fully shadow one cell. It doesn't matter which cell.
3. The current I can measure with my multimeter is insensitive to the shadowing of one cell or 2, UNLESS the two cells are in DIFFERENT segments of the module.
So if I shade 2 cells in the same segment, I get a small or zero reduction in the current measured this way, but if I shade 2 cells, one in each of the 18 cell segments, then the current drops about 50 percent. This is what I had expected to see if I shaded only one cell.
4. Both of my panels act pretty much the same way, so unless I got extremely unlucky, I believe this to be the normal response of these modules when measured with a multimeter this way. Since each panel pumped out more than 8.5 amps in decent sun, they both are nearing the spec of 9.14 amps Isc claimed by the sticker on the back of the modules. In short, I believe they both are working as intended though maybe not as I expected.

I am still not fully understanding the electrical mechanism at work here, but have some suspicions as to why the panels don/t appear to be acting the way I expected
.
1. When you connect a panel to, say, a depleted battery, the voltage of the panel as a whole will be pulled down close to the battery voltage as the circuit through the battery draws current from the module. I have not used a battery here, but instead have used a multimeter to draw current from the panel which will also pull down the voltage during the current measurement, That voltage is probably quite low, near zero but not quite zero as there is a small 'burden voltage' across the shunt in the meter.
Is it possible the bypass diodes do not act as expected at this very low voltage? Unfortunately, I have only one multimeter and cannot measure the voltage and current from the module simultaneously.

In short, I still don't quite know what is going on here, but my original concern, which was to be convinced both my panels are working as they should, is satisfied. Because they both work the same, and I find it hard to believe they are both defective in exactly the same way, I conclude they are not defective.
I just don't yet understand what is going on. I think I will have do some more reading and thinking and wire up a controller and a battery then do some more playing around. At this point, though, I feel I could proceed to mounting the panels on the camper.

Thanks again for LittleHarbor for taking the time to reply, it is appreciated.

[jflorey2]

When you connect a panel to, say, a depleted battery, the voltage of the panel as a whole will be pulled down close to the battery voltage as the circuit through the battery draws current from the module.

Correct. It will be pulled down to (say) 13 volts if you connect it directly to a 12V battery.

I have not used a battery here, but instead have used a multimeter to draw current from the panel which will also pull down the voltage during the current measurement, That voltage is probably quite low, near zero but not quite zero as there is a small 'burden voltage' across the shunt in the meter.

Also correct. Very close to zero.

Is it possible the bypass diodes do not act as expected at this very low voltage? Unfortunately, I have only one multimeter and cannot measure the voltage and current from the module simultaneously.

It sounds like they are working as expected; shunting current around an area not generating much current.

At this point, though, I feel I could proceed to mounting the panels on the camper.

Yep. Reaction of panels to shading is an interesting academic point but won't affect real operation much (unless of course you have partial shading.)

[littleharbor]

When you test Isc. The voltage would be near zero the same way when you test Voc. there will be no current flowing. Take a 12 volt automotive bulb and connect it to the panel to put a load on it . You should will see current flowing and voltage as well. Do you have a DC clamp meter? with the bulb in series with your volt meter you can see voltage on your meter and with the clamp meter read current. I have to admit I'm not sure what to use as a load to use to see full output. It does sound like your panels are fine, though.

[bcroe]

Isc and Voc are at least a sanity check, not a complete check. Blocking a cell removes its ability to deliver
useful current, but will have almost no effect on the open circuit voltage. So blocking a cell or 2 has little
effect on Voc.

With 2 cell groups using 2 bypass diodes, blocking a cell takes out that cell and that section can by bypassed.
The capability then will be the same current, but at only the voltage produced by half the cells (in the unblocked
section). Cover cells in both sections and you have no load current capability left.

A more complete test is checking current capability while maintaining most of the voltage. Equipment to run
the complete curve is a little difficult. But just connecting to a 14V battery and observing load current is a
pretty good spot check. Bruce Roe