Is Voc a Good Check?

keyturbocars

Hi, I've been busy and distracted the past few months, and I am just now getting around to installing my 2.1kW Evergreen solar array made up of 10 Grade B 210W panels. I've got the Unirac rails all mounted on the roof of my shop. Today, I was going to try to mount the panels. Before I went through all the trouble of hauling them all up on the roof and mounting them, I figured it might be good to do a quick check of each panel to make sure there are no problems. I lined all the panels up against a chain link fence I have and they are all facing south. Skies are clear and no shadowing. All panels are tilted at the same angle. I then started to go through with a digital voltmeter and check open circuit voltage (Voc) on each panel. A couple panels were real close to each other and another was almost 1.0V different. I went back and rechecked a panel and the voltage had dropped some. Then I realized that maybe I better wait until all the panels come up to temperature, so I stopped and came in to write this e-mail. When I try to recheck them, I will make sure that they have all had a chance to sit in the sun and warm up to the same temperature first.

Is Voc a good check just to try to spot potential problems (mismatching) between panels in an array?

I realize that testing under load would probably be best, but I don't have a good way of doing that right now. I thought a quick Voc check to see how well matched all the panels are would be good. I don't want to find out later after everything is all mounted on the roof that there is a problem with one or more panels.

Thanks,

Edward

mike95490

Re: Is Voc a Good Check?

Better test:

Set your meter to AMPS, if the range is within what the panels are rated at.
Run each panel for a minute or two, to let internal parts heat. not too long though, all that power is going into your meter ! Amps should start high, and very quiclky level off 15% below their start.

keyturbocars

Re: Is Voc a Good Check?

I waited about 1 hour and went and checked them again. They all came back with Voc readings of 20.4-20.8V. That's within 2% so there doesn't appear to be any big discrepancies in terms of Voc.

Thanks for the idea Mike. My old Fluke DVM has a 2A limit. I have a cheap Chinese multimeter that shows 10A max, but the leads are broken on it. I feel better now that the Voc are pretty close. I don't think there's anything wrong with any of the panels. I hope to get them mounted tomorrow, and then I need to do a lot of trenching by hand to bury some conduit. I wish I could rent a power trencher, but the rental place said that the extremely rocky soil we have here would not be good for a ditch witch. When I dug a trench for my wind turbine, I broke the hardwood handle of the pick in half, because I kept smashing into big rocks. My arms and hands hurt for a long time after that. Not looking forward to this new trench for the solar wiring conduit! I now have a pick with a fiberglass handle, so hopefully it doesn't break (or my arms)!

mike95490

Re: Is Voc a Good Check?

Your Voc is nearly pointless. Without a load, even the cr#&*($st apnels will give you voltage. Get leads for your amp meter before you mount the panels.

When you say "Grade B" are they new or used ?? New, I'd likely just mount, used, I'd test first.

BB.

Re: Is Voc a Good Check?

Yea, you really need to pull some current. If you are going to be working with DC a lot--I would suggest this cheap $60 DC Clamp Meter (plus DMM functions). Just short each panel and put the DC clamp on the lead. You can measure hundreds of amps with the clamp meter (vs 2-10 amp maximum for the typical DMM).

meridiansolar

Re: Is Voc a Good Check?

I got a Ideal Brand dmm from lowes and my panels read zero amps when I use a 24v dc drill as a load. I think I don't know how to use the DMM or it is broken. I just cut the wire and put the DMM in line with the circuit, does it matter if it comes before or after the load?

BB.

Re: Is Voc a Good Check?

Should not be a problem with measuring current by just cutting the wire somewhere and connecting a DMM (set to amps, plug into COM / 10AMP plugs, generally).

It may be possible your drill was not correctly switched on (or other wiring/DMM connection errors).

DC Current Clamp meters are 100x better than having to cut wire and insert a DMM.

-Bill

meridiansolar

Re: Is Voc a Good Check?

the drill worked fine when the multimeter was removed from the circuit, maybe I'll just return it and get a clamp type.

BB.

Re: Is Voc a Good Check?

Even with a Current Clamp Meter--you still need to separate one wire from the bundle to measure the current through that one wire (for example, you may have to remove the outer jacket of cable and separate the wires).

If you put the clamp around both wires (+ and -, Hot and Neutral, etc.)--The clamp will only measure the difference in current (typically zero amp difference) in the wiring.

-Bill

mike95490

Re: Is Voc a Good Check?

AC clamp meters are very different from DC clamp meters.

To measure the DC amps from a panel, the amp meter IS the load, no other load needed. You are effectively shorting the panel + to -, which is harmless to the panels - even a spec for it : Isc (current short circuit)

keyturbocars

Re: Is Voc a Good Check?

Thanks for the feedback. These are brand new Evergreen panels. They were just "cosmetically imperfect", so that's why they were sold as Grade B. They are probably just fine, as Evergreen had to test each one to sort it out at the factory. I was just trying to double check to make sure I didn't get a dud panel. They should all be fine as they are brand new from the factory. Obviously, installers that mount 100's and 1000's of panels are not going to retest and recertify every new panel they install. I was just trying to be extra careful, but it's probably unncessary in this case.

bmet

Re: Is Voc a Good Check?

Dont ask BB about new panels. Just don't do it. He won't tell you a thing

keyturbocars wrote: »

Thanks for the feedback. These are brand new Evergreen panels. They were just "cosmetically imperfect", so that's why they were sold as Grade B. They are probably just fine, as Evergreen had to test each one to sort it out at the factory. I was just trying to double check to make sure I didn't get a dud panel. They should all be fine as they are brand new from the factory. Obviously, installers that mount 100's and 1000's of panels are not going to retest and recertify every new panel they install. I was just trying to be extra careful, but it's probably unncessary in this case.

niel

Re: Is Voc a Good Check?

voc is a good limited test. it won't indicate how much amperage the pv is capable of, but when the proper voc is shown it does indicate there aren't any opens that would show a lowered voltage due to a section being bypassed. of course when seeing 0v that could be a diode or something even more serious that is unreachable and therefore unfixable. if a pv is fully illuminated, but shows say 6v or 8v lower than it should this is a section that has been bypassed and that open circuit is not usually fixable if the problem lies around the cells themselves.