Voc and Isc are pretty much hypothetical numbers which do not change and neither of them is ever where you will be operating your equipment. Voc is the voltage you will read with a voltmeter between the plus and minus connections with nothing else connected to the panel. Of course, the panel is useless if you don't connect it to anything and the moment you do, the voltage will drop below Voc. Isc is the maximum current you will ever get out of your panel and that condition occurs when the plus and minus connections are shorted together through a very, very low value resistor. If you measured the voltage between the two leads at this point, it would be zero. Practically speaking, you will never get as much current out of you panel as the Isc spec because the devices you run with your panel will not operate at zero volts. Voc and Isc should be considered upper bounds that you will not quite achieve in normal use.
" Voc and Isc should be considered upper bounds that you will not quite achieve in normal use. "
I am not sure I have seen full 8.2 when at 25C but I have seen 8.1 so that fits. Perhaps there will still be chances this summer to see that or not if we get another heat wave (I believe we are allowed two heat waves per summer and we had one already--- mena661 sneers at what I call a heat wave ) I get 8.2 lots of times when temps are more like 20C.
Of course in cloud effect it goes way over Isc but that is outside the idea of "normal" although it is not at all "unusual"
My main gripe remains that if you suspect your solar isn't working as it should, how can you test it except on perfect days? You just have to wait and see instead of being proactive. Don't like that.
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My main gripe remains that if you suspect your solar isn't working as it should, how can you test it except on perfect days? You just have to wait and see instead of being proactive. Don't like that.
I think this relates to what Mex was advocating as a baseline reference test of the panel Isc. If you see those watts go missing and know what the panel Isc reference is, you can then retest the panel at Isc and eliminate the panel as the loser of those watts.
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BFL13 wrote:
“I am feeling especially stupid so need more elaboration, please.”
I feel your frustration. I admirer someone that ask questions when they don’t know the answers. I spent most of my life working with people like that. They asked the hard questions, they are called scientist and a lot of them tried to explain things to me that are far over my head.
I’ll try to shed some light on some of this. Some may laugh but I don’t care.
First Ipmax and Vpmax mean nothing unless you are using a MPPT controller, forget about them!
This may sound silly but I’ll try to use a car engine to explain some of it. Voc is like the engine running at max RPM out of gear, it makes noise (Voc) but does nothing. Isc is somewhat like the engine just as you stall it when pushing against a wall; max torque but you go nowhere. You use your engine in between these points. You use your panel in between Voc and Isc.
If you use a MPPT controller then this applies. Ipmax and Vpmax are the points that the panel puts out maximum power (maximum watts), the point that your engine puts out maximum horsepower. That is the point that torque time RPM is at maximum. Then there is this magic box – the MPPT controller. It converts the maximum power to the maximum current that your battery will accept. Sort of like your transmission converting engine power to maximum torque at the wheels.
I have a home system and have noticed that temperature is a big deal. On a 90* day I get a bit more than a 10% drop compared to 70* (sorry about the *F values) the day before and the day after. There could very well be more haze on the hotter day as a contributing factor, but I'm comparing totally clear blue-sky days. On a 90* day my 4 kW array hits about 3.5 kW peak, on a cooler day it gets close to 4 kW peak.
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.....I think this relates to what Mex was advocating as a baseline reference test of the panel Isc. If you see those watts go missing and know what the panel Isc reference is, you can then retest the panel at Isc and eliminate the panel as the loser of those watts.
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How do I know if it is under-performing from bad solar conditions or from a bad panel etc unless I have one of those solar condition meters linked earlier?
I already have 8.2a (on the Trimetric)as my baseline for good solar conditions. This all started when I was only getting 7.8a (on the Timetric) with what looked like a nice blue sky.
If I am getting 8.2 on the Trimetric - when controller is passing it all through as though direct - will I also get 8.2 across the panel wires with the multimeter? Same with the 7.8? If so what's the point of taking the Isc like that with a meter if that's what I am already seeing? ( I will try this out to compare them next chance I get- later next week-as at least one thing I can do)
There must be something here where you have all that extra voltage with non-MPPT controllers like mine that lets you have full Isc without having full Voc conditions?
Good info about temperature. I don't have those temperature vs specs graphs for my particular panel that the MPPT controller sizing guide said you need to get the voltage to expect at your lowest camping temp to be safe from exceeding the max input voltage rating of the controller. A graph like that would be handy for knowing what happens on each side of 25C.
* This post was
edited 07/28/12 10:47pm by BFL13 *
Isc is the maximum current the panel can produce when the voltage across the panel is equal to 0VDC (output shorted).
The panel can never achieve this maximum current value when operating normally since the battery bank has some stored charge (hopefully) which means a voltage is developed across the bank.
The state of the battery bank charge, which determines the voltage across the battery bank, also determines how much current (charge) can be drawn from the panels.
A severely discharged battery bank will draw much more current than a battery bank that has a good charge stored/available.
If the battery bank voltage is at or near its correct fully charged value, very little current can be drawn from the panels.
So don't fret when you don't see maximum current flowing from the solar panels on those bright sunny days unless the battery bank voltage is low.
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I found the "temperature coefficient" for the Isc of my panel.
0.053%/degree C.
So does that mean at 15C instead of rated 25C, my Isc will be 0.53% of 8.2a higher = 8.243? I was seeing 8.3 at 15C instead of 8.2 at 25C on the Trimetric (which rounds off to one decimal place)
If you want to see more (or the max) current from your panels, just turn on all of your 12 VDC loads which will cause the battery voltage to lower due to a loss in stored charge...
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) I get 8.2 lots of times when temps are more like 20C.
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