a dumb question about testing volts!

System · October 2010

Hi!

I'm new here (and to solar energy) and have found this community to be helpful and active, which is great! I'm experimenting a little and ordered a 100w monocrystalline panel from ebay with such specs:

Pmax: 100 W + - 3%
Vmp: 18 V
Voc: 21 V
Imp: 5,55 A
Isc: 5,94 A

Ok so this was a cheap purchase but the panel seems to be professionally made although there's no brand.

Now onto the stupid question. The panel arrived yesterday and I figured I should test if it's working properly or not. I live in Finland and
we had no sunshine then nor now (and prolly won't have in a couple of days). So anyways I hooked up my voltage meter with it and tested it indoor with average room light condition and the meter said 6 volts. Is this normal? I understand I should test it outside in a direct sunlight but then again I thought voltage shouldn't change that much in a little worse test condition. Shouldn't the volts be almost stable and it's the amps that change more depending on the amount of light?

Any help much appreciated!

BB. · October 2010

Re: a dumb question about testing volts!

I don't have a panel to test myself--But testing indoors is going to be difficult to see if the panel is good...

--some numbers:

• sunlight on an average day ranges from 32 000 to 100 000 lux
• TV studios are lit at about 1 000 lux [i.e. 1000 lumens
per square metre]
• a bright office has about 400 lux of illumination
• At sunset and sunrise, ambient outdoor light is also about
400 lux (if the sky is clear).
• moonlight represents about 1 lux
• starlight measures a mere 0.00005 lux

A bright office is about 400/100,000 or 0.004 (0.4%) of full sunlight. At those light levels, the self discharge of the solar cell (there is some internal resistance between +/-) might keep the output voltage from reaching Voc (internal leakage current).

If you have a 100-500 watt flood light--you might try shining that at the panel.

Also, what is the short circuit current (multi-meter set to Amp/Current mode)?

5.94 amps * 0.004 = 0.024 amps or 24 mAmps (not very much current)

-Bill

System2 · October 2010

Re: a dumb question about testing volts!

Thanks Bill!

I forgot to mention the short circuit current I believe was 0,1a (or maybe it was 0,01a?). So that should be normal according to your numbers. I'll see if I can find a powerful floodlight to test it with. I sure don't have one but I'll ask around. I want to be sure the panel is working ok asap so I can inform the ebay seller and return the panel if necessary.

Does anyone know how dim light level really effects the voltage? It can't be directly proportional with lux. Otherwise my multimeter should read about 0,084V (21V * 0,004). Like you guys who have solar systems up and running, when it gets darker in the evening do the volts drop quickly below battery charging level or will the amps drop first?

BB. · October 2010

Re: a dumb question about testing volts!

The short circuit current output of a solar panel is pretty much proportional to light levels.

Voc is, more or less based on the physics (photons, electrons, holes, etc. in the silicon) which creates a 0.5 volt level (per cell) and a current 0.0.....0x amps per photon.

Because there are resistive and probably other effects that recombine holes and electrons at the crystalline levels... You probably end up with something like xxxx ohms of parallel resistance (i.e., resistor between + and - of the cell)... If we assume a current of 0.01 amps and 1,000 ohms. If we assume that the values of current/voltage are 0.024 amps and 5 volts:

R=V/I= 6 volts / 0.024 amps = 250 Ohms of parallel resistance

Is that a "real value"--I don't know.

Take a look here for some good pointers to solar cell physics...

From the solar cell Wiki page, a cell may have a shunt (parallel) resistance on the order of 150 to 1,000 Ohms per sq.cm... (at less than 0.55 volts).

So, 250 ohms is probably not a bad estimate for parallel resistance of your panel... (saying that we are within a factor of 2 of the real value--vs being off by a factor or 10 or more).

-Bill

mike95490 · October 2010

Re: a dumb question about testing volts!

skarr wrote: »

Like you guys who have solar systems up and running, when it gets darker in the evening do the volts drop quickly below battery charging level or will the amps drop first?

I see the amps drop to nil, then the volts drop to zero just a few minutes later

System2 · October 2010

Re: a dumb question about testing volts!

Thanks again guys!

Ok yesterday I had a chance to test it outside - we had a whole 15mins of sunshine! Voc was 22,35 and Isc 5,15 so I guess it's working properly. When we were testing the current, my friend said he saw a small spark somewhere in the junction box. I didn't see it cause i was holding the panel.. Should I be worried? It didn't affect the volts/amps. There's two diodes in the junction box rated 10A I guess? Are they there to block backflow drain from battery, and if yes, why are there two of them?

BB. · October 2010

Re: a dumb question about testing volts!

They are probably by-pass diodes... The diodes are in parallel with part of the cells so that if they are shaded while other parts of the panel/array are in full sun the current can by-pass the shaded cells (instead of causing damage).

What caused the spark? My guess that the screws were not tight while you were testing the panel? Or, if you used fine stranded wire, perhaps a strand or two of wire shorted across the connection points/diodes?

Not good thing to cause sparks--but probably did not hurt anything.

-Bill

mike95490 · October 2010

Re: a dumb question about testing volts!

or the box internal connections to the panel could be flakey

BB. · October 2010

Re: a dumb question about testing volts!

If the internal box connections are flaky--then the panel is of limited value... You could try "repairing" re-soldering--but you should not have to do that with a new panel.

-Bill

a dumb question about testing volts!

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