Low light 2 panel 2 behavior

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Hello,

great forum,

i have a "small" test system, to move around for fun and in case of small emergency for a few hours. (small computer 30w, led light 15w, small tv 50w, no fridge nor coffee machine of course)

200Wc (2 pannels of 100 wc in 12V) http://www.lorentz.de/pdf/lorentz_sm_la100-12s_en.pdf
125 Ah no name closed calcium battery 12v
Phocos controller CXN20A
Victron PSW 300w inverter

i bought the first panel, 100Wc, it was working ok. So a month after i bought a second one to put it in parallel with the first (with a 10A fuse on each +).

But they are not working the same in low light. Here i have in january a lot of day with clouds, i don't see the sun but the first panel reach still 0,45A of output. At a irradation of 100W/m2 theorically it should output 1,2 A, so i guess i have currently around 50w/m2 of irradation, so very low. Can't do a lot but it is working a little bit and i am happy.


But the second panel i just received, alone in the same configuration, output only 0.15 A. Exact same model. As there is no direct sun currently, i can't test it with more irradiation. I have seven days to resend the panel if there is a problem, but i don't know is there is one!


Can i conclude something about the second pannel? defective or not?

Or is normal at very low irradiation to have a difference of output between two same panels? Is it some kind of lottery? Is it that the first panel is perfectly balanced, i was very lucky, and the second one have a "weakest link hard to awake" cell that show only in bad irradiance, nothing to worry about? Nothing the manufactor can do or guaranty? When i test the open voltage of both during the day, nothing connected, i have 21v for the good but only 20,6 for the bad (Voc is 21.2, but i tested 21.6/21.8 on both panel the only time i saw the sun, didn't had time to test the amperage output then).

At my location, my average annual energy collecting is between 160wh (0,50 derating) or 200wh (0,70 derating) per day per panel. Max monthly average in
summer with 15° tilt is 400Wh best case, min monthly average in winter is 60Wh with 50° tilt badest case. i already know that my system will be useable only during 8 month.

But currently the first panel collect 30wh per day on a bad cloudy day, and the other only 10wh. I am nitpicking i know. I was considering adding a third panel to help in winter but if it will only add 10wh in bad day, for a total of 50wh and not 90wh like i thought (3 x 30 ) now i am less in a hurry.

When the sun come back i will see if the second panel is really working ok in direct sun. If not i send it back, problem solved.

But if is ok, can i still complain and ask for a exchange for the behavior in low light ?

what's your experience with your panels in very low lights? Are they behaving all the same? Do some "fail" more rapidly than others?

thanks.

Goofy

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Low light 2 panel 2 behavior

    What does the warranty say? "Money back if not 100% satisfied"? Because you're not satisfied and that's what matters!

    It is difficult to test panels under low light because that's not what they're spec'd at. But you should expect two identical panels to be very close. To that end, the specifications call for a tolerance of +/- 10%, but that is on the rated Wattage under "full sun" conditions.

    See if they'll give you an exchange/refund. It's a good test of their customer relations. ;)
  • System2
    System2 Posts: 6,290 admin
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    Re: Low light 2 panel 2 behavior

    For now i had only a copy/paste reply (sunday!) stating i should check the panel in full sun to see if they are working correctly . I can't do that right now so i came here to ask the expert for advice.

    Yes i can send the panel now and have my money back but i would still have to pay the transport fee from my pocket and take all the risk on my part if the panel is broken during transit, so i would do this only if i know that the panel really doesn't work.

    Tomorrow i hope i would be able to really test them and i will contact the technical guy with more info directly and not the commercial guy. If i can prove it doesn't work i think they will be willing to take the full cost of a replacement, but i need proof.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Low light 2 panel 2 behavior

    The way to test your panels:
    1. place in full sun (facing square to sun) and measure voltage... Should be close to Voc (hot panels, Voc will drop a bit).
    2. Cover solar panel (dark). Set your DMM to 10 AMP DC full scale (assuming panels are 8 amps or less current rating). Clip amp meter to solar panel leads. Remove cover. You should see around Imp or a bit more (Isc--short circuit current rating). If current is less than ~75% of rated current, I would wonder (or you have a hazy day, etc.). Less than 50%, you might have a bad panel.
    3. If you have more than one panel, all panels should behave similarly in same sun/conditions (i.e., if you have 3 panels showing 90% of Imp, and one panel with 60%--that panel is probably bad).
    There is one more test that can catch the last 5% of problems... Basically connect the "12 volt" panels (really around Vmp=17.5v) and measure the current at battery voltage. You have to be careful to not connect the panel backwards to the battery--if backwards, it will immediately fry the panel and make it worthless.

    If you have parallel panels connected to a charge controller--A quick test is to run one string at a time and check its output with the charge controller (battery needs to be below ~80% state of charge and/or you need some loads on the battery bank so the charge controller takes 100% of panel energy). All strings should have ~equal results.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Low light 2 panel 2 behavior

    if i were you i'd observe the pv to see if any unforeseen shadowing is occurring that could account for this as even a little shadow can devastate output. if this isn't the case then send it back citing the other identical panel shows much more production indicating a faulty pv.
  • System2
    System2 Posts: 6,290 admin
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    Re: Low light 2 panel 2 behavior

    i had to wait two more days for the sun to show. The second panel wasn't working, it could not have more than 0,20 A. The first one i saw up to 4,13 A. Not looking good.

    But suddenly, i saw the light!

    I was always testing one panel at a time but for the second panel the - side was already hard connected to a Y connector i added. The + side has also a connector, but not completely fixed at this time, sometimes it was on, sometimes not. I did this because i wanted to be a smart guy and have a system i can carry and be able to plug one or two panel when i wanted and not have to look for the needed connectors. But as soon i tested the second panel without this Y connector on the - side, guess what, it is working... perhaps a little bit better than the first one.

    So now i am looking for real Y connectors for solar panel.

    Goofy ( yes, that's me)
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Low light 2 panel 2 behavior

    glad to hear it is working for you. you confirm that diagnosing things without being present is a difficult task to do and we do not always come up with the correct answers or paths that can lead to a proper diagnoses of the problem. it's good to see you reverse engineered what you did to find the problem.:D