Why High measurement(s) on used panels?

balee123
balee123 Solar Expert Posts: 86 ✭✭
I measured a used panel with a volt ohm meter and have unexpected results. I'm just measuring across the panel outputs directly.


Panel Specs

Voc 24.6 V
Isc 7.6 A
Vmp 18.9 V
Imp 6.9 V


Measured

Voc 40 V
Isc 12 A


Is this possible? Could panels be damaged? I'm used to seeing values that are closer to Voc and Isc for other panels when measured this way.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Why High measurement(s) on used panels?

    Double check your meter--I have seen meters with failing batteries that give strange results--As well as meters that are broken give crazy but somewhat consistent results that leave you scratching your head.

    Otherwise, it sounds like the panels are miss-labeled or miss-wired (some have junction boxes where you can jumper cells into series or parallel connections and change the Voc/Isc).

    We have seen one set of panels here that were not per the label--and it was not obvious until we saw the light coming through the rear of the panels and figuring out that there where 2x as many cells in series as was assumed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Why High measurement(s) on used panels?
    BB. wrote: »
    Double check your meter--I have seen meters with failing batteries that give strange results--As well as meters that are broken give crazy but somewhat consistent results that leave you scratching your head.

    Otherwise, it sounds like the panels are miss-labeled or miss-wired (some have junction boxes where you can jumper cells into series or parallel connections and change the Voc/Isc).

    Miss-wiring wouldn't do it; all that would do would be to trade voltage for current or vice versa.

    His measured Voc X Isc is 480 "Watts". The published panel specs show 187 "Watts". I'm betting on a low battery in his DMM.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    Re: Why High measurement(s) on used panels?
    ggunn wrote: »
    His measured Voc X Isc is 480 "Watts". The published panel specs show 187 "Watts".


    Actually, Vmp 18.9 V X Imp 6.9 V should be 130 watts.

    These are usually 36 cell modules. I've seen a few 72 cell mods rated at nominal 12volt (2 parallel 36 cell strings) with jumpers which could be set for 24 volt use.(1 string of 72 cells) I think SunWize had some like this. They would typically have semicircular cells in 4 rows of 18 with a series or parallel jumper connection in the J-box.

    The measurements of Voc 40 V and Isc 12 A would reasonable for a fairly hot module not quite perpendicular to the sun. (oops- the amps should only be 3.5 or less)
    I don't see any published panel specs in this thread. I'd double check the meter then confirm readings on another module. (if you have one)

    Alex
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Why High measurement(s) on used panels?
    Actually, Vmp 18.9 V X Imp 6.9 V should be 130 watts.

    These are usually 36 cell modules. I've seen a few 72 cell mods rated at nominal 12volt (2 parallel 36 cell strings) with jumpers which could be set for 24 volt use.(1 string of 72 cells) I think SunWize had some like this. They would typically have semicircular cells in 4 rows of 18 with a series or parallel jumper connection in the J-box.

    The measurements of Voc 40 V and Isc 12 A would reasonable for a fairly hot module not quite perpendicular to the sun. (oops- the amps should only be 3.5 or less)
    I don't see any published panel specs in this thread. I'd double check the meter then confirm readings on another module. (if you have one)

    Alex

    I know that max power is Imp X Vmp, that's why I put Watts in quotes; since he is only reporting Isc and Voc I was trying to make it an apples to apples comparison. If a module were internally wired differently the voltage and current (whether Voc or Vmp and Isc or Imp) would both change but the product of voltage and current would not. I do not agree that a module with the specs he reported:
    balee wrote:
    Panel Specs

    Voc 24.6 V
    Isc 7.6 A
    Vmp 18.9 V
    Imp 6.9 V

    could show a Voc of 40V and (or) an Isc of 12A under any conditions. Something is amiss in either his measurements or the specs of the module. If he would show the dimensions of the module we could figure it out pretty quickly. With module dimensions and the voltage and current numbers quoted we could get an idea of module efficiency and compare it with what is "normal". One set of numbers or the other would give unreasonable results.

    The ratios of his reported Voc and Isc are off from the specs he quoted by virtually the same factor (~1.6) and both in the same direction (high). If I were to bet I'd wager that his numbers are off due to a low battery in his DMM; a low battery will push less current through the reference resistance in the DMM and skew all the readings high.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    Re: Why High measurement(s) on used panels?
    ggunn wrote: »
    If I were to bet I'd wager that his numbers are off due to a low battery in his DMM; a low battery will push less current through the reference resistance in the DMM and skew all the readings high.

    I agree.

    The only explanation I could see for the voltage at 40voc (if the meter were good) would be if there were two circuits in series and the panel temp up around 63c. Of course a "used" panel could be compromised in some way where the voltage would be lower than spec so the temp could be lower and still see 40v. On my first read I doubled both volts and amps. That was the "Oops" I went back to change. I've got no explanation for the 12 amps. Quantum module? :D:blush:

    Sorry. I guess that's what can happen when I figgure while I've got the flu.

    Alex
  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    Re: Why High measurement(s) on used panels?

    Thanks to all for the advice/discussion. The simplest explanation may be correct. My 9V meter battery was in the 4V range. Will check for sure this weekend.
  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    Re: Why High measurement(s) on used panels?

    It was the meter. Everything looks good/appropriate now. Thanks again to everyone.