Junction box help

I need help please!
Can anyone tell me why I am getting half the voltage from my panel once the Junction box is attached?

Comments

  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Can you tell us a bit more about your system/setup?

    In general, solar panels are current sources--They output (for example) 5 amps into the load. If you have a PWM type charge controller (the simpler/less expensive controllers), then the Voltage at the solar panel/array will be that of the battery bank when charging (i.e., panel voltage will drop to ~13-16 volts). If you have a Vmp~30 volt panel or higher, it will only be ~battery voltage).

    It has happened that people miswire the solar panels when connecting them in series (2 panels in series, add 3rd panel wired backwards)...

    There can also be other issues (misprogrammed MPPT type solar charge controller).

    If your panel is ~22 volts in sunlight (typical Voc -- voltage open circuit), and you connect it to a PWM controller (or directly to the battery bank), you will see ~13 volts (or even less if the battery is well discharged/you have DC loads on battery bank). That is perfectly normal.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    It is not hooked to anything. I got 7 panels used. One is missing the junction box. 24V, 170W.
    When I put my meter to the bus wires I get 32.7 V. Once I attach the new junction box I only get 11.7 V.
    Someone thought the panel might be damaged and the bypass diode is reflecting that, but why would the wires show full voltage?
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Junction boxes typically contain 2 or 3 bypass diodes. I you have the wiring backwards, the diode(s) will short out that section of panel.

    If you have a bad diode (diodes can fail either open or sorted), that will cause a similar problem.

    https://www.solar-electric.com/learning-center/general-solar-information/solar-power-technical-tips.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    edited August 17 #5
    I get all that, my  question is would the voltage be different at the bus wires coming from the panels vs after attaching the junction box and measuring  the output of the cables?
    When you say wired backwards, 4 strips coming out of panel, 4 clips in junction box.
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Your markings on the panel appear to be correct... You should get the ~22 volts Voc (voltage open circuit).

    The question is the J-Box... Does it include diodes (2x)? Have you used the diode check function on a DMM to see that the diodes are in good shape (not shorted or one installed "backwards")?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    The wires themselves actually put out 32.7V.
    DMM? The junction box actually had 6 diodes
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    edited August 18 #8
    Set your meter to 10 Amp full scale current, then connect to A-B, B-C, C-D, and A-D ... See what current you read (panel should be in full sun, noon-sun and you should get ~7-9+ amps in full sun. It is possible you will get 1/2 that if not noon-time and/or a little haze. Mostly I am looking for similar Amperage for each test.

    By the way, the voltage between B-C, is that ~11 volts (picture cut it off).

    And what is the cell configuration on the front of the panel (3x16 cells or something else)?

    -Bill

    PS: DMM = Digital Multi Meter
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    edited August 21 #9
    the voltage from B-C is 32.7. These are 24V panels. The voltage written on the back was with some cloud cover when we got 10.9 and 11.1.
    Cells configuration is 10x6 cells.
    So on a whim we connected only B and C wires to the junction box. Got the full 32.7 Volts. The other panel all have the cables from the junction box in the two middle spots. This new box and every other Junction box i have seen has the cables coming from the two outside spots. Is my panel wired backwards or do i need a special junction box?
    What risks am I taking if I only hook up those two wires?
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Some panels (few, some, nearly all???) panels could require that "B-C" be shorted together to get String A-B (~16 volts) connected to String C-D (another ~16 volts). It sounds like the two strings are electrically isolated until you connect them at B-C.

    The diodes would go across A-B and C-D (reverse biased--Not conduct current under normal conditions)--Assuming the above guesses are correct.

    But things do not really sound "right" above... It appears you could have three series strings... If A-B and C-D are ~11 volts... Then I would have thought B-C would have been ~11 volts too... 

    If you set your meter to 10 volts full scale. What current do you read when A-B, B-C, C-D are each tested with your meter (sun on panel, don't really care if noontime full sun, or other sun/angle... They should all read about the same current).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    or were they using a special type of junction box? This is the junction box on the other panels.
    we have the meter on 220V. the other choice is 2V. We get A-B, 10.9 and C-D 11.1 and B-C 32.7-in the sun.

  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    10 Amp full scale current.

    Not volts scale.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    I guessi should add, measure A-D too.

    Both voltage and current.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stff503stff503 Posts: 8Registered Users ✭✭
    I do not have a DC amp meter
  • stff503stff503 Posts: 8Registered Users ✭✭
    A-D got nothing
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