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  • Difference in voltage

    If there was any damage to my panel, would I see a difference in voltage from the wires coming out of my panel to the voltage coming from the junction box?

  • #2
    Maybe, maybe not. Some damage can result in loosing 33% of your voltage UNDER LOAD. Measuring open circuit there is often enough leakage to show OK voltage, but power can be way down.
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    • #3
      any other reason why I am getting different readings? Would the junction box be considered a load? Would the bypass diodes be blocking it?

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      • #4
        Originally posted by steff503 View Post
        any other reason why I am getting different readings? Would the junction box be considered a load? Would the bypass diodes be blocking it?
        There should not be anything inside that junction box that would be considered a "load".

        Unconnected to anything each panel should provide close to the Voc voltage in direct sunlight. The Vmp would be measured by a CC again if the panel is in direct sunlight.

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        • #5
          So why then do the wires give me 32.7 and once the junction box is attached i only get 11.7?

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          • #6
            Originally posted by steff503 View Post
            So why then do the wires give me 32.7 and once the junction box is attached i only get 11.7?
            I would say there is something in the junction box that is opening a couple of the circuits. Maybe the diodes are bad and only allow on set of cells to produce. A "load" would not reduce the Voc by 2/3rds so my guess is an open circuit.

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            • #7
              The cells in a typical panel are all in series. The diodes will sit 1/3 and 2/3 of the way through the series string to bypass shaded sections. You are clearly connecting something wrong, or one or more diodes are bad or reversed. If you look at the panel from the front you may be able to visualize the path of the energy flow through the panel. you need to connect the positive to the negative lines to maintain continuity of the series string. In the other forum you posted a pic of the raw backside of the panel. If you can connect a jumper from to the center negative to the positive you should see full voltage at the outer pos. and neg. tabs. My guess from the numbers I see is that you have a 60 cell panel. Can you verify this?
              Last edited by littleharbor; 08-18-2018, 05:22 PM.
              2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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              • #8
                yes that is correct. I do get full voltage without the junction box. Should I then assume it is the box and get a different one?
                Am I correct that if there was a damaged cell, I would not see full voltage at the wires?

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                • #9
                  Not so much the box, but possibly the diodes, They should all be pointing the same direction. Maybe a bad one. Can you see if they are all oriented the same way?
                  Is this J box intended for a 60 cell panel?

                  As to having a bad cell, you really need to put a load on the panel to determine You need more than open circuit voltage to test that. The problem arises when you add the J box. I'd focus there.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                  • #10
                    OK I really need to understand this. Below is a pic of the wires coming out of my panel. Noted are the voltages at those wires. When i hook my junction box with only the center two wires I get the full 32.7 V. On the other panels, wired the same way, the junction box has the cables coming from the two center spots. All the junction boxes I see or have, have the cables coming from the two outside spots.
                    Is something backwards on my panel or am I using the wrong junction box.
                    Also what harm if any am I risking by only hooking up the two center bus wires to the junction box?
                    Attached Files

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                    • #11
                      The problem is your output terminals are in the center, but you have tried to use a junction
                      box expecting them at the outside. And the 3 bypass diodes must be connected to 3
                      locations (pairs of terminals) that each read about 11V. One pair I believe will be end
                      to end. The cathode (bar) goes to the more positive of the pair in each case. Bruce Roe

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