Panel shows voltage on its own but 0 in series.

diWhy said:

I'm making up 7 panel stings. I've come across one that's doing something I'm totally baffled about.
I can measure voltage when it's not connected to anything but if I connect it in series with just one other panel, then it outputs zero voltage.
All I can think of is there is something in the small black box the cables are coming from that disables the output when the voltage goes higher than the panel can output on its own?

Stumped.

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Check your meters DC scale to make sure that your combined series string VOC falls in the parameters

But they must be connecting since I get voltage when I test the panel on its own. What am I missing here?

Bananaman321 said:

Check your meters DC scale to make sure that your combined series string VOC falls in the parameters

View attachment 234268

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It does. As I added panels, the total string voltage kept going up by 30+ (depending on sun) volts.

diWhy said:

But they must be connecting since I get voltage when I test the panel on its own. What am I missing here?

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a lot apparently
How do you know the mc4 connector is connecting to the other connector?

Pop the cover off the black box that has the diodes in it.
Pick a lead, doesn't matter which one.

Now connect that lead to the other panel with its black box popped off too. Check continuity between the two leads at the black boxes of each panel. If its good then disconnect those mc4 connectors and connect the opposite ones. Check continuity on those wires at the black boxes.

One should show bad. If so you have found your problem. If not its aliens or @Hedges fault


*addendum* If you can't get the covers off the black boxes you can straighten out a paper clip or use a stiff wire and unscrew the threaded end off the mc4 connector after they are connected and use the wire as a probe extension for your meter to check for continuity that way.

As @Crowz pointed out just because your testing leads have made a connection doesn't mean other mc4's are, mc4 isn't a standard hence why it's advised to use pairs made by the same manufacturer.
Simply put you have two possible scenarios either the panel isn't making a connection when paired in series or the diodes are shot in either case the fix is cheap either new mc4 connectors or new diodes.

I believe the leads are ok because I also tested them with extensions and still saw the 30+ volts.
It does make sense what you're saying about different connectors so I'll check and check again tomorrow .

ThaiTaffy said:

As @Crowz pointed out just because your testing leads have made a connection doesn't mean other mc4's are, mc4 isn't a standard hence why it's advised to use pairs made by the same manufacturer.
Simply put you have two possible scenarios either the panel isn't making a connection when paired in series or the diodes are shot in either case the fix is cheap either new mc4 connectors or new diodes.

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yea I have 2 solar panels that don't have compatible MC4s, it's pretty interesting. had to cut both off and put mine on it