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On my sailing boat we've installed four solar panels (two times Vmp = 17.64V, Voc = 21.6V, Imp = 5.67A, Ioc = 6.07A, Pmax = 100W and two times the same but 75W), which we connect all in parallel, then to an MPPT system from Mastervolt which serves the 12V batteries and autopilot.

The solar panels are not exactly equal in voltage (Voc 21.6V and 21.2V) but in parallel that averages out. They're 2nd hand and were in working condition.

When we connected them in parallel, we accidentally had the system through the MPPT Mastervolt connected and all four minuses (3 panels, 1 Mastervolt) got connected with one solar panel's plus. Accidentally (we did inspect the voltage and pluses prior, but still screwed up). Then we connected the rest, all pluses (so we thought), saw a spark, disconnected, reviewed, and found that one panel didn't give voltage anymore. The bypass diodes (it had two) were burnt and split in two.

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I read somewhere that short circuiting solar panels should not break them as the Imax can never get higher, but apparently we broke it anyway. This lead to some questions:

  • clearly these were wrongly connected, what happened here and why did it fry the bypass diodes? There was no shadow, current should not flow through the bypass diodes, right?
  • without bypass diodes, good panels should still give power, correct? [After frying, that panel gave zero on V meter. Is it possible one (of three) bypassable cells was already broken and the bypass was always in use?] Edit: this was wrong, they still have 20+ V.
  • these exact type schottky diodes are not available anymore, is it safe to select any diode with the same or higher specs (these were 80SQ045, rated 8A, 45V), as long as it is 'schottky' type? If I install two diodes in parallel (there's room), does that improve performance?
  • could it be burned solely because it's 10y old? The other panels seem still fine.

enter image description here

I tried to find answers to the above online and did find how these things are supposed to work, and that they break, but no info on how or why it breaks, let alone what happens when you screw up like we did. Hopefully someone here can help solve the mysteries or if not, help with the main question, selecting proper new bypass diodes.

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    \$\begingroup\$ You wired some things up wrong and there was smoke and now you create false assumptions to question why things failed? Clearly significant current flowed and the diode failed. It's not about age or whether there was sun or not. If you have any doubts, replace the diodes and repeat the experiment but this time measure the current. You will have your answer. \$\endgroup\$
    – Kartman
    Apr 11, 2021 at 9:14
  • \$\begingroup\$ @kartman Not 'false' assumptions, on this very site, there's a post that says that short circuiting does not damage solar panels, generally. I short circuited (stupid, sure), and things broke. If max current in these solar panels is capped at 6A, I'm still confused why the diode broke, rated 8A. And more importantly, what new diode I should get. \$\endgroup\$
    – Abel
    Apr 11, 2021 at 9:30
  • \$\begingroup\$ Also, I like to learn from my mistakes. If it should break, fine, but please help me understand. If it shouldn't break, then what is a likely cause so I won't repeat it (this part didn't have fuses, now they will). \$\endgroup\$
    – Abel
    Apr 11, 2021 at 9:33
  • \$\begingroup\$ You didn't short circuit the solar panel, you connected one backwards against three others. The current from those three solar panels was enough to have excessive current flow through the diode. The diode conducted because it was wired backwards (along with the panel). When diodes are overloaded they usually fail short circuit. If there is enough energy they overheat and the epoxy splits. I frequently repeat tests where things have blown up to determine the exact mechanism. See your friendly solar company for suitable diodes. Just about every solar panel needs one. \$\endgroup\$
    – Kartman
    Apr 11, 2021 at 9:41
  • \$\begingroup\$ @kartman, thanks, makes sense now, guess you could post that as answer. I couldn't find the info from the company. Hence I wondered if it's fine to replace schottky with (other) schottky, same type, same Vf, A & Vmax values. My guess is: yes, as these are just functional (redirect current only), and not part of a micro controller or something. \$\endgroup\$
    – Abel
    Apr 11, 2021 at 9:55

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You didn't short circuit the solar panel, you connected one backwards against three others.

The current from those three solar panels was enough to have excessive current flow through the diode.

The diode conducted because it was wired backwards (along with the panel).

When diodes are overloaded they usually fail short circuit. If there is enough energy they overheat and the epoxy splits.

Even though the diode is rated at 8A, they are also thermally limited. You might find if you pass 8A through one, it will get too hot and die. Further on in the datasheet there are other specifications that also apply, so you can't simply say that the diode will pass 8A and survive. It would require heatsinking to achieve that value. As well, the 8A spec is 'absolute maximum rating'.

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  • \$\begingroup\$ Excellent, thanks, that explains a lot. If the diodes are absent, and the solar panel works, there should be a current over the outer wires, right? I mean, they only bypass if one strip of cells has shadow, or fails. Meaning to ask, without bypass diodes, there should be a current (weak current if some cells have shadow). It also seems I should better get 15A diodes for replacement, to be on the safe side. \$\endgroup\$
    – Abel
    Apr 11, 2021 at 11:34
  • \$\begingroup\$ Actually, I just had a chance to measure that assumption. The panel with the broken diodes 'just works'. Which answers the other half of my confusion in the original question, and now I'm less worried I've to replace the whole panel. \$\endgroup\$
    – Abel
    Apr 11, 2021 at 11:41

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