Solar Panel problem - diode shorted?

[Tommohawk]

So I have 2 x 100W TopSolar brand panels on the roof of my campervan, wired in parallel, connected to a Renogy Voyager charger, and 100Ah battery + inverter.
this all worked great until a week ago, and I notice I have no charging voltage. I separated the panels, and found one was working fine, but the other appeared to be shorted - pretty much zero ohms in either direction.
The junction box is potted, but I've managed to remove the panel connections - there are just two conductor ribbons on the panel. With the box removed, the panel is generating 20+ volts, same as the other. But the box itself appears shorted - I'm guessing there is a single bypass diode potted in the box which has gone bad.
So I'm wondering, can I maybe carve open the box to reveal the dud diode, and replace it?
Also, given there is only a single diode (I assume) is it doing anything with the panels wired in parallel? It wont work as a bypass presumably?
Unfortunately, assuming I can fix the panel, I don't think I can wire in series, as that would give 40V and the controller has a max input voltage of 20V.
Replacing the panels would be a real pain, as they are bonded to the roof. So repairing is partly to save ££, but also to save labour (and more eco-friendly than dumping existing ones!)
I'd be really grateful for any input.

 

[BiduleOhm]

Your analyze looks spot on

If you don't put this panel in series with other ones then you can omit the diode altogether.

 

[Tommohawk]

Your analyze looks spot on

If you don't put this panel in series with other ones then you can omit the diode altogether.

OK, that's reassuring. The only concern is that I thought bypass diodes are supposed to minimise the risk of overheating? Isn't there a risk that if I install it without any diode, current from the good panel could pass through it if it's shaded and cause damage?

 

[BiduleOhm]

No, the panel would act the same with a working diode if it's in // with the other one; it's only useful when you put panels in series.

 

[Tommohawk]

No, the panel would act the same with a working diode if it's in // with the other one; it's only useful when you put panels in series.

OK - well, that was my conclusion. You would think someone would make panels without diodes for those who only want a parallel setup then - to avoid the risk of one failing. That said, if you have a proper junction box, then diode replacement is quite easy I guess.
So next step is try and locate the diode in the existing box and remove it - which I think will be near impossible if its completely potted - or maybe just get a new junction box.

 

[dianea]

If the box is safely removed from the panel, I would destructively drill out a nice section to install a replacement diode, drill and thread in new connectors, and re-pot with epoxy.

 

[Tommohawk]

If the box is safely removed from the panel, I would destructively drill out a nice section to install a replacement diode, drill and thread in new connectors, and re-pot with epoxy.

Yeah I'd like to keep the junction box if I can - but I think drilling it out could make a right mess. If I new what the innards looked like it would be easier. Also, as above, is there any point in a diode in this setup?

 

[BiduleOhm]

If I new what the innards looked like it would be easier.

If you have a dentist or vet friend that can easily be done with a quick x-ray :D

 

[Tommohawk]

Nice idea... sadly I don't know anyone with an x-ray!

 

[Tommohawk]

Right - just to tidy this up! I guessed that the diode might be in the centre of the 3 horizontal ribs in the above pic. So I drilled with a 5.5mm right there and struck gold! It was difficult to see what I was actually drilling but it appeared grey/ceramic. At a depth of about 6mm I could see what remained of the connector wires either side, and nothing remaining in the middle. I checked these stubs individually - each was connected to the respective output, and more importantly no longer connected to each other... if that makes sense.
Checked the output with a V meter and all good! Had a bit of fun with a dry solder joint on my first effort at reassembly, and usual mess with sealant but all OK now. My only dilemma is whether to drill out the other one just in case it shorts too.
thanks for the input folks - it was very helpful!