I done a bad thing: need advice diagnosing shorted PV panel (output = half voltage)
 

Play with fire and you get burned...

Play with a nice commercial PV panel & batteries without taking the time to put a fuse inline first ... and you'll eventually screw up by reversing polarity to the batteries and frying your panel. :(

The panel is a new Sharp NE80EAEJ - 80w module, 21.6 open circuit voltage. Got it relatively inexpensively on eBay because the frame was damaged after manufacturing, but the cells & glass were still OK.

So enough back story. Here's how I SNAFU'd it:

This afternoon I mistakenly hooked it up BACKWARDS (reverse polarity) to a nearly fully charged 12v battery and the instant I connected the lead I knew I done a bad thing from the size of the arc.

Now the panel is putting out exactly half its rated voltage. Fine. So I guess I blew out a connection somewhere between the two strings of cells, right?

I identified which string seemed to be isolated, and dug the potting material out of the junction box (see photo) and found what looked like a "blown" connection. After cleaning the material out and soldering it back together I now have ZERO volts output.

So I disconnected the join and I'm back to half panel voltage. And... now I'm a little confused.

I should add: the blown out connection seemed like it may have had some crumbly material in it between the copper traces - maybe silicon wafer? Was it maybe a diode?

Any ideas for me?

(I'll be away this evening - will reply tomorrow or later tonight if anyone has any suggestions or questions....)

http://ecorenovator.org/forum/attach...1&d=1239481743

Bummer.. hope someone knows what that is. I don't see how you could get your symptoms.

Sounds like you blew a bypass diode.

Maybe... except for my 0 volt situation. My understanding of the discussion you linked is that it's OK to omit a blocking diode provided a charge controller is used, and bypass diodes are optional.

I'll have to play around with it some more tomorrow. If I spend the time to actually trace & map the wiring in the entire panel, I should be able to figure it out.

But what happened when I soldered that burnt connection is a mystery - I'm confused as to why it didn't solve the problem.

I think your pot box there has the diodes in it and that you some how shorted past them? if you disconnect the leads coming from those bits of goo, you should be able to hook a multi meter up and find out what side has power and what side does not as alot of panels are 6 volt strings, allowing you to reconfigure them to 6-12-18-24 volts each.

It my have been a blown blocking diode, which the bypass diode from the other cell would see as an open and bypass, so you get half the voltage. When you soldered the connections, could that have made a short your MM was measuring? What's the resistance of it when V=0?

Ding ding ding! Except not one, but "one and a half."

Did a bit of research, and the panel does have (used to have) built-in bypass diodes.

Had to do a bit of learnin': I knew what a blocking diode was, but not a bypass diode.

Bypass diode: If part of the panel is in shade, current will back-feed into the shaded cells, reducing panel output and potentially damaging the shaded cells from overheating. A bypass diode shunts current around the shaded cell(s) (its resistance increases so the current seeks the path of least resistance through the diode). This panel has one bypass diode per string of 18 cells.

The first diode I dug out yesterday had clearly blown apart when I shorted the panel. But I dug up the 2nd one this AM and discovered it had fused together, creating a dead short around half of the panel - thus the 1/2 voltage. (Yesterday when I soldered the first diode together - not realizing it was a diode - I then had shorts around both strings, thus 0 volts.)

So I separated it the fused diode and the panel has full output again. Now I just need to replace 2 diodes ... and get a fuse.

Thanks for the tips everybody.

Very nice solar panel adventure Darin. :)

What are you going to use the panel for?

It gets used for all kinds of things depending on the season. Charging the batteries on the sailboat in the summer, the car's battery if I'm driving alternator-less, random batteries if I'm running something in the house from an inverter...