Help me troubleshoot - low solar voltage

[ThereAndBack]

So the situation is that I checked my battery monitor today and noticed the expected amp in were rather low for daylight hours when we should be in bulk state. I checked the individual mppt controllers and found that one of them was not on. It had not been working for the past two days.

The reason it reports is that the voltage coming in from the solar is too low. The controller is reporting 11v.


Steps I've taken so far:
Measured the voltage of the individual panels with a multimeter (both 40v)
Measured the voltage at the output of the MC4 style parallel splitter (40v)
Measured the voltage at the solar input of the mppt (11v)
Measured the voltage between the negative solar input at the mppt and the positive solar input before the circuit breaker (11v)

Now here is the funny thing... If I flip the circuit breaker...
Measured the voltage between the negative solar input at the mppt and the positive solar input before the circuit breaker (40v)


I don't understand how the voltage is changing when I flip the breaker open. Any insights?

[sailorboy1]

Originally Posted by ThereAndBack

I don't understand how the voltage is changing when I flip the breaker open. Any insights?

When it happened to me it was a burnt MC4 connector. It still had a connection so there was voltage, it just would carry current.

[ThereAndBack]

Sailorboy1 was actually spot on.

I also learned (and didn't understand previously), that the solar controls ground out the PV - to the Batt -. They're all tied together.

To help anyone attempting to diagnose this problem in the future. I found one MC4 connector that was a lot more difficult to separate than the others. That should have been the first clue, they should come apart easy.

Then I found physical evidence upon close inspection:


Notice the bubbled plastic indicating burning (caused by some resistance). I tried to get the thing apart but ended up just having to cut it off, and I was unable to pull the metal out of the MC4 connector presumably because it has melted to that point.

Thanks to sailorboy for nailing this issue.