Determining Positive vs Negative wires

Bob Bullis

New Member
I need some help. In this photo to the left you can see my PV wires running from my roof panels showing both positive and negative wires in red and black respectively. On the right you can see my leads from the other side of my van connected to my MPPT 1-5kva. Notice both wires are black. When I installed these wires, I taped the positive wire so I could tell which wire was positive on the end of the wires without terminals. I did not tape the positive wire on the other end as shown in the photo because they had terminals already attached. Now I'm not sure which is which and I need to put the correct connector terminals on the red and black wires. My question is: how do I determine which terminal goes on which wires? Can you tell from the connectors which is the positive wire? The wires are not labelled. I'm afraid I'll blow up the MPPT unit if I do it backwards. Can someone help me out?
Bullis PV wires.jpeg


Participation Medalist
Disconnect the wires at the SCC end.
Strip your solar panel wires so they can make contact in your MC4 connectors as shown.
With a DMM at the SCC end, see which is positive, which is negative. This might require two people, especially to hold panel wires to MC4 connectors.


Solar Addict
Use your multimeter. there is a setting that will buzz when a circuit is complete. You may have to use an extension wire to get both ends of the wire you want to test.

Bob Bullis

New Member
Thank you both for your advice. I easily found the polarity with a multimeter and the panel was producing 12.5 volts. After reattaching the PV wires, to my MPPT nothing was happening perhaps because the unit had shut off due to low battery charge. So, I plugged in 120AC to charge the battery and the entire panel is flashing warnings and a warning beeper is going off. I cannot figure out how to reset the unit. I've turned it on and off to no avail. Is there somewhere to trip a reset? Not sure what to do. Manual is useless for solving the problem. It doesn't even say what all lights flashing means and doesn't mention the beeper alarm.


Participation Medalist
I've turned it on and off to no avail.
You did not specify your exact procedure but I strongly recommend that you disconnect the solar panels so that the battery is connected first. Some SCC's set their operating voltage on startup if they have an "auto voltage" feature (setting the actual voltage, i believe is a way to prevent a wrong charge voltage from occurring while messing around with things).

And what is the battery voltage?

Do you have any info on what SCC you have? Does the manual indicate what the lights flashing indicate? Are any of the lights labeled?


I'm a little unsure about this. Figuring out the polarity of the wires coming off the solar charge controller using a voltmeter sounds a bit iffy. The solar charge controller shouldn't be sending power down these leads, should it? But since you got a 12v signal, I guess it's sending power down the wires.

I had to figure out the polarity of the leads coming off my solar panels with a voltmeter.