question

marleekreative avatar image

75/15 input voltage incorrect

Hi there. I have been running my 75/15 in my car for over 12months, I have a 250w 36v PV on the roof, and up until last week, it run perfectly, keeping my crank and AUX batteries fully charged. My monitoring system alerted me to the fact my battery (AUX) was almost flat, so I checked out the system and found the PV input was only reading 13v, it should be reading 36v or so. Further investigation just now, shows 36v at the panel, 36v at the cables into the 75/15 when they are unplugged, when they re connected however, it only reads the 13v which I assume to be battery voltage. I am 99.9% sure this PV input reading is always 36v-40v when its daylight, irrespective of that, my batteries went flat.

Anyone offer some advice on what could be wrong here? Should I factory reset my 75/15 or is there another option or something I am missing.


TIA

voltageinput current
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2 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Hi M,

You most likely have a high resistance joint/crimp/MC4 on the PV side.

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I did check the cable and the crimps. I test 36-40v at the PV, on the controller side of the MC4's AND to the end of the cable, removing the cable from the controller. So it appears to me that the controller is turning down the PV voltage..

So it appears to me that the controller is turning down the PV voltage..

Or you could look at it the other way... The high resistance joint can not allow enough current to pass through it, and the voltage drops off.

You can test the wiring with a light globe (in place of the mppt)..

I do have another brand controller capable of handling 36v PV input, so I'll see what it offers.

marleekreative avatar image
marleekreative answered ·

I found the solution. When I ran the cable from MC4 into the car, through a bung in the floor, the cable was too short, so I put in a soldered connection (which I had completely forgotten about), and during testing and troubleshooting this weekend I found it again, I found it the be the cause and corrected the join. No more heat shrink in line solder joins (you know the ones, where the heat shrink contains a small round of solder), it is now twisted AND properly soldered. @klim8skeptic, that would explain why I was seeing the full 36v on my multimeter, but as soon as the 75/15 started to draw power from PV to charge the batteries, it failed as this dodgy join was, well, a dodgy join.

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