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jdsquared asked

Losing voltage at MPPT

I have three 330 watt SunEdison panels in series hooked up to a 150/45 MPPT, and a 48v (16 cell) BYD battery. I get 113 volts from the panels when not connected, but as soon as it's connected to the MPPT I show about 73 volts average. I did get a max voltage of 81.68 earlier according to my Victron Connect app, and a max of 412 watts on a full sunny day. Same numbers yesterday.


Are these expected numbers? I had 8 250v panels connected for three years, I guess I never noticed a voltage drop, or paid attention. But we crushed those panels in an accident, so I have replaced them this week with the above used panels.


The batteries are charging, but slowly. They are not reaching an absorption charge.

I check them all in pairs, and the voltage goes from 73 volts to 55 volts. Which wasn't enough to float charge the batteries. Which I just realized is bad, because I wanted to get one more panel to make it series parallel.


I guess it's always been like this, I was looking back the past month and my history, and I was getting 58 to 60 volts average, when it should have been twice that with the other panels.

MPPT SmartSolar
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3 Answers
anthony avatar image
anthony answered ·

What is the VOC and VMPPT of the 330W panels you are using?

Have you made off any of the MC4 connections on the solar panel cables?

What country are you in and what are the weather conditions?

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jdsquared avatar image jdsquared commented ·

Voc is 46.2 VPT is 37.7

I am in southeast USA. The sun is low, but strong all day.

I have a bunch of cables, two brand new, and there's no difference in voltage through any of them, in any length (i've combined a few together to test.)


Just to clarify, I am getting 113 volts unconnected, just using the meter in the ends of the connectors. So it's when I connect it to the MPPT it drops considerably. The Victron distributor says that's what happens under load, but going from 113 volts to 73 is a huge drop, isn't it? I have a fridge hooked up through an inverter, and everything else is off or 12 volt leds and fans. If I unhook the inverter, the MPPT goes into absorption mode, even though my battery is down to 49 volts

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jdsquared avatar image jdsquared jdsquared commented ·
The long term problem, too, is I am not getting enough charge from 3 panels, and need to go to four. But connecting a 4th in series is too much voltage for the MPPT, and 2s2p, the voltage drops to 55 volts in the MPPT, which won't float charge my batteries (unless it's 5v above the battery SOC
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kevgermany answered ·

Guessing, from your voltage figures, that at least one of the panels is faulty. Try to check by connecting only two in series at a time. The bad panel is the one that makes no difference or gives an improvement when it's out of the system.

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jdsquared avatar image jdsquared commented ·

I kind of stated that in the question. I've checked them individually, all at about 33 volts, and in pairs, which adds to 73v, but drops down to 55 from the MPPT.

I also got sent to a local distributor, who just says that's what happens and it's working.

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4pierre answered ·

are they used panels? I would support the above hypothesis that you have 1 bad panel in there. Voltage alone might not tell you. If you have an amp meter you could take and short the each panel to itself and measure the current on each. The one with largely lower current would be the culprit. Otherwise if those are 72 cell panels try just two in series, if no two in series will charge in full sun, you have two panels with a problem. Next If its not the same wiring as before or that was damaged in the crushing, it could be that undersized wire is the loss of power. There are good voltage drop calculators that wouldn't hurt to punch in your wire, length, voltage and observed current to see if you are in a desired V drop range.

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