MPPT 75/15 low input voltage

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Emanuele D asked Sep 12, '21

MPPT 75/15 low input voltage

Hello to everyone,

I'm writing because recently I encountered an issue with my solar system (correctly working untill some months ago) and I hope that someone here can solve it.

My setup is as follow:

- Semi flexible MONOCRYSTALLINE solar panel 105W with this characteristics

V Pmax (V) 20,16
I Pmax (A) 5,26
Pmax (Watt) 105,99
Vca (V) 23.04
Icc (I) 5,63
Eff. cell % 15,42

- Victron energy MPPT 75/15

- Green Power AGM Battery 12V 100Ah

I live in north Italy and the results of a test made today at 2PM are these:

Mutimeter voltage directly out of the panel 19.6V

Current directly out of the panel 2.6A

Multimeter voltage at the end of the cables 19.5V

Current directly at the end of the cables circa 2.6A

The I connected the cables to the MPPT 75/15 (with battery not fully charged 12.8V connected and no loads) solar voltage displayed by MPPT is circa 14.6V, the same result with multimeter directly on the MPPT terminals. Current showed is 0.3A.

So the MPPT doesn't charge the battery..

After that I disconnected the battery from the MPPT and the voltage displayed is 19.2V and 0A.

Fuse is ok.

What could be the problem? Damaged solar panel or MPPT??

Thank you for your answer.

MPPT Controllers voltage input current

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Answer 1 · 2021-09-12T15:54:49Z

Alexandra answered · Sep 12, 2021 at 03:54 PM

@Hardee87

Not enough solar power.

So with resistance inbetween the pv and mppt you simply dont have enough pv to get 5v above battery voltage to get charging going.

You should be getting the v max from your panel with nothing connected, so either the irradiation in your area is way lower than the STC or the panel is hot so under performance (derating) is already an issue.

Dont forget the battery and the mppt itself are a load.

The 5w you are seeing is the power the mppt is making for its own consumption so it is trying.

If it was working and now not, then the suspect is the panel since the flexible ones are not great lifespan wise anyway.

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Answer 2 · 2021-09-13T18:26:45Z

Emanuele D answered · Sep 13, 2021 at 06:26 PM

Alexandra thank you for your answer.

As I sad I measured the voltage directly on the panel and it was 19.6V, not so bad. The battery was 12.8V, add 5V for a total of 17.8V and the regulator should work in this conditions. Voltage at the end of the cables is 19.6 so no voltage drop in between panel and regulator.

In August I was in Sicily and the sun was beating a lot, the system didn't work.

By the way today I bought a cheap PWM regulator and tomorrow I'll install it and see if it works.

Many thanks

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