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flyingfishfinger
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Joined: 12/09/2020
Location: United States
Posts: 118

Posted: 10:27pm 07 May 2025

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Hi,
I thought I'd break this out of the MPPT thread and show some fun stats along with some questions. I built this a few years ago, it's been running since 2022 at low power levels and occasional usage, plus a spectacular explosion last summer that I posted about in poida's original MPPT thread.

We got Starlink at our offgrid site, so now I can pull data and visualize it locally. A Raspberry Pi queries the MPPT using poida's special "^" commands to get data, sends them over SSH to a local file at my apartment where another Raspberry Pi plots it out to a little HDMI screen I can look at.

Here's a screenshot of 48 hours of data (ignore the hours on the x axis, I didn't rescale it and it's built for 24 hours). In the voltage plot, red is battery voltage and blue is solar voltage.

I labelled some interesting trends that map to MPPT settings in green, along with a bit I don't understand just now in orange.

You can see that as the ambient temperature increases, the temperature offset slowly drops the Absorb target voltage. You can also see that after 3 hours (current setting) in Absorb, the MPPT transitions to Float before it's retriggered.

You can also see that when nobody is there, we hardly use any power at all (Starlink draws ~30W and our bear fence maybe only a few W), so when I had the current limit set to 5A (first day shown) I barely spend any time at all in Bulk in the mornings before power starts dropping in Absorb mode!

But, why does the input voltage jump up at first, then drop back down, then pop back up in the mornings and evenings?

EDIT: I think I know why. I have "NIGHT" set to 15W, so anything between 0 and 15W generates no load on the cells and therefore the voltage goes up until 15W is hit. Then, because there's little light it can't generate much power yet so the voltage drops again until there's enough light to bring it back up to near Vmppt.

Second question, why does the solar in voltage have a bowl shape to it around midday?

Thoughts and observations are welcome!

Cheers,
R

Edited 2025-05-08 09:55 by flyingfishfinger

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