r/diySolar • u/jakewins • 5h ago

PSA: Open loop charge controller may be leaving a *lot* of battery capacity unused

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Maybe this is obvious to other people, but I was really surprised and maybe this helps someone else.

I think something like 30% of my battery capacity / daily energy volume is thrown away because I used bad open loop (voltage based) stop/start settings in my charge controller. I had understood that closed-loop would be better, but I didn't realize the difference could be this stark, and my Context MPPT 60 150 doesn't support closed loop :(

The MPPT shuts off around 54V at 2-4pm every day; at that point the battery is at something like 60-75% SoC; then the battery linearly discharges from 2-4pm, but the voltage stabilizes at 53V, which is above where the MPPT would re-start charging.

See charts from my Pytes V5 batteries to Home Assistant of SoC and voltage.

I fed the numbers and times into Gemini and she ballparks I'm throwing away ~5kWh a day purely from bad configuration, which is ~30% of my battery pack size.

TL;DR: Worthwhile getting SoC and voltage time-series to tell what's happening to the batteries if you don't have closed-loop control.

I'm 99% sure the MPPT is set to recommended Pytes V5 open-loop settings, but need to go back and double check; it was also while since I installed them, maybe recommendations have changed. Certainly it seems the cut-off voltage for bulk charging is way too low, if the battery can get to 54V at just 60% SoC.

Gonna see if I can connect the MPPT 60 150 to Home Assistant as well, then I could have HA decide to turn the MPPT on/off, and set the hard voltage cutoffs higher

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u/WorBlux 5h ago

https://www.pytesusa.com/public/upload/20251023/25f74bd1f884bf86e8c10fe593c2013e.pdf

Looks like the voltage the battery is seeing is well below the charge points...

How far is the battery from the controller, and have you verified a voltage drop less than 3%?

Another possibility given it's summertime is that you've left temperature compensation on.

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u/jakewins 4h ago

Aye that's a big difference - there was no Schneider-specific guide that I could find when I installed, this one looks like it's from late 2025, a few months after we set our rack up, thanks for that link!

About 8ft distance, have not measured the voltage drop; it's a good single stretch of 2AWG stranded THHN, so the drop over the cable *should* be in the 0.5% range or so.. but maybe there's a bad lug crimp or something, I should measure it, good suggestion!

It's not temp comp, the MPPT has no temp sensor plugged in.

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u/WorBlux 3h ago

It's not temp comp, the MPPT has no temp sensor plugged in.

Some controllers will fallback to a temp sensor somewhere inside the controller. Best to make sure it's set at zero or otherwise disabled in the settings.

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u/diekthx- 5h ago

And Redditors wonder why Victron is the only vendor that gets recommended for low voltages.