Smartshunt deviates % badly at low Soc

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seriusrod asked Jan 13, '24

Smartshunt deviates % badly at low Soc

I have a home system with a 18s LiFePO4 560Ah battery. It has a smartshunt for Soc calculations an everything goes through it. DVCC is active and the smarshunt is configured as the battery monitor. Voltage and current share is active. And the BMS is not configured as controlling.

The ve.configure sustain voltage is 52.74 and disconnect at 51.3 (as well as cut-offs).

What I've found today is that the system entered maintain at the minimum 14% configured, but the battery voltage was as high as 58.9v. That is 3.27 per cell.
Consumed AmpH was reading -348.2Ah and to be coherent with the behavior the battery should had 404Ah capacity.

In fact, the two JKBMS where better calculating the Soc at 40% than the shunt.

100% sync is working ok, but it seems to deviate while discharging. I'm attaching the SM config below. Does anyone know if it is something that I'm configuring wrong?

PS: Sorry, I can't attach anything. The web always responds "No link in attached response".

Battery capacity: 560Ah
Charged Voltage: 61.2v (Absorption is 62.1v)
Discharge Floor: 10%
Tail current: 3%
Charged d. time: 3m
Peukert: 1.05
C. Efficiency factor: 98%
Current threshold: 0.10A
Time-to-go avg period: 3m

SOC

2 |3000

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Modifications Space

Node Red Space

Answer 1 · 2024-01-18T11:57:44Z

matt1309 answered · Jan 18 at 11:57 AM

Have you set min SoC in ESS settings?

I wasnt aware that the system used SoC for entering "maintain"/sustain based on SoC unless set in ESS settings.

I would say maybe reduce your peukert exponent closer to 1 maybe 1.02 and increase efficiency to 99% however I thought the behaviour of entering sustain was based on voltage.

One thing to check is that you weren't drawing a large current at the time (this will drop the voltage temporarily when it's under load which may have caused it to dip under maintenance) tbh the voltage differences you mentioned with sustain being 52.74 and battery at 58.9v i doubt this is the cause. I suspect there's a SoC limit set somewhere in ESS settings

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seriusrod commented · Jan 18 at 01:46 PM

Thanks for your help, Matt.

Yes, a 14% Min. Soc is set at ESS. What I was complaining about is that while discharging, the smartshunt Soc value lowers more than the real value. So when it arrives to real 40% it is showing 14%. So ESS cuts battery discharge.

Since then, I've already raised the efficiency to 99%. I might try reducing the peukert. I've configured like this based on Victrons documentation for a known LiFePo batteries brand. But It's true that most other installations I've seen have it lower, at 1.03 or 1.02.

Also I have the Soc when bulk finished at 95, as a fellow's recommendation.

I find that the documentation for all those settings is so limited. For example none of them specifies what they do while charging as well as while discharging.

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matt1309 seriusrod commented · Jan 22 at 06:15 PM

Ahh i see that makes sense. Yeh I'd definitely try tweaking the peukert. I set mine at 1.03, I was the same originally set mine to 1.05 and asked a similar question to you then went right down to 1.01, both were causing odd behaviour, 1.03 seems to be the sweet spot for me.

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