SOC measurement issues

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nickdb asked Oct 20, '20

SOC measurement issues

With a BMV712, shunt that is correctly wired and an ESS MP2/Smart solar MPPT setup (victron gel batteries), I have continuous issues with reported SOC and what it does with system performance.

I only use the top 30% of battery for use overnight.

On a good day, SOC gets to around 97/98% when the shunt will sync the SOC to 100% (my shunt charge voltage is midpoint between float and absorb at 55.4V).

The problem comes in if there has been a bad light day, or like now in warm temps (31C) the inverter decides to not do a bulk/aborb cycle as the voltage is at a temperature adjusted float voltage and can't trigger the sync. The only change in the environment is temperature and with increased cooling loads ESS hits the minimum SOC threshold very early in the morning.

The inverter stays permanently in float. Unless I manually sync the SOC, it will continue to slowly leak, which means there is less discharge by night.

I have several installs all doing the same thing where before the increase in power required at night and the increase in temps it worked fine.

Trying to get assistance with this is near impossible.

The shunt should be able to do a better job of measuring the SOC.

Multiplus-II battery charging ESS SOC

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klim8skeptic ♦ commented · Oct 20, 2020 at 12:16 PM

Trying to get assistance with this is near impossible.

Well somebody designed and installed your system.

Can they not help?

What was the design parameters of your system?

Your batteries do have a little wriggle room regarding their absorb/float voltages..

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Answer 1 · 2020-10-21T12:39:40Z

JohnC answered · Oct 21, 2020 at 12:39 PM

Hi @nickdb

If your conditions change, change your settings. Particularly the 'Charged V', for that I use 54.5V for hot weather (below Float). I see no point in syncing while still in Absorb, because it hasn't finished the main charge. Then of course a suitable 'Tail' setting, where it has actually come close to a flat tail. Then set to sync there. The default Tail setting isn't useful for me - way too high.

If your SOC is pushing downwards continually from one day to the next, then try changing the Charge Efficiency upwards. If you get that right, it won't head straight to 100%, but taper off to it as the Tail current tapers off.

These devices can give a great SOC result if set up correctly. Manual syncing should be a rarity..

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nickdb ♦♦ commented · Oct 21, 2020 at 01:06 PM

@JohnC

The problem with lowering the charged voltage on the shunt is that with solar, if the voltage drops or surplus for charging is limited then it is easy to trigger a false sync.

I could try extending the duration and really lowering the tail current (I'm at 1,8% now for gel) but it would still be easy to trigger the thresholds.

I will adjust the efficiency and see if that helps at least with the gradual leak.

I was trying to understand what about temperature has triggered the deviation from "normal" functioning.

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JohnC ♦ nickdb ♦♦ commented · Oct 21, 2020 at 02:38 PM

@nickdb

A false sync is likely too high a Tail setting.

A higher ambient temp will greatly increase batt reactivity, so everything happens faster. This may be relevant? Efficiency and Peukert may change too, I don't know which way nor how much.

My own true tail is probably close to 0.4% (floodeds). Impractical down that low, and I ran 0.6% for a while. Works better at 0.7% as I have a little variability in my Amps. I find the Trends graphs in VictronConnect great to determine that.

I use 2.7% to drop my mppt from Abs to Float. By then my SOC is ~99%, so the job is mostly done. Then I never see V above Float, so a sync V must be lower. The Tail lowers too with the lower Charged V, so consider that too.

I've had cloudy days when I've never reached Abs, but have made 100% and met sync parameters about the same time. I once worried about that, but no longer.. I trust it now, and it's surprisingly reproducible.

You'll get there.. good luck.

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