SOC jumps to 100% before tail current setting is reached.

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curtis1222 asked Dec 29, '22

SOC jumps to 100% before tail current setting is reached.

I have a small ESS system consisting of

1.6Kwp of PV via 2 MPPT 100/20 controlers

Multiplus II 48 3000

BMV 702

I keep having issues with the SOC jumping to 100% too early resulting in the SOC becoming inaccurate, the batteries discharging too much then cutting off and sustain kicking in. Looking at the VRM data I can see that when I charged the batteries fully with the multi the SOC jumps to 100% even though the tail current hadn't dropped below the setting in the BMV. The BMV is selected on the venus device as the chosen battery monitor.

Pictures below include the graphs and BMV settings. My understanding of the maths behind it is that the current needs to be less than (124 * 0.04) = 4.96A as well as voltage over 52.8V and 3 mins of both values being true.

Is there something simple I am missing?

ESS BMV Battery Monitor SOC

screenshot-2022-12-29-at-175115.png (305.5 KiB)

screenshot-2022-12-29-at-175325.png (717.0 KiB)

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JohnC answered · Dec 30, 2022 at 03:55 AM

Hi @curtis1222

With settings like that, you're inviting premature sync with open arms. Try to understand the dynamics of what can happen. Even at solar startup in the morning, or when a cloud passes over.

Let's take the cloud. Do your Amps sometimes go below the 4% with a heavy cloud passing? Maybe not every day, but when it does, does the Vbat stay above 52.8V for 3 min? Bingo..

Pb batts, these are my settings.. Just enough to avoid the 'cloud sync'. The low Tail is my summer setting. Raised to 0.7% in winter, YMMV.

Your high Peikert and CEF probably need a little work too, to avoid a 'natural' early rise to 100% SOC.

1672371933241.png (24.7 KiB)

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Modifications Space

Node Red Space

Answer 1 · 2022-12-29T19:06:48Z

pwfarnell answered · Dec 29, 2022 at 07:06 PM

The logic you understand for synchronization to 100% is correct ONLY if the tail current is reached before the Amp hour counter has got to 100%. In your case, the BMV has counted back enough Ah in about 15-20 minutes. However, the current plot shows that the starting SOC should have been much lower. I can think of 2 possibilities, the first is that you have some loads connected directly to the batteries that are not measured by the BMV. The only connection to the battery negative MUST be the shunt, nothing else, not the ground connection. All negative connections to all loads, charging sources and ground must be to the system side of the shunt. The other thing that could be changed is the charge efficiency factor, reducing this to say 85% or 90% may help but in my view, your error is larger than this, check the wiring first.

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curtis1222 commented · Dec 29, 2022 at 08:21 PM

Thanks for your quick reply, I was not aware of the algorithm using the Ah counting as well, the more I think it does make sense. I have no loads/chargers on the battery side of the BMV although I did as you suggested and checked connections. I found 1 slightly loose battery connection and also found there to be a 0.4A charge indicated by the BMV with no batteries connected.

I have now calibrated the BMV and checked the tightness of all connections, I shall monitor the system over the next couple of days and see if this has helped to resolve the issue.

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Answer 2 · 2022-12-30T02:42:53Z

kevgermany answered · Dec 30, 2022 at 02:42 AM

Your graphs show the values from the system, not the BMV. You need to select the BMV in VRM custom widgets.

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Answer 3 · 2022-12-30T10:05:05Z

curtis1222 answered · Dec 30, 2022 at 10:05 AM

Thanks for the replies, I have calibrated the BMV as it was showing 0.4A charge with nothing connected. Below is a charge from this morning which looks much better.

As John said I need to fine tune my settings to remove the jump to 100% but at least now it’s reaching the set tail current before the Ah has reached 0

63814cd9-4edc-437b-bd72-2f55ddce639d.png (346.1 KiB)

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pwfarnell commented · Dec 30, 2022 at 10:33 AM

Drop the tail current down to 1% or just under as others have mentioned, mine is 0.8% for AGM, the SOC will then increase another 3% or so before synchronisation.

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