Synchronisation of SOC with Multiplus switched off

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rue-1 asked Jan 26, '22

Synchronisation of SOC with Multiplus switched off

My simple System-Set-UP:

Multiplus 1200/24, Lithium-Batterie with a simpel BMS without CAN, MPT100/20 Raspi 3B with Venus OS 2.80 /different Versions, now Large with Node-Red

Problem: During winter I need several day to charge the battery only with MPT - Multi is switched off and disconnected from grid. The battery-SOC rises slowly from day to day.. after switching on the Multi the SOC of the Multi is read out, which results in nearly full battery by voltage but an SOC of 50%, which is the discharge-limit in winter...

Solution?

From Version 2.80~33 there is an Update:

"Extra battery current: for systems without a battery monitor but with Solar chargers, the current as reported by the solar chargers it synced to the Inverter RS or Multi RS, where they'll use it for their SOC calculations."

So far I remember my problem was fixed with that update - today I'm not sure. Because I switched to Venus OS Large and had the problem again with actual SW V2.80~41-large-25. Is it planed, that the changing SOC due to solar charging via MPPT is

caculated by Venus OS and send to Multiplus after switching on?

Thanks for all the great work an possibilities, which is offered here...

Rue

Venus OS

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Modifications Space

Node Red Space

Answer 1 · 2022-01-26T20:12:29Z

bathnm answered · Jan 26, 2022 at 08:12 PM

The SOC sync works by syncing the MultiPlus (Ve.Bus) SOC to the SOC from the battery monitor the Cerbo is using.

If something else is happening then (1) do you have up to date software on the MultiPlus, (2) check with the newer beta release.

Please note that the Inverter RS or Multi RS change referenced is to allow the SOC updating to work for those new products, The SOC update from Cerbo to Venus OS has been working and in place for some time, well since before my system went live 18+ months ago.

A few things possibly too check.

What do you have under settings, system for your battery monitor?

I suspect due to not mentioning any BMV-712 or Smart BMV in your system the Battery Monitor might be set to MultiPlus. If so that is your issue. There is no SOC sync going on as the Venus OS cant sync SOC back to the MultiPlus which has come from the MultiPlus. The solution is to install a BMV-712 or Smart BMV in the system. Connect this via VE.Direct to the Cerbo and get battery details including SOC from there.

While a little out of date take a look at this manual. Section 4.2 on SOC might help explain things.

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Answer 2 · 2022-01-26T21:19:07Z

rue-1 answered · Jan 26, 2022 at 09:19 PM

Thanks for answering so quick..

Indeed I have no bms for SOC caculation, because of this remark from your linked manual:

. If the systems consists of an inverter/charger, MPPTs and a GX device, then there is still no need to add a dedicated battery
monitor

So for workaround I can leave Multiplus (FW 490) switched on- but the topic is still unclear for me: is their a possibility to let the increased SOC by charging with MPPT write by venus to the multi, if its switched on?

Because from my point of view the SOC is recalculated in the Venus OS on the Raspi if the SOC increases (in my setup without a DC-System) by MPPT-charging ...with Multi switched of (Mode 4)

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Answer 3 · 2022-01-26T22:14:31Z

bathnm answered · Jan 26, 2022 at 10:14 PM

Hi, Unfortunately the VenusOS does not itself track SOC. This is done by the MultiPlus. They key is that the VenusOS does not track SOC or send SOC to the MultiPlus in your setup, it just passes the MPPT current.. This allows the MultiPlus, which is tracking SOC to take into account the current from the MPPT in its SOC calculation. This does mean the Multiplus must be switched on and operational to calculate SOC.

The solution as you say is leave the MultiPlus on so it can get the current information from the MPPT via VenusOS, or install a separate BMV, which will feed SOC information to the VenusOS, which in turn will synchronise with the MultiPlus.

Getting that changes to have Venus track SOC I suspect will be challenging and adds I suspect little value in the wider Victron eco-system when there are other options like adding a BMV.

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Answer 4 · 2022-01-27T07:52:21Z

rue-1 answered · Jan 27, 2022 at 07:52 AM

From my experience SOC is moving with Multi switched off. Even in Dashboard of VRM SOC is moving by MPPT. Who is tracking the SOC ?

Thanks for you answer again..

screenshot-2022-01-27-at-08-46-48-node-red-dashboa.png (81.4 KiB)

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Answer 5 · 2022-01-27T09:32:49Z

bathnm answered · Jan 27, 2022 at 09:32 AM

I'm afraid I do not have any idea why your SOC is changing when the MultiPlus is switched off. I would check what the setting of battery monitor is under settings; system. What I do know is that from everything you have shared the MultiPlus is the source of SOC, therefore no syncing is going on from VenusOS->MultiPlus just from MultiPlus->VenusOS.

It is therefore nots uprisings hen yous witch the MultiPlus back on that the SOC reverts to what it was prior to the switch off, as it has no other knowledge or idea what the SOC is now.

All the recommendations from Victron are that if there are other sources that the MultiPlus can not track or that the MultiPlus is not on when the rest of the system is on, an external BMV should be added to the system.

If it was my system and this is causing issues, as it certainly will due the MultiPlus having been off has not got the correct understanding of the system SOC, I would install a BMV in the negative path, just before the battery to give you a correct understanding of SOC at all times.

I very much doubt you will get Victron too hange the system so that the VenusOS is the master SOC tracker.

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Answer 6 · 2022-01-27T09:34:48Z

rue-1 answered · Jan 27, 2022 at 09:34 AM

actual charging: PV-MPPT-charging - SOC increases to 47,5% - values on node-red-dashboard directly taken from victron-System-node..or also from Dashboard in the Portal...

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bathnm commented · Jan 27, 2022 at 10:12 AM

Maybe so. I note two things.

Battery voltage is 25.53, that would equate to just under 3.2v per cell. That is certainly not a SOC of 50%. I would say that is a SOC more like < 25%. See attached discharge curve for a lithium cell. As the charge/discharge curve for lithium is flat, a SOC of anywhere between 90% and 30% the voltage should be around 3.3v per cell (26.4v)
Knowing what I know of the VenusOS and where SOC is manipulated and utilised, that SOC value could have come from anywhere and may not be accurate.

So unless you can actually quantify and absolutely know where and how that SOC value is calculated when the MultiPlus is off I would not trust it. Digging round wiki pages, dbus and various python scripts will allow you to find this out, should you have the technical programming skills to do so.

Please share a copy of the System; Settings screen which shows what device the VenusOS is using to manage SOC. Only with that can one get a better idea of what is going on.

I can not stress this enough, in your situation with the MultiPlus not able to be authoritative for SOC, due it out being switched off when charging is happening. The recommendation is a BMV to be connected in the negative feed directly before the batteries and connected via VE-Direct to the VenusOS and the VenusOS set to use this to manage battery information, aka set as the device under System, Settings, Battery Monitor. You will then find under System; Settings System Status (if the menu option is unlocked) that VE.Bus SOC synced with Battery will be ON. At present I suspect from everything you have shared that you will find your system has that OFF. See my Screen Shot.

I do not know how else to help you on this topic, so wish you all the success in your offered adventures.

I have just set my system to use the MultiPlus as the battery source and this is what I got, when teh MultiPlus was off!

As soon as I turned the MultiPlus on I started to get meaningful data one again.

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Answer 7 · 2022-01-27T09:44:17Z

rue-1 answered · Jan 27, 2022 at 09:44 AM

Mayby I go back to standard Venus OS (without large) to check this issue- because I thougt my issue was solved there with this remark:

From Version 2.80~33 there is an Update:

"Extra battery current: for systems without a battery monitor but with Solar chargers, the current as reported by the solar chargers it synced to the Inverter RS or Multi RS, where they'll use it for their SOC calculations."

But I don't want to miss the convienience of the node-red-possibilities...

Thanks for your thougts and remarks!

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bathnm commented · Jan 27, 2022 at 10:18 AM

As I shared earlier. That change is related to just two models of device, the Inverter RS and Multi RS (new products). It ensures like situations with the MultiPlus, where they do not have full knowledge of all current flows, that it's internal SOC can be synchronised with a device that does have that knowledge, aka a BMV. It brings those devices inline with the standard MultiPlus/Quatro devices.

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Answer 8 · 2022-01-27T09:52:25Z

rue-1 answered · Jan 27, 2022 at 09:52 AM

Multi is off- SOC now 48%And after switching on, the SOC is resetted to 47%..

OK- solution will be, switching Multi on, if there is PV-Power available..or install a Smart-BMV

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bathnm commented · Jan 27, 2022 at 10:30 AM

Per my earlier post. I really would not trust that SOC at all. In my opinion that SOC is way too high for the voltage being reported. I would if at all possible connect to the mains and do a full charge to reset everything. Failing that monitor the voltage closely as most Lithium batteries will not like dropping below 2.8v per cell, and as you can see from the discharge curve, they can drop very quickly down to that at the bottom end of the discharge curve. If your battery has an inbuilt BMS then hopefully the load disconnect will trigger before you have a problem.

I would also very quickly get a BMV installed and then do a full charge to reset everything.

If you have no mains available at all, shut the entire system down and isolate the batteries, with no load or charge source, until you can resolve. That will stop any further discharge.

Note that the MPPT will consume a few Ah over these long nights as they do not fully shut down. For example the self discharge rate of a 75/10 is 8mA@24v, as defined by the datasheet. So on a long dark night, that could easily push upwards of 2Ah it will drain from the battery. You also need to take into account the PI, assume that is drawing .5A, that could be a further 5-7A of draw over night. So assume you have 10A of draw through the night. You only generate the equivalent of 9Ah through the day, then you will over a 24hour period be 1Ah down, not 9Ah up. I have no idea what other draws you have on your system.

So with the load of the MPPT self discharge, the Raspberry PI and maybe other small devices, the MultiPlus even when on when there is PV-Power will still not give you a true reflection of SOC as not all loads or charging is captured. If you are going down that route, shut the whole system off at night so there are no loads on the system. While they may be small, over a number of days they get big. It is due to this that I suspect your SOC on the MultiPlus has drifted so much. With plenty of solar and a full charge each day, these constant loads or noise to the system. In low solar generation these constant loads become more of an issue.

As an example I have a consistent parasitic load of around 34w. My system was at 100% SOC on Sunday at 13:00. Right now it is at 85%. I have 1.2Kw of Solar, but am generating next to nothing in the south of the UK. This is the last 24hours of my system.

Look at all of that overnight load that the MultiPlus will not collect and consider if it is off.

When my battery is heading down towards 40% SOC, I will either make a trip to the boat and go to a Marina to re-charge or shut everything down remotely and have zero monitoring. So again need to visit the boat and bring her back to life. Thankfully no unplanned visits or shut downs have been need yet, but it has got close.

Until you can sort the SOC issue, watch voltage like a hawk and when you start getting down to 24v have some form of back up plan in place, as they are heading towards a very low SOC.

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rue-1 bathnm commented · Jan 28, 2022 at 09:35 PM

Please don't worry about my battery: i use a 7-cell-lithium-ion-battery- 3.6V is cell-voltage for around 50%. After charging to 28.2V with MPPT or with Multi the SOC recalibrates to 99% and the discharged Ah-Hours are aequivalent to SOC. I'm doing battery-testing since 18years on my job...

Thanks-Ruediger

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