question

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)

Maybe so. I note two things.

1. 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)
2. 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.

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.