question

Are your batteries suitable for series connections, maybe they'll need an external balancer ?

Try setting charged voltage to 28.7

1. DC power

This question keeps coming up. The DC power is calculated from the other information available, it is not measured. The Shunt measures everything flowing into and out of the battery. The Multiplus as far as I know does not have a super accurate DC current measurement. Therefore, the Cerbo calculates the difference between the reading from the shunt, the estimate from the Multiplus, data from any other known sources if fitted such as an MPPT and this difference is the calculated DC load. An additional factor is that the time that readings are taken varies from device to device, so when you have fluctuating or changing loads, the calculated DC power can vary. For example if the Multiplus increases power draw from the batteries and the calculation of the DC power is still using an older lower value for the battery current from the shunt then the DC power is wrong.

2. SOC problems

Copy us a screenshot of your Smartshunt settings.

I Set it to 27.4, with a tail current of 0.5%, yesterday, as per documentation, and then watched the charge terminate.
By the time the Multi dropped into float, 27.7, the battery current was below 1A, consistently, so I am pretty sure the battery was full and as you would expect showing 100%.

What's the thinking behind lifting that setting? I was under the impression that the battery percentage was supposed to be reset to 100 when the voltage is above Charged and the tail current requirement is met. My system had settled at 100% shortly before that but only perhaps 5 mins.

I am happy to change the setting but I would like to understand why, and what I am looking for.

I am wondering if the bulk and float voltages are a bit too low but I think I checked with Ultramax when setting up... Not that I made a note of the conversation.

For me the battery is full when absorption is finished. So I set the "full" close to absorption voltage. Works fine for me.

Seems logical and will not do any harm... I will give that a go thanks.

In other news...

I suspect my problem is partially to do with charge settings, they were a little shy for the battery, and also linked to temperature and load.
At C/2 discharge I am seeing a voltage drop at the terminals that is greater than I was seeing a few months ago. This caused me to set a very low disconnect voltage, which is why the internal BMS shut the battery down before the Multi switched off. Neither impact capacity but the capacity reduction associated with temperature, a thing apparently, isn't being compensated for in the shunt.

I have temperature monitoring but its done by the Multi. The shunt doesn't have that info as far as I know so couldn't trim the capacity even if it is supposed to.
Do you know it capacity trimming / derating is built in? Adding a temp sensor is hardly a big deal.

The second thing is that the volt trop associated with load looks like it is temperature dependant as in, colder battery lower terminal voltage but also bigger drop with a given load.
Is that plausible or am I grasping? Temp is 14C right now with a terminal voltage of 26.16 @ -8.3A

that would be about 2.77A per string
at -60.7A per string the terminal voltage fell to 25.92 within about 20 Secs. I ran that load for about 60 Secs, and then dropped back to the circa -8A base load and the voltage recovered to 26.12 within a couple of mins.

That doesn't seem excessive to me but if I am trying to set a disconnect at to capture say 15% SOC as an absolute minimum capacity the drifting voltage will mess that up.

BTW battery currently at 26.21V with a 200W draw. SOC reads 53% which is perhaps a few percent optimistic but not that far out at this stage. No charging since Last night.

I am beginning to think this isn't some horrible capacity issue but likely several smaller things conspiring to confuse both me and the shunt.

You have two multis in series ? Could you post a diagram of your wiring, I just can't imagine what you are trying to achieve.

:-) Boxes are not my thing... I would love to say I think outsude them but the truth is I never lerned whre they were... Started with an easy solar / BYD 13.8 kW, absolutly standard install. Thn needed more battey capciy and higher load capacity, occasionally. Couldnt afford another 12K for second battery and another 5000/48/100, also wanted redundency which you dont get with 2 identical units in paralell. SO....

5000/48 feeds 5000/24, ACout to AC In. Solor feeds 48 pack, genertor feeds 5000/48, house connected to 5000/24.

The result is 8kW availability plus 3.8kW genny, with genny on and both units running in assist... Pretty cool.

Two transfer switches allow, will allow, the house to run from 48/24/genny or any combination of all three. Its just a chain. A Cerbo manages things handling what is on and what charge the 24 is taking.

When I present a high load, the AC limit on the 24 is used to split the site load between both invers. The ac limit on the 48 keeps the genny happy.

Would a couple of Quatro's and 3 times the battery capacity have been better... Technically, of course' but i am just some dood on the same wage as you so that wasn't happening, sadly.

Still fiddling with Node Red to hang all this stuff together. Technically now sorted but with manual control. Working on automated triggers now to replace my input.

BTW. Battery SOC from the shunt looking way more realistic after playing, as per previous posts/suggestions. Early days but beginning t think I may have a handle on it

Good to talk, thanks.

What I have....Victron Systeml.pdf

To be fair the transfer switches are still in a box, wiring is as Pri. There is a CCGX on the 48 and a Cerbo on the 24. The Cerbo hosts Large OS with NodeRed handling the control.

Data from the 48, and some control, is via Modbus TCP, custom nodes are available in NodeRed on the Cerbo to manage the 24. Charge current control on the 24 is achieved by manipulating DVCC, which isn't an option on the 48n because the BYD BMS has control of that.

The DC-DC isn't in place, in fact I don't have one yet, still looking at options . My Cerbo and Ethernet/WiFi all run from the 24 DC. Adding resilience there, is/will be, a good thing.

Two multi's in parallel have to be identical and cant fail over if one of them has a problem.
Organised this way I have way more flexibility and auto failover right back to the genny.

Ok, if this works for you it's fine for me. Glad I could help with your soc problem.