question

please can someone from Victron answers me on this

Hi

I had exactly the same question and could not find a definitive answer on the forums, So I have just finished testing this.

My Test Setup:

Multiplus II - Firmware Version 497 (5000VA, 230V, 48V)
Cerbo GX - Firmware Version V2.91 Latest ESS Assistant.
Blue Nova Battery (560Ah) Can BMS connected to Cerbo GX on BMS-Can Port

My Findings:

Both ESS "Dynamic Cut off" Values, and the Multipluss-II Ve.Configure III "Shut-down on SOC" values are active.

Whichever one is triggered first will cause the Inverter to Shut-down.

In my case the Default ESS - Dynamic Cut off Curve values were causing the premature shutdown.
The default 0.005 C value of 52V was to high (My average load of 0.1C was triggering the 52V shutdown at 35% SOC)
Lowering this to 51V has resolved the issue.

Now my "Shut-down on SOC" value of 20% is the first shut-down trigger, with my battery voltage still at 51.4V (I may even set this down to 50V as I suspect a sudden load spike may decrease the battery voltage enough to trigger the shutdown)

In the case of a BMS connected Lithium battery (and I presume a BMV712 battery monitor on a lithium battery without BMS as well) the Dynamic cut off values can be set lower, but don't go to low, so that if the BMS-Can communications fail (or the SOC calculation is wrong), these values will still shutdown the inverter and protect your battery. The exact Voltage at specific C ratings will depend very much on your particular battery.

The idea behind dynamic cutoff is that the battery voltage is a function of current. So this provides a more real-world limit than just a single low voltage limit for the battery.

This has nothing to do with a low SOC shutoff. You need both.

PS: Victron states that the Dynamic Cut Off Values are a Curve. In my case the Premature shutdown did not happen at exactly 52V but rather somewhere on the curve between the 0.005C-52V and the 0.25C-50V Value.

Something else that I am thinking about is that as my battery loses capacity over time, the SOC calculation may start to be wrong. In this case some years down the line the Dynamic Cut of Values, may come into effect again, and protect my battery from damage.

So as @Kevin Windrem states, Both SOC cut-off and Dynamic Voltage/Current cut-off values are very important.

My conjecture is:
That when you always cut off on SOC (ex. 20%), you are in effect always over time losing capacity in that lower 20% (Actually you lose capacity over the whole battery but you don't notice it as you never discharge to 0%)

The Upper 80% of the battery is always put back into the battery at for example 96% efficiency, which is the calculation that the Battery Monitor uses. (Unless the BMV is really smart and has some super intelligent way of calculating the actual SOC, taking age and cycles into account)

Maybe someone can comment on my conjecture? Am I on the right track with the SOC over time?