To explain first: I have an external BMS with a DIY battery that is not coupled with the Multiplus II.
I found an error in the calculation of the SOC by the battery monitor in the ve.configure, I think. It is set to 48.2 volts with SOC 90% for the transition from bulk/absorption.
However, the following happens: Always at 0.4 volts before the voltage is reached, the SOC switches to 90%, the battery remains in bulk mode. Then the SOC stays at 90% to 48.20 volts, then the battery jumps from bulk to absorption and the SOC continues to count up. It would be right if the SOC followed the green line.
This always leads to problems in the DESS, because the SOC jumps like that. I have tried several voltages, the jump in the SOC value always happens 0.4 volts too early.
The battery monitor in the Multiplus is basic, it is not an accurate shunt and is an approximation. For any system it is best to add a shunt based battery monitor such as a Smartshunt that is very accurate and does not rely on approximations.
Even if this solution is not as accurate as a shunt, there still seems to be a bug in this program, right? I know several users from other forums who use the Battery Monitor from ve.configure and are satisfied with it, but all of them are annoyed by this error, which makes an exact SOC calculation and thus also a use in DESS impossible. The question is this: Will Victron change anything about this error or not. When I have this answer, I will think about buying a shunt.
This forum is staffed by volunteers, Victron staff ocaisionally contribute but it is not an ask Victron forum and more often than not a direct answer will not be forthcoming on future developments.
My personal judgement is the hardware for DC current measurement in the Multiplus is not accurate enough, the Multiplus does not have a shunt built in. Therefore, the SOC is more likely to drift so it needs someway of knowing a fixed reference point to resynchronise to correct drift and make it repeatable. The set to 90% at end of absorption is this synchronisation point, it is a guesstimate that is all because of the hardware limitations. No amount of programming will correct this. Therefore a shunt is the solution if you desire more accuracy.
Thanks for the answer, unfortunately it is unsatisfactory for me for the following reason.
From the description ve.configure it can be clearly seen that the SOC value is set to the freely defined value (e.g. 90%) when switching from bulk to absorption. However, this is incorrectly implemented, as the battery remains on bulk while the SOC is set to 90% too early. This has nothing to do with the accuracy of the current measurement or the absence of a shunt, but is simply incorrectly programmed or incorrectly explained.
But I can already see that this discussion here leads nowhere, as it ends in the same result as with my dealer. The error is not fixed and I am forced to buy a smartshunt, what the heck, I will survive it even though it annoys me.
I think the built in shunt is accurate enough. I’ve got a calibrated shunt on a DIY BMS and the Victron is within 5%. I think the SOC calc is adequate too. My Multiplus does the charging. I don’t use DESS as I live near mountains and the forecast is normally way off.
Like you @BuBu I have noticed this behaviour. However in my case MP SOC is held at 95% when it drops out of bulk and enters absorption. Below is a graph from June
The blue line is MP. It sticks at 95% for 1 hour, then goes into float and jumps to 100%. The purple line is my SOC calc, which continues.
This behaviour has changed, though. I must have done a FW update as today I got the graph below. A little more linear and no real jump.
