I’m looking for help, my system that has been running for 9 years satisfactorily has just started behaving starngley and I’m trying to work out why. The System is 4 x 90Ah 12v Victron smart batteries in sereis and BMS, 2 Multi’s in parallel and a colour control.
I’m running DESS and the battery discharges accoring to schedule and then for the last 2 days for no apparent reason the SOC from the victron Smart BMS and Multi in the system drops to around 10%. The system then says it needs to charge, so starts drawing power to return the batery to at least the minimum SOC or the DESS SOC.
See the attached Graph of SOC and DC current and voltage.
For better understanding you have to examine how the battery voltage and current behaves when the SOC passes from 60 to 10%. It could be that your battery has a problem (wear ?). Examine if the DC connections between batteries and inverter warm up more than before.
As @molenman says you need to look at the data for the few hours where this happens not for a week, it is too coarse to see the detail, plot a 6 hour data window or finer. The Multiplus SOC is calculated from current in and out so you need to confirm what was flowing out during that time. It could be DESS pushing a discharge.
Your batteries are in parallel so you have 360Ah of battery capacity.
Looking at that chart, at 07:00 for some reason the SOC dropped from 60 to 0% so the batteries started charging. After just under an hour of charging at 100A the batteries reached absorption voltage of 14.2V, so end of bulk, so the Multiplus SOC jumped to 95%. I assume you have the battery monitor set to go to 95% at end of bulk. So the amp hours recharged agrees with the 60% SOC.
The drop to 0% SOC was nothing to do with discharge.
What I can not answer is why this happened. Some of the Victron battery monitors reset to 0% if they loose input voltage so the battery monitor function in the Multiplus could have developed a fault pr it could be a genuine loss of voltage.
What exact BMS do you have, Victron call.most of them Smart. Is it a Lynx. Do you have a SmartShunt or a BMV battery monitor. If you do, see if they have a settings “SOC on reset” and if they do change this to “Keep SOC”.
The VE Bus BMS does not report battery SOC, the battery SOC is coming from the Multiplus and there is no setting for SOC on reset. As noted above, you may have an intermittent failure in the Multiplus, I can not advise further other than checking all wires. It will also be out of warranty if it is 9 years old. One thing you could consider is adding a SmartShunt to the system to do the SOC measurement and connect that to your GX device and choose that as your battery monitor. You will then have something reliable for your SOC.
I have 2 multi’s in parallel, would it be worth configuring the secondary multi to be the master and visa versa? I’m assuming the Master unit calculates / provides the SOC to the system in the BMS doesn’t.
