I have been operating an off-grid PV system (i.e. without feed-in) for a few years now, which I recently expanded to 3-phase. Installed are:
1x ColorControl
3x Multiplus 48/5000/70-50
1x SmartSolar Charger MPPT 250/60
1x SmartSolar MPPT VE.Can 250/70
1x DIY 9.6kWh LiFePo4 battery with third-party BMS
1x EVCS wallbox with display.
My problem is that the SOC regularly ‘runs away’. Then the battery is already full, the two MPPTs go into absorption mode, but the SOC is somewhere, just not at 100%. If the battery is now allowed to discharge until the SOC is at 0% and beyond, until the system switches to the grid, the charge level is temporarily correct again.
This behaviour wasn’t really bad for me, as I know how to interpret battery voltages and charging power. I have now recently added the wallbox. Unfortunately, the shifting of the SOC sometimes means that the minimum SOC (70% for me) for charging the car is not reached. This means that the solar system cuts out and the car is not charged.
Do you have any ideas on how to get this solved?
Incidentally, I deliberately decided not to use a shunt after I read that this should not be necessary in such a constellation because MPPTs and Multiplus already have one installed.
I am afraid that once you have a complex system it is best to have a shunt installed to directly monitor the charge and discharge of the battery. The SOC on the multiplus does not always give reliable figures. There are I believe settings on the GX device to allow solar charge to be allowed for as this improves accuracy, not sure where to find this though.
With the 3rd party BMS, Does this have both a battery shunt and a data connection to the CERBO?
The cerbo should be getting accurate SOC information from the BMS.
The Multiplus are incapable of tracking the SOC in a system with PV, as the soc charge they measure only comes from their own charger.
IF there is no data connection from the BMS to the Cerbo, then you need to add the shunt to get the SOC to track the battery charge anything like accurately. SOC is derived from an intergaral over time of the battery current, so long term drift in SOC is normal. This is normally reset to 100% at a specific voltage, or other set of conditions (by the BMS) usually on a daily or weekly basis.
@MikeD: No, the 3rd party BMS unfortuneately has no connection to the BMS.
For a system which is incapable of tracking the SOC, it doesn’t work that bad. But the important point is, as you mentioned, that the SOC has to be reset to 100%, if the MPPTs reach the max. voltage. I wonder why victron does not do so (in the systems without Smart Shunt) , as it would improve the accuracy considerably.