The hardware in my setup:
- 3x Multiplus-II 5000/48 in 3 phase, firmware 556
- 20kWh Pytes batteries (4x E-BOX-48100R)
- 2x Fronius Symo PV inverter (10kW + 12.5kW)
- Cerbo GX MK2, currently running VenusOS v3.60~76)
The system is running off-grid due to pending grid connection, the PV Support assistant is loaded in the Multis.
Disclaimer:
Due to pending grid connection, extra Multis & batteries, the system currently has too much PV inverter power and too little battery capacity to be compliant with the AC Coupling and Factor 1.0 rules.
I am fully aware of this so please do not simply blame me for that.
Please continue reading, I’ve countered this 
First thing I noticed when building the system is that, even though Fronius power regulation via Modbus is possible, in an off grid situation with the PV Inverter Support assistant loaded, the system still uses only frequency shifting to throttle the Fronius inverters.
Can someone clarify the strategy used by the Victron Multi & GX to regulate charge power based on battery SoC and AC Load (in an off-grid situation with Fronius PV on AC out) ?
The reason I’m asking:
Yesterday my batteries were charging from 70ish% SoC, my electric motorcycle was (single phase) charging with 3kW and batteries were reporting max 200A charge current.
All perfectly fine and PV was happily sourcing the power.
When battery SoC reached 80%, the Multis started limiting the PV power by frequency shifting and keeping the battery SoC at 80%.
This behavior I didn’t encountered before, but I hadn’t charged my motorcycle there before either.
When the motorcycle finished charging, battery charging resumed.
I am assuming that the Multi’s were limiting PV power to keep the batteries at 80% SoC while a large load was present, so that if/when this large load were to drop there would be spare room in the batteries to take in the excess PV power while the PV inverters ramp down.
This topic is to try to understand how/when this mechanism is triggered, for reasons explained below.
TL;DR section:
Because there’s currently too much PV power (and also because I like to limit charging the batteries to 90% SoC) I’ve built a Node Red flow that regulates & limits the Fronius inverters.
Second reason I made this is to keep the AC Out frequency at 50Hz by limiting the PV inverters via Modbus before the Multi’s go into frequency shifting mode.
Currently configured hard limits:
- max 12kW PV power (to protect the Multis) - this means charge power + AC load power
- max charge power of 6kW (to protect the batteries) - even if they report 200A max charge current at 50V
With these hard limits, the system is again (somewhat) within spec of the AC Coupling and Factor 1.0 rules.
When battery SoC is at my Target SoC - 10% (so 80% with my usual settings), I’m further limiting the PV power to slow down the charging.
So far this has worked perfect, but with the extra AC load of the motorcycle charging it seems the Multi’s throttling mechanism kicked in earlier than I anticipated.
If I can find the Victron algorithm (used by the PV Inverter assistant) I can modify my Node Red flow to kick in just before the Victron algorithm, so output frequency stays at a clean 50Hz.
Even better would be if the PV Assistant or GX would do the PV throttling using Modbus and keep frequency shift throttling as a last resort.
I’m not sure if the question is clear and my explanation makes sense.
Constructive comments welcome