question

You will have some control over the system without a GX device but integration with a BMS will most likely require a GX device of some kind. A GX device would also be required to intelligently prioritize battery power over grid power. You can build a inverter config to ignore AC input completely based on criteria such as battery SOC and/or load but enabling the AC input takes a while and your inverter might overload on heavy loads before the AC input is accepted.

A brief glance at REC BMS pages looks like some of their products would interface via CANbus to a Victron GX device, but some will provide "do not charge" and "do not discharge" to feed into battery protects (REC shows contractors for these but Victron makes solid-state ones also.)

When connected to the grid, the load voltage will be equal to the grid so you might not be label to eliminate the low voltage from your grid. You might look at separate chargers and inverters so you never actually use the incoming grid power directly.

If you plan to run both a generator and the grid, consider Quattro instead of Multiplus. It has two AC inputs so you can connect the generator to one and the grid to the other. With a Multi, you would need to use a transfer switch to route either grid or generator to the Multi's single AC input. My ExtTransferSwitch enhancement to the GX device might be useful in that case.

There are a number of GX devices available from Victron or you can run Venus OS on a Raspberry PI. You'll need adapter dongles for the VE.Bus, VE.Direct (MPPTs, etc) and CANbus interfaces since the PI does not have any. So the PI may or may not be less expensive.

Thank you for your reply :)

I see that I would need a Cerbo GX to provide the control. I'm considering REC BMS to have the CAN information so I can stop/start charging from the PV (priority)

I'm also considering the multiplus II (PMP482505012) due to price (2 multiplus II are cheaper than a same power Quatro on my local providers) ... I would need 2 in parallel for the power demands. I will have to read a bit more since I was not fully aware of its AC input/charger bypassing the inverter and feeding (my low voltage and unstable power) to the loads.

AC/Generator inputs would by manually switched on (if ever needed) I could consider adding a programmable logic controller (or use some spare I/Os on my hot water and bore hole pump controls) to connect AC power on lower cost times/if night-time and I predict a low battery state.

I rather have a cerbo GX and support over setting it up than having to have 3 panic attacks on RPIs systems... even my NAS system runs on a really old laptop but I had help setting it up. my programing skills are frozen on smart relays (programable logic controllers)

I would prefer to fully isolate from the grid further down the line and only have a generator as a emergency or high demant situation. I'll try some more reading before going on.