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xpim asked

REC BMS config for a multiplus II + mppt chargers setup

Hi all!

I'm in the working of choosing and preparing a solar/battery storage system due to poor grid with issues ans instabilities.


I'm looking at getting 2 multiplus II 48V - 5000VA hooked in parallel with a (yet to be calculated KW) lifepo4 battery bank.

due to cost constraints I will probably have to make my own pack with prismatic cells and I've been checking the details on BMS systems to maintain battery health. I've searched around this forum and I'm inclined onto REC BMS.

I've seen that I will need the VE.Bus to USB to config the multiplus II inverters for parallel and charge options (I'm still grid connected and will probably use while I set up all the needed the PV arrays) and later on, probably a generator if/when power demands are higher for any reason (welder, woodchipper, low PV production)

my problems now is: how can I config the inverters to stop charging/discharging with the REC BMS ? I can't seem to find the proper information.


I also have not decided on the solar MPPT charger(s) and would like to also have control on the charge limits. I don't want to push the lifepo4 cells to the very limit. I need them to last a long long life.

this system will have to power my house, water heating system, bore hole pump and a few other things. I already have power management in order to avoid having every system coming up at the same time but the induction stove top will have to be monitored (to stop the bore pump or water heater).

do I need to ad a Cerbo GX to control my system or just with the inverters I can set up the charge/discharge parameters?


also, the system will have to be modular. adding extra PV arrays later since I need some work done on the roof of this house before adding PV panels and I'm still trying to make some calculations on actual battery capacity + PV power to make all this work with no issues (I'm moving power habits to during the day)


any input would be greatly apreciated. I unfortunately cannot hire a company to hook my systems up. I need to re-wire parts of my house, still need work done on the roof and all quotes I get from the local companies are very much inflated by 60% with half the power I want to have plus they advice grid connected which here is a problem. I have a realy high voltage drop even with just a 700W toaster. I drop from 230V to 218V at the fuse box... and daily power cuts... I need to get "kinda" off-grid

BMS
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3 Answers
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Kevin Windrem answered ·

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.

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xpim avatar image
xpim answered ·

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.



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xpim avatar image
xpim answered ·

Hello all!


I'm pretty much almost ready for ordering parts for a half system,

Iv'e been reading the equipment manual and I still have some question marks here and there but I'm working on them.


I'm going for:

48Volt 16S lifepo4 EVE 314Ah cells with a REC BMS kit

1* multiplus II 5000W 48V

1* MPPT 250/100

1* cerbo S GX

fuse galore

combiner PV box (I'll make it myself. they cost to much ready made and are just a couple of wires, fuses a switch and over-voltage protectors...)

12* 460W mono half cut PV panels in 4 series and 3 combined

a bunch of wires, ground rods and a main electrical panel.


after having this online and after raising more money to add more solar panels (probably with another mppt) and the extra inverter. double the battery also to support everything in this house and my sisters workshop.


I "may" have a petrol generator for high demands or other issues if ever anything comes up later on.


I've user the victron MPPT calculator and added the PV parameters on the panels I'm going for and it said to use a 250/70 or a 250/85 MPPT charge controller but: when I add up the total PV power its higher than the controller maximum output... I'm afraid to be dumping a charge to big/to small but if it does not pull all the power I may add another string before adding a second charge controller and PV array.


I'm working now on trying to understand settings and parameters on the inverter and charge controller/BMS and how will the cerbo work those parameters in...


I'll also get the USB/MK3 cable but will I need other cables to config other parts of the system?

the HDMI output on the cerbo, will work with any LCD monitor or does it really need the touch? mainly for configurations since I wont probably need much status viewing. I'll probably connect on a computer to check system status (still not sure if it works)

I'm working also on wire sizing for every part:

I'll probably have 10mm wire from the combiner box to the charge controller

70mm for the battery bank to the bussbar and same for the inverter. will make 2 bundles of wires from the start. if I see its dropping voltage and heating up I'll double the wires but on my power demands it sould be safe. I'll fuse the crap out of every part just because...


right now I'm just affraid to pull the trigger on this and I need to be a little bit more sure on my selections.


oh also: I'm only using a shunt for the battery on the REC BMS. I hope I can get the REC BMS to dump the battery state onto the cerbo since I'm a smidge limited and I don't want to add the victron shunt and have it be redundant... (the only redundancy here would be fuses and the second inverter when I get the funds for it)



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