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Pontus Lindberg avatar image
Pontus Lindberg asked

Planning a 3 phase system

I am currently planning a battery setup for my home and need some input. Planning is perhaps an exaggeration - I am trying to learn what it would take to build something like this :)


I already have a 27 kWp grid-tied solar setup, using a Growatt 25 kW inverter - but I am not planning to use the panels directly with my battery system for now (string voltage is ~750V and I do not intend to rebuild this any time soon). As I understand it, what I want is called an "AC coupled" system.


As of now my plan is this:

A homemade 71 kWh battery bank using REC BMS.
3x Victron MultiPlus-II 48/5000/70-50
1x Cerbo GX
1x Victron Interface MK3-USB
1x EM24
3x Current transformer 100/5
1x Smart Shunt 500A/50mV
3x RJ45 UTP Cable 1,8 m
1x VE.Direct Cable 1,8m


What else do I need to get this working?

My ultimate goal is to become less dependant of the grid, but also having to possibility to do some "arbitrage" for times when the electricity price moves a lot.

Multiplus-II3 phase
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daniel-feist avatar image daniel-feist commented ·
EM24 is an older model and requires LAN AFAIK. Suggest looking at EM530 or EM540 which are newer, have a backlit display, faster refresh frequency and don't require LAN.
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Pontus Lindberg avatar image Pontus Lindberg daniel-feist commented ·
Thanks, I´ll look into it.
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3 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

for the shunt, Victron do a range of smart shunts that would work with this type of system. The contactor is not needed, as the Cerbo will shut down the inverters on low battery condition, once it has communication with the REC.

As your total inverter power will be 15kW, you could reduce the PV input to the grid tie inverter to this level - or limit it by programming. The balance of the P|V system could then be fed to an MPPT charger to help charge the batteries.

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Pontus Lindberg avatar image Pontus Lindberg commented ·

Thanks, I´ll have a look at the smart shunts. So the Cerbo has the ability to shut down the multis in case of a problem reported by the REC BMS? Sounds great, did not know that.


About your last comment, I´m not sure what you mean here. Is your point that I should use DC (using a MPPT charger?) to charge my batteries without converting DC (solar) -> AC -> DC (batteries)?

As far as I can tell, Victron product only support up to 450V DC (correct me if I´m wrong), and my solar setup is ~750V.

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Kevin Windrem avatar image
Kevin Windrem answered ·

You can AC couple your existing grid-tied inverter to the input or output side of the Multis.

There is no limit to the size of the PV inverter if put on the input side, however if the grid goes down, you loose the solar contribution.

Solar power on the output side is limited to the total power of the Multis (15 KW). This is explained in Victron documentation but essentially, the limit insures the Multi and batteries can absorb the full solar production if a heavy load switches off. Eventually the PV inverter will be throttled back but until then the energy needs to go somewhere.

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Pontus Lindberg avatar image Pontus Lindberg commented ·

I will go for the input side of the multis for now. Might reconfigure once I can afford 3 additional multis so they can handle my full PV system.

We have an incredible stable grid where I live. I can only remember one outage for the last 10 years or so.

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Kevin Windrem avatar image Kevin Windrem Pontus Lindberg commented ·
Another possibility would be to split your PV array and run separate PV inverters one attached to the input side and one attached to the output side. Limit the PV inverter on the output side to 15 KW. Put the rest on the input side. That way you have some solar contribution with the grid down.


Or as others suggest you could rewire some of your array to feed a MPPT controller connected to your batteries. Yes, you'd need to reduce the voltage on that part of the array.

I'd cost each option and select the one that fits your budget.

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Pontus Lindberg avatar image
Pontus Lindberg answered ·

Another question that just popped up. Do I need Multiplus-II GX-devices?

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Kevin Windrem avatar image Kevin Windrem commented ·

You indicated you will include a Cerbo in your system so the GX device inside the Multi would not be used as you can have only one GX device in the system. Just get the regular Multis.

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Pontus Lindberg avatar image Pontus Lindberg Kevin Windrem commented ·
I just figured perhaps they had to be GX-devices to be compatible with the Cerbo GX, but apparently that is not the case, thanks!
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