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What happens with two Multiplus in parallel not linked with each other

Background: I have an old Multiplus 48/5000/70 and I would like to increase the available power with a second battery bank

I'd like to know what happens if I connect the output of a Multiplus to an existing grid formed by another Multiplus. Would it be any different if any or both of the Multiplus is a Multiplus II instead. note that in this example the two Multiplus would not be linked together in parallel, it cannot work as this would require a single battery bank.

Take this schema from the MultiGrid datasheet.

1635441890321.png

In this schema, Let's replace the MultiGrid by a Multiplus II (as I understand the Multiplus II is an improvement of the MultiGrid). Let's replace the PV inverter by another Multiplus and the PV array by a separate battery bank (that too has solar panels).

What would happen in such a scenario ?

edit: an answer below tells us that it's impossible to connect the output of two Multiplus together without linking them. But then, if the Multiplus is capable of feeding back current to its AC input, the second Multiplus can be connected via its AC input.

If it doesn't work by replacing the PV Inverter by a Multiplus, can I take the PV inverter and plug it to some batteries instead ?

Thank you

Multiplus-IImultigridgrid parallel
1635441890321.png (32.9 KiB)
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3 Answers
shaneyake avatar image
shaneyake answered ·

You can not link the multiplus outputs without them being in parallel.
Based on your other post, you are trying to use another battery bank that you already have.
It is possible to have them in series but might not be worth the work to get working 100%
You can just have the second unit run until battery low and never push back to main inverter to make things simple.
I have a micro-grid running like you would want. Here is my blog post about it.
https://shaneyake.blogspot.com/2021/09/micro-grid.html

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Thank you, that looks a lot like what I wants to achieve.

1635511205109.png

In your diagram, you have a "Distribution Box" that seems to connect the outputs of the two Multiplus together. But you say that it's impossible to connect the two outputs together, how do you do that? What's in the "Distribution Box" ?

or perhaps I am misinterpreting the schema and you have the second multiplus AC input connected to the distribution box, and the Multiplus has the ability to inject back current in its AC input. If that's the case how do you manage :

  • When the second Multiplus (say during a sunny day) has more energy than what can accept the garage loads and the house loads, it will inject more energy than the system can take. Does it regulates itself somehow?
  • Is the first Multiplus able to take the excess energy from the AC migrogrid to power its own batteries for later use, and possibly inject back in the public grid?
1635511205109.png (64.4 KiB)

Distribution Box is my main electrical panel. Garage Inverter is connected via AC-IN to Distribution Box. Yes, multi can output power via AC-IN

This is where is starts to get tricky. My system via software maintain both batteries at same SOC always. So depending on solar the load will be equally split between both inverters or more on the inverter that has more access to PV. House is East array, Garage is North-West.

1. It has 2 modes of operation, when grid and when island. When grid is present then the second multi will export as much power as it can. If it isn't used by the house or the garage it will be exported to the grid, basically just max feed-in. This is done via the inverter Over-Voltage feed-in system, this is how normally ESS will feed-in DC coupled PV.
If there is no grid, Over-Voltage feed-in is disabled. The system will then control the feed-in, to keep both batteries at equal SOC and then once full to keep battery load equal. Once the house battery gets to 99% or greater the garage will power it's own loads but won't push any power to the house. It also has a mechanism to prevent overload, so if one inverter is pushing out more than 4kw the other inverter will take some of the load.

2. Yes, if the garage is pushing back more than is being used the house will store that into its battery, once the battery is full it will push to grid.

This is a complicated system and I would not recommend you try and set it up unless, you understand it won't work out the box and have time and energy to develop your own control system. Maybe one day Victron will respond to frequency shifting and that would make everything way simpler.

Thank you, I'm trying to wrap my head around that. if I understand, you developped a small control system that will take data from both the Multiplus and configure it to balance the loads.


In particular, when there is no grid, you say you control the garage multiplus to feed back to the house. I suppose you connect to both Multiplus using VE.bus and tell the garage multiplus at regular intervals to feed back energy to the AC-IN until the house batteries are at equal state of charge. Is there a way to tell a Multiplus to feed back a specified power level at a point in time ?


Also, how do you detect if there are loads exceeding 4kW to tell the garage Multiplus to feed back some power? is that your control system that does this or is it managed by the Multiplus itself? If so, I suppose your system must react quickly to loads. What interval do you use for this?


Do you have a protection in place in case your control system goes down and you told the garage Multiplus to feed back power. At some point I suppose it can provide too much power and that can overload your house Multiplus if the control system is not there to stop it?


House = Master, Garage = Slave

Yeah, so when there is no grid is when things get a bit complicated.

Both systems are full systems in my case, eg. Have Multi, Cerbo, MPPT, battery.
You will need 2 cerbo's, one for each system. The control is all done via MQTT.
You can't have multiple standalone inverters on one VE.Bus.

When grid is lost, you need to immediately disable Overvoltage-Feed In. Else the slave won't obey the setpoint if battery full. If you turn on UPS function in the the slave would disconnect and then reconnect with can be helpful but I found turning it off works better as it will stay connected to microgrid. The slave stops exporting once the Master SOC goes above 98%

Correct, the house is mostly set to 0W for it's AC-IN setpoint and the garage setpoint gets updated every 5 seconds or less. Yes, how ESS works is that you are controlling the AC-IN power this is how victron is doing it as well, you will need to run in ESS Mode 3 tho, so you can control the inverter directly.


Can you exceed 4kw, yes but kind of. If the load is on the output of the Slave, then yes, the slave will buffer and then balance, This isn't a problem because the slave will ramp to max power then the rest will come from Master. If the load is on master, then you can go over but you are now running in the peak power of the inverter so you only have a few seconds or it will overload and shutdown. So far, I can balance fast enough that this isn't a problem but I wouldn't run loads that are more than 6kw. Any kind of big change in the system does take some time to leveled out.

As you power calculations I simply do AC_OUT - AC-IN = INVERTER_POWER
Then for balancing it is pretty simple
ERROR = MASTER_INVERTER_POWER - SLAVE_INVERTER_POWER
Error is then feed into a PID control loop which outputs the new slave setpoint.

I try to update as fast as I can but Victron NodeRed is around 5seconds which has been fast enough for my loads. With 6kw being the biggest load I see. Pulsing loads like Stove suck but I have been able to get it to work reliably with the PID tuning.

As for protections there are a few. Slave Cerbo will automatically got to 0W setpoint if coms are lost. Slave is monitoring incoming Frequency if it goes above 53 hz it disconnects completely. I am playing with the Idea of using AU grid-code as it has the ability to reduce output based on Frequency.

If you are pushing too much power to Master then the load on both isn't balanced and they will auto rebalance. So I have never had that problem even tho I am over the 1:1 rule as I have 2kw of AC-Coupled Microinvers as well but I am typically far within the 4kw limit always.


screen-shot-2021-11-08-at-110447-am.png
You are going to get transientes, this even happens with the inverter in normal ESS mode.

Yellow = Master Power
orange = Slave Power
Blue = Slave Setpoint
Red = Balance Error.

On the balance Error, you will see it isn't 0 and this is because I have a lot of DC loads on the Master, Whole house as HVDC lighting CCTV, Network equipment are all DC. The PID balance setpoint takes into account the current DC loads so that the batteries are discharge at the exact same rate.

Kevin Windrem avatar image
Kevin Windrem answered ·

You can not connect the outputs of two Multi inverters together unless they are programmed to work in parallel and linked with a VE.Bus cable. Victron requires the units to be the same model and version and run the same firmware. In addition, proper load sharing requires equal length cables.

PV inverters are designed to tie (connect in parallel) to a grid which includes the output of a Multi. As far as connecting batteries to the solar input to the PV inverter, you'd need to consult with the inverter manufacturer. You'd need to find a way to charge the batteries.

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tcircuns avatar image
tcircuns answered ·

Hi Shaneyake,

I'm going to install my first Multiplus II 48/5000. I've been looking for options on adding a new one in a future, but maybe using different batteries.

After reading your blog, in my case I was thinking on a simplified version of your one, as I was only thinking on using frequency shifting from master inverter to control exported power of slave one (connected in grid-parallel).

That should be giving me the sum of power of both Multi and the posibility to have both fully controlled by master one.

I am not sure if that's possible, but when I read your post, specially next paragraph, I thought that is possible:

"Other Grid code, may need to be set on the inverter to allow frequency shifting of the master to work correctly. I have slave GX monitoring input frequency so that if the Master shifts the frequency the slave will reduce it's export to the Microgrid and eventually disconnect. This is to prevent the master inverter and battery from getting overcharged or overloaded, This is only used if comms failure or to control Enphase system. You also need to disable Overvoltage feed-in via Modbus on the slaves if the grid is lost. This allows more control over export when batteries are full or close to full."

There I understood what I was thinking about is possible, but I'd like more details about that.

- Is that a control I could implement on slave Multi using NodeRed?

- Is that GX NodeRed firmware an standard Victron one anyone could install and use?

- Could that work fine as the unique master/slave configuration?

- Maybe I should set a maximum power export limit, in order to not sending too much power to master Multi.

Does that makes sense?

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Okay cool,


I will need a bit more info to help you.

Are you offgrid?
How are you planing on connecting the second inverter? Connected to grid? Connected to AC-OUT of first inverter? What do you mean by grid-parallel?

Frequency shifting should only be used as a backup and not the primary means of control.

Control system is implemented on both Cerbo's Master and Slave.

Large Firmware is available to everyone, currently in Beta. It is developed by victron and includes Node Red.

Not sure what you mean "unique master/slave configuration" Like can they be diffrent? Yes, can you have multiple slaves? Yes.

The master has to be the biggest node in the network, It should not be possible to overload it with another node. Exporting from slave to master is only a problem when No grid and High SOC on the Master. Control system should stop all exporting from Slave to Master in this condition. Slowing decreasing export till SOC is 99%, starting at like 95% depending on battery size.