MPII not accepting other MPII on AC In

Q&A and troubleshooting
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1

Dear Victron fans

We’re stuck with a problem that I need help with. I hope you can give us some ideas.
The existing three-phase system with Multiplus II 5kVA works fine for some years. The PV panels are all DC-coupled. The system is entirely off-grid.

In preparation for winter, we aim to connect a single-phase backup battery system comprising a Pylontech US 3000C and a Multiplus II GX 5kVA, to the main system (also known as “chaining” Multiplusses). By itself, both systems work fine. Later, it is also planned to operate a single-phase generator.

The AC out of the Backup Multiplus (acting as “Generator/Grid/Shore”) is connected to the AC in of the Main Multiplus L1.

    flowchart
         Multiplus_GX_Backup -->Multiplus_Main_L1
         hidden(a) ~~~ Multiplus_Main_L2
         Generator["Generator (planned)"] --> Multiplus_Main_L3
         style hidden display: none,font-size: 0

However, the AC Input on the the Main Multiplus system does not activate.
The input is seen on the Main system just fine. Here is a screenshot from the Cerbo:

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Further, VeConfigure shows “zero seconds left” for the Mains connection. But: that never ends.

Screenshot_2025-10-05_17-48-52
Screenshot_2025-10-05_17-48-52186×267 1.95 KB

When forcing “AC Input Ignored” to false with NodeRed/VEBus, it immediately jumps back to the Ignored-state.
During the short time, when AC Input is not ignored, it is also shown in the overview:

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But then falls back to no connection:

image
image492×284 83.1 KB

We wouldn’t expect that the sine wave of the Multiplus II GX is bad, that another Multiplus won’t accept it, right?

In VEConfigure we of course already tried all the usual suspects (as seen in the generator training and in the generator FAQ).

  • Enabled Weak AC Input
  • Disabled UPS
  • Reduced the AC Input Current Limit
  • Enabled the Dynamit Current Limiter
  • Accept wide range of input frequency
  • Grid code is None (off-grid)
  • Disabled Power Assist
  • On the GX: AC Input Types: Grid, Shore and Generator

What else can we try to make the AC In connection work?
Is there any (VEBus) value that we can read to find out why the input is ignored?

2

Yes. You need to turn off switched as a group in the three phase

Why don’t you just use the inverter to charge to charge your current bank directly (using the generator)? Rather then through ac in.

1 Like
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Okay, that makes sense. I have the feeling that this switch is not very well documented, isn’t it?

As I’m currently not on site, I’ll have to do that with Remote VE Configure, and obviously, I’m a bit more careful and get this warning message:

Changing the VE.Bus configuration of the remote system is disabled.

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For me, it’s now not clear why it’s disabled by default. The temporary shutdown is OK.

EDIT: Appears to have worked, the AC input is now shown constantly, and draws some energy.

The issue is not the generator, but the two different battery banks. The backup battery system is of a different BMS and voltage than the main system.

4

Yeah, switched as a group is the normal use of a system for synchronization purposes particularly when there is a grid connection and phases etc and (ess. Assistant)

But since you are none of those you would be able to remove it for your use case.

Not sure much documentation is needed. Your system either uses it or not. And the warnings are pretty clear when you disable it as you saw.

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