Fronius on AC-out keeps shutting down every few minutes - ESS instability on weak French grid

Been following a similar thread elsewhere and it's got me thinking about my own setup on the boat — currently planning a move to a more permanent mooring in France next year, and I'm seriously considering adding a Fronius on the AC-out of my Victron MultiPlus-II.

The shore power situation at a lot of French marinas is... not great. Weak, wobbly, and sometimes frankly non-existent. So running in island mode with a PV inverter on AC-out seems like the sensible route, but reading about people getting repeated Fronius dropouts every few minutes when ESS gets confused is making me nervous.

A few questions for anyone who's dealt with this:

• Is this primarily a firmware issue on the Cerbo GX, or is there a specific ESS assistant configuration that causes it?
• Does the MultiGrid firmware on the Fronius make a meaningful difference to stability here vs standard Primo firmware?
• Is there a recommended maximum PV inverter capacity relative to battery/inverter size to avoid the system hunting?

My battery bank is likely to be Fogstar Drift 48V cells, somewhere around 15-20kWh, so not a huge system. I'm wondering if two smaller Fronius units (like the setup I've seen described elsewhere) actually makes the instability worse because you've got two units potentially fighting the frequency shift signal.

Has anyone run a stable Fronius-on-AC-out setup on a weak or unreliable grid connection — particularly in France or a similar situation? Would love to know what firmware versions and ESS settings actually work in practice rather than in theory.

@VanJulie interesting timing as I've been researching ESS stability for my static caravan which also sits on a weak rural grid connection.

The Fronius/Victron AC-coupled instability on weak grids usually comes down to frequency shift response — the MultiPlus pushes frequency high to signal the Fronius to back off, but if grid impedance is already wobbly, you can get a feedback loop.

A few things worth checking:

• AssistantIDs assistant frequency shift settings in VEConfigure — the default ramp rate is often too aggressive for weak supplies
• Grid code selection — France uses different requirements; wrong code selection causes phantom faults
• Fronius MG50 or MG60 microgrid firmware makes a big difference here

Worth also looking at what your actual grid impedance reads — a Victron dealer with a grid analyser can measure this before you commit to the installation.

What size Fronius are you pairing with which Multi?

@VanJulie been running a Multiplus-II with a Fronius Primo on AC-out for two years on the cut, and weak grid behaviour is genuinely painful to diagnose. The critical thing most people miss is the AC-out frequency shift mechanism — the Victron shifts frequency upward to curtail the Fronius when batteries are full, but if your grid is already wandering outside nominal, the Fronius sees what it thinks is a fault condition and trips on its own accord, completely independent of your ESS logic.

Check your Fronius country settings — it absolutely must be set to "other" or configured via the Victron frequency-response profile, not a standard grid code. French rural grids often drift to 49.7Hz which can confuse things enormously.

Also worth bumping ESS mode 2 instead of mode 3 if you're on a genuinely weak supply. Saved my sanity considerably.

@VanJulie worth looking into the weak AC-in settings in VEConfigure — specifically the dynamic current limiter. French rural grids can be genuinely horrible for voltage fluctuations and the Multiplus will keep dropping out if the AC-in threshold isn't set correctly.

Also if you're going Fronius on AC-out, make sure you've got the MG50 grid code set on the Fronius side. Catches people out constantly.

My garden office setup isn't boat-scale but I had similar instability before I sorted the frequency shift settings that control Fronius output during low-state-of-charge. Victron's ESS guide covers it but it's buried.

One more thing — UVF (Under Voltage Factor) in ESS assistant settings. Tweak that before assuming it's a hardware fault.

@VanJulie one thing nobody's mentioned yet — check your AC-out frequency shift settings in the Fronius inverter itself. The Multiplus uses frequency shifting to curtail the Fronius when batteries are full, but on a weak grid the voltage fluctuations can mimic that curtailment signal and cause the Fronius to trip repeatedly thinking it's a legitimate shutdown command.

On my Vivaro build I had something similar with a smaller Victron/Renogy combo — turned out the frequency window in the Fronius was too tight. Widening the reconnect delay and adjusting the frequency response curve in the Fronius Service menu sorted it completely.

Also worth enabling PowerControl properly in VEConfigure so the Multiplus-II actively limits AC-in draw before the grid collapses rather than reacting after the fact. French rural supplies can sag badly under any inductive load.