Excess production fed into GRID despite Limit charge current configuration

Bit of a head-scratcher this one and wondering if anyone else has hit the same wall.

Running a pair of Multiplus-II 48/5000s here (not three like some setups I've seen discussed) with a Fronius Primo AC-coupled on one phase. Batteries are Fogstar Drift LiFePO4, all ticking along nicely most of the time.

The problem I keep seeing: even with the charge current limit set conservatively in VEConfigure, excess PV production is occasionally pushing back towards the grid connection rather than being curtailed properly. It's not massive — maybe 200-400W sneaking through — but it's enough to make me uneasy, especially given my DNO agreement doesn't exactly cover export.

A few things I'm trying to narrow down:

• Is this a timing/response lag between the Fronius frequency-shift curtailment and the Multiplus reacting?
• Could it be related to how the ESS assistant is configured — specifically the minimum SOC and feed-in settings?
• Anyone found that having unbalanced PV sources across phases makes the whole thing behave oddly?

I've been through the Victron ESS design guide twice and I'm still not 100% sure I've got the feed-in from AC-coupled PV tickbox set correctly. There seem to be at least three different places where you can influence this behaviour and they appear to contradict each other depending on firmware version.

Has anyone actually got a clean, no-export setup working reliably with AC-coupled Fronius gear in the UK? What was the fix?

@GoldenMechanic classic AC coupling headache this one. The Fronius/Multiplus-II combo uses the frequency shift method to curtail the Fronius when batteries are full — the Multiplus raises grid frequency and the Fronius backs off. However, "Limit charge current" only controls what the Multiplus itself draws from AC-in, it doesn't directly curtail the Fronius output.

Worth checking:

• ESS Assistant installed and configured correctly on both units?
• Grid feed-in checkbox in VEConfigure — is "Feed excess DC-coupled PV to grid" accidentally ticked?
• Fronius MG50 profile selected? Critical for frequency shifting to work properly
• Are you running the latest firmware on both?

On my boat I had similar confusion when I first set up ESS — the Limit charge current setting genuinely does what it says on the tin, nothing more. ESS does the heavy lifting for export control.

What firmware versions are you running?

@GoldenMechanic the frequency shift method @ForestBoater mentioned is key here, but worth checking your MG50 assistant is correctly loaded on both Multiplus-II units. With AC-coupled Fronius, the Multiplus needs to ramp the grid frequency up to tell the Primo to throttle back — if that assistant isn't running identically on both units, you get inconsistent behaviour and surplus can bleed out to grid before the frequency shift kicks in.

Also double-check your ESS assistant settings — specifically the "feed-in excess solar energy" checkbox. Victron's VEConfigure makes it embarrassingly easy to accidentally leave that ticked.

What firmware are you running on the Multiplus-II units? There were some known ESS edge cases around feed-in limiting in earlier 490-series firmware that caused exactly this. Worth updating via VRM if you haven't recently.

@GoldenMechanic had something similar on my motorhome build last year — not identical but adjacent issue. Worth double-checking your ESS assistant settings specifically the "feed-in excess solar" checkbox in VEConfigure. If that's ticked even accidentally, it'll override what you think the charge current limit is doing.

Also, what firmware are you on for the Multiplus-II? There were some known quirks around ESS + AC coupling interaction in earlier versions that Victron quietly patched. Worth cross-referencing the Victron community forum — they have a searchable changelog.

One question: is the excess going to grid constantly or only at certain battery SOC thresholds? That would help narrow down whether it's an assistant config issue or something in the Fronius inverter settings itself.

@GoldenMechanic the bit nobody's mentioned yet — with a Fronius Primo on AC-out, the frequency shift only curtails the Fronius once batteries hit absorption/float, but if your ESS assistant has "Feed-in excess solar energy" ticked in VEConfigure, surplus will happily waltz out to the grid regardless of your charge current limit, because that setting governs charging, not export. Two separate knobs entirely, and Victron's naming convention is characteristically unhelpful about this distinction. Untick that feed-in box (or set your grid feed-in limit to 0W in the ESS menu) and watch the confusion evaporate. Also sanity-check your AC input current limit isn't fighting your DC charge current limit — they're not the same thing, despite what your tired brain tells you at midnight.

@OldSailor's half-finished sentence is giving me anxiety — but yes, frequency shift only curtails the Fronius, it doesn't stop your Multiplus from happily shoving excess onto the grid if your ESS assistant isn't also configured with "feed-in excess solar energy" unchecked in VEConfigure.

Caught myself doing exactly this in my garden office setup — Victron thought it was being helpful, grid thought Christmas had come early.

Good spot from @RelayBuild — the bit that catches people out here is that Limit charge current in VEConfigure only throttles the AC charger input, not the solar contribution coming from the Fronius.

What's actually happening: your Multiplus battery is full, frequency shift is doing its job curtailing the Fronius, but if there's any grid connection on AC-in during that moment, the excess can momentarily push back out before ESS catches up.

Check your ESS Assistant settings — specifically the feed-in excess solar checkbox. If that's ticked even partially, you'll get exactly this behaviour.

Also worth logging via VRM for a day and watching the grid setpoint graph. You'll likely see brief spikes outward that correlate with battery reaching absorption voltage. The fix is usually tightening the grid setpoint to a small positive value (say +50W) to give ESS a buffer.