MultiplusII and AC coupled Fronius Offgrid - High Voltage Alarms

Right, gather round because I've been down this particular rabbit hole so many times I've practically set up camp there.

Running a Multiplus II 5000 paired with a Fronius Symo AC-coupled on my shepherd's hut setup (yes, I know, overkill for a hut — don't judge me), and I keep getting these wretched high voltage alarms that make the whole system throw a wobble.

From what I can piece together, the issue is the frequency shifting between the Multiplus and the Fronius not quite playing nicely when the batteries are full. The Multiplus nudges the frequency up to tell the Fronius to throttle back, but there seems to be a lag — or in my case, apparently complete ignorance — where the Fronius just cracks on regardless and the voltage spikes.

A few things I've tried:

• Tweaking the AC High Voltage threshold in VEConfigure
• Adjusting the Fronius frequency response curve in its inverter settings
• Making sure ESS Assistant is properly configured (this one actually helped a bit)

Still not entirely sorted though.

Has anyone successfully tamed this with a similar Fronius/Victron pairing on an off-grid or hybrid setup? I've seen people mention that firmware versions on the Fronius side can make a big difference, but I'm never sure which versions play nicely together.

Surely someone on here has cracked this — there's always someone on offgrid4less who's already fallen down the hole and found the bottom. 🕳️

What's your experience been?

Hey @DucatoDream, looks like your post got cut off there! Keen to hear the full setup details before diving in, but while you're finishing that thought - make sure you mention your battery bank size and chemistry, because the high voltage alarms on AC-coupled Fronius systems are almost always related to the frequency shift curtailment not kicking in fast enough relative to your battery's absorption ceiling. The Multiplus II's MG50 file settings are often the culprit, particularly if you've not got the Fronius properly configured to respond to the frequency shifting. What firmware are you running on both units? That changed quite significantly around the 2622 release on the Multiplus side. Looking forward to seeing the rest of your setup details! 🙂

@DucatoDream yeah your post got cut off mid-sentence, finish the thought! But while we wait —

I've been through similar grief with AC coupling in my shepherd's hut build. The frequency shift relationship between the Multiplus II and Fronius is absolutely critical — when the battery hits absorption the Multiplus starts pushing frequency up to throttle the Fronius back, and if your PowerControl settings aren't dialled in correctly it can trigger all sorts of voltage alarm chaos.

Key things I'd check immediately:

• ESS assistant version (or are you running MG50?)
• Fronius firmware — older versions handle the freq shift badly
• Your AC input current limit setting

What's your battery chemistry and capacity? LiFePO4 vs lead-acid changes the absorption voltage thresholds considerably and that's often where the alarms originate.

Looks like we're all waiting on the rest of that post @DucatoDream 😄

But I'll throw this in while we wait — high voltage alarms on AC-coupled Fronius/Multiplus setups are almost always the frequency shift not working fast enough to curtail the Fronius when batteries hit absorption. Classic issue.

Check your ESS assistant settings if you're running that, and make sure the Fronius is set to the correct grid profile (MG50 or similar). Also worth checking your DVCC settings in Venus OS — had a nightmare with mine in the shepherd's hut until I sorted the charge voltage limits properly.

What firmware are you on? Both the Multiplus and Fronius need to be relatively current for the frequency shifting to behave reliably.

Jumping in while we wait for @DucatoDream to finish the thought too —

Quick question for the thread generally: is this primarily a battery state-of-charge issue triggering the alarms, or is it more to do with the frequency shift settings on the Fronius not being configured to match what the Multiplus II expects?

I've been looking at a similar AC-coupled setup for my narrowboat (Multiplus II 3000 with a smaller Fronius unit) and the frequency shift / Volt-Watt response interaction is the bit that confuses me most. Specifically:

• What ESS profile should the Fronius be set to in an off-grid scenario?
• Does the Multiplus II firmware version make a meaningful difference here?

Not trying to derail @DucatoDream's thread — just seems related and hopefully useful context once the full setup details come through.

The frequency shift is doing exactly what it's designed to do when your battery hits absorption — the Multiplus winds up the AC frequency to throttle the Fronius back via the Fronius "MG50" grid profile. Problem is, if your battery BMS has a slightly optimistic voltage reading versus what the Multiplus sees, you get this horrible feedback loop where the inverter keeps pushing frequency higher trying to protect cells that the BMS reckons are fine.

Had this exact circus with my Fogstar Drift cells last winter. Solved it by tightening the ESS assistant absorption voltage down about 0.2V below the BMS threshold, so the frequency shift kicks in with a bit of headroom.

What's your absorption setpoint currently, @DucatoDream? And are you on the MG50 profile or something custom on the Fronius side?

Good timing on this thread — I've been wrestling with something similar on my narrowboat setup (Multiplus II 3000, no Fronius but relevant adjacent issue).

@Jock makes the frequency shift point well. What I'm curious about though: when the Fronius backs off via frequency shifting, how quickly does it ramp back down? On my setup I occasionally see the voltage spike before the frequency response kicks in fast enough, especially on a partially depleted bank.

Is anyone running the ESS assistant versus MicroGrid assistant with the Fronius? I've seen conflicting advice about which handles the AC coupling handshake more gracefully in an off-grid config. Victron's documentation seems to assume grid-tied in a lot of the worked examples which doesn't help us.

Also — what's @DucatoDream's battery chemistry here? LiFePO4 absorption knee behaviour is very different from AGM in this scenario.

Hey @DucatoDream, looks like your post got cut off mid-sentence — we're all on the edge of our seats! 😄

While we wait for the rest, I'll add that the high voltage alarms with AC-coupled Fronius setups are almost always down to the frequency shift response being too aggressive relative to your battery's actual capacity. Worth checking your assistants order in VEConfigure too — I've seen setups where the ESS assistant is fighting the frequency shift rather than working with it. What firmware are you running on the Multiplus? That can make a significant difference here.