Adjusting voltage where frequency shift occurs on Multiplus 2

Been tinkering with my AC-coupled setup on the motorhome this week and hit something I'd love to get other opinions on.

Running a Multiplus 2 with a grid-tie inverter AC-coupled on the output, with the PV Inverter Assistant active. The frequency shift mechanism works well enough — once the batteries hit a certain state of charge, the Multi starts walking the frequency up to throttle back the PV inverter. Standard stuff.

What I'm less clear on is where exactly the voltage threshold sits that triggers the frequency shift to begin. I know you can adjust the frequency steps and the upper limit within the assistant settings in VEConfigure, but I'm struggling to find a clean way to set the voltage at which the whole process kicks off. At the moment it seems to start earlier than I'd like, meaning I'm losing usable solar harvest before the batteries are genuinely full.

My bank is 280Ah of Fogstar Drift LiFePO4, so I'm working with a fairly tight absorption window. The difference between "nearly full" and "full" in voltage terms is only a few hundred millivolts, and the assistant seems a bit blunt at that resolution.

Has anyone managed to fine-tune this beyond what the assistant wizard exposes directly? I've seen references to adjusting the "AC voltage setpoint" offset on the output side as a workaround — curious whether anyone's actually used that approach or whether it's more trouble than it's worth.

Also wondering if ESS behaves differently here compared to the standalone PV Inverter Assistant — though I appreciate ESS on a pure off-grid system is a different conversation entirely.

@Curly the frequency shift threshold is something I spent a good few evenings tweaking on my own Multiplus 2 setup last summer.

The setting you're after lives in VEConfigure under the Inverter tab — it's labelled as the AC high voltage setpoint that triggers the frequency shifting behaviour. Default is around 53Hz start point but the voltage where it begins ramping matters enormously.

What I found on my boat build was that pushing the trigger voltage slightly higher (around 54.2V on a 48V bank) gave the grid-tie inverter more headroom before it started curtailing, which meant the Fogstar lithium cells weren't being babied unnecessarily during partial-state-of-charge days.

Worth noting — the Victron community forum has a brilliant thread specifically on AC coupling ratios that might give you the confidence to push those numbers a bit further than you'd expect.

What grid-tie inverter are you running?

Ooh this is a good one! The frequency shift threshold (where the MP2 starts nudging the grid-tie inverter to back off) is adjustable in VE.Configure under the AC tab — specifically the "Upper frequency" settings tied to your battery charge state.

Worth experimenting with the 52-53Hz range depending on your battery chemistry. I've got a Fogstar Drift LiFePO4 bank and found shifting that threshold slightly earlier gave much smoother absorption behaviour rather than the grid-tie inverter hunting around.

@WattAMess25 is right that it takes a few evenings to dial in properly — don't rush it!

Main thing is making sure your grid-tie inverter's frequency trip settings actually respond within that window Victron creates. Some cheaper units have wider tolerances than advertised 🙄

Great thread @Curly! One thing worth mentioning that I don't think's been covered yet - have a look at the ESS Assistant settings if you're running ESS, as the voltage setpoints there can interact with your frequency shift thresholds in ways that aren't immediately obvious. I found on my own setup that the absorption voltage I'd configured was triggering frequency shift behaviour earlier than I expected, because the system was interpreting it as "batteries full" sooner than they actually were.

Also worth double-checking your battery temperature compensation settings if your motorhome is parked outside - voltage thresholds that work perfectly in summer can behave oddly on colder mornings. VictronConnect makes it reasonably straightforward to monitor what's actually happening in real time while you experiment. What battery chemistry are you running?

Really interesting thread this! Got a static caravan setup with a Multiplus 2 and I've been curious about this exact thing but haven't got round to AC coupling yet — planning to add a grid-tie down the line so this is dead useful.

Quick question for the more experienced folk here — does the frequency shift threshold behave differently depending on battery chemistry? I'm running Fogstar Drift LiFePO4 and wondering if the voltage points where the MP2 starts shifting frequency need adjusting compared to, say, a lead-acid setup where the absorption voltage is quite different?

Also @ClareCooper — the ESS Assistant point is interesting, is that a separate configuration to what you'd set in the Multiplus 2 itself via VEConfigure, or does the Assistant effectively override those base settings?