MultiPlus 3000/ 48V, frequency distortion when starting a RV A/C

Running a Truma Aventa on my MultiPlus 3000/48V setup and getting some odd behaviour I'd love to pick apart with you lot.

My bank is a 280Ah 48V LiFePO4 (Fogstar Drift cells, homebuilt). The Truma pulls around 1.2–1.4kW at full belt — fine in theory, well within the MultiPlus's headroom. But on startup I'm seeing the frequency swing noticeably before it settles. The Truma's inverter-driven compressor seems to confuse the MultiPlus briefly, like they're both trying to figure each other out.

It's not tripping anything, but my Cerbo GX logs show the frequency dipping and spiking in that first 10–15 seconds. Doesn't happen with other loads — angle grinder, kettle, all fine.

A few things I'm wondering:

• Is this a PowerAssist tuning issue?
• Could it be related to the AC out waveform interacting badly with the Truma's own inverter board?
• Anyone tweaked the UPS mode or charge current settings in VE.Config to smooth this out?

I've seen mentions of similar behaviour on boat installs where two inverter-type devices are essentially talking over each other on the same circuit. Running a similar setup on my garden office feed but never seen this there — though I'm not throwing compressor motors at it either.

Would be really interested to hear from anyone running modern inverter-driven A/C units off a Victron system. Particularly if you've had to dial in specific settings to keep things happy.

@GoldenGaffer what BMS are you running on those Drift cells? I'm asking because I had something similar starting a Webasto on my narrowboat — turned out my BMS was momentarily throttling discharge during the inrush current spike, which I reckon was causing the inverter to briefly lose its footing on frequency.

Also worth checking — have you got Power Assist enabled in VE.Config? I found that made a noticeable difference with high-inrush loads on my MultiPlus setup.

What does VictronConnect show for battery voltage during the startup moment? If it's dipping hard below 46V or so, that's probably your culprit rather than anything wrong with the Truma itself.

@GoldenGaffer the Truma Aventa's compressor startup is notoriously spiky — I've seen it pull a brief ~2x running current surge that catches inverters off guard, even beefy ones like the MultiPlus 3000.

Worth checking a few things:

• VE.Configure → UPS mode disabled, and ensure your AC input current limit isn't artificially throttling available power during the startup transient
• Dynamic Current Limiter — if you've got shore power connected, this can actually help smooth the handover
• What's your DC cable sizing and bus bar arrangement? A voltage sag at the battery terminals during that surge will cause the MultiPlus to do all sorts of peculiar things

I ran into similar gremlins on my cabin setup before I properly torqued down every busbar connection. Embarrassingly simple fix, but the frequency wobble was genuinely baffling until I found it.

What does VRM show for DC voltage at the moment of the distortion?

Classic MultiPlus gotcha — check your transfer switch current limit in VEConfigure, but more critically, enable PowerAssist if you haven't already; it lets the MultiPlus borrow from the AC input momentarily during that compressor spike rather than collapsing under it.

Also worth bumping your AC output frequency tolerance setting — by default it's tight, and the Truma's variable-speed compressor can briefly confuse the inverter into thinking something's wrong.

On the BMS front (as @DerekMoore89 was getting at), if yours is set to cut out below, say, 44V under load, that startup surge will trigger it before the MultiPlus even knows what's hit it — I had exactly this nonsense on my static van until I widened the low-voltage disconnect on my JK BMS.

@GoldenGaffer really interesting one this — I've been eyeing up the Truma Aventa for my motorhome build so watching this thread closely!

Quick question though: have you checked your AC input current limit settings in VictronConnect? I ask because when I was commissioning my MultiPlus II on the garden office, I accidentally had it set too conservatively and it was causing all sorts of weird frequency wobble under heavy inductive loads.

Also — what's your DC cable run like between the BMS and the MultiPlus? I've read that even a slightly undersized cable can cause enough voltage sag during compressor startup to confuse the inverter's regulation loop. What gauge are you running?

Curious whether the distortion happens on shore power pass-through or purely on inverter mode too — that would help narrow it down quite a bit, I'd think?

@GoldenGaffer also worth checking your BMS discharge rate limit — if those Fogstar Drift cells are set conservatively, the BMS might be briefly throttling output just as the compressor kicks in, which compounds the inverter's own wobble into something truly spectacular.