Victron Multiplus keeps switching to passthrough during the day - anyone seen this?

Had almost the exact same saga on my narrowboat last summer — turned out to be the AC input voltage dropping just low enough to trigger the passthrough threshold. My Multiplus was set with the lower AC limit at 207V and the marina shore power was wobbling around like a drunk on a Friday night.

Couple of things worth checking:

• AC input voltage sensitivity — crack open VE.Configure and look at your lower input voltage limit. Victron defaults can be a bit hair-trigger on older installs
• Transfer switch settings — there's a "dynamic current limiter" option that catches people out regularly
• Grid code settings — if yours was installed or updated recently, some UK grid codes are more aggressive about dropping to passthrough

The frustrating part is it often looks fine on a multimeter but the Multiplus is seeing momentary dips you're missing. Hook up VictronConnect or Venus OS if you've got a Cerbo and watch the AC input voltage graph over a few hours — you'll likely catch it red-handed.

Also worth asking — are you on solar with a separate MPPT, or is the PV going through the Multiplus itself? On my tiny house build I had a configuration conflict between the MPPT absorption cycle and the Multiplus that caused similar head-scratching before I sorted the system voltage settings.

What firmware version are you running? There was a period where certain builds had quirks around this.

Lefty59 | 847 posts | ⚡ Solar & Wind Hybrid

@BoxerWanderer that's a really common one on boats and motorhomes where the shoreline cable or connectors have seen better days! Worth also checking the AC input current limit setting in VEConfigure — if it's set too low, the Multiplus can get confused when there's a sudden load spike and temporarily prefer battery/inverter mode before snapping back to passthrough.

Also have a look at your "UPS function" toggle. With it disabled, there's a deliberate transfer delay which can look like passthrough switching when it isn't quite the same thing.

What's your shore power source — a marina bollard or a generator? Makes a difference in terms of where the voltage sag is likely originating. VRM portal logs would tell you exactly what the input voltage was doing at the moment of each switchover.

VivaroWanderer | 1,203 posts | 🔋 Off-Grid & EV Charging

Worth checking your AC input current limit setting in VEConfigure — if it's set too low, the Multiplus throttles back and can behave erratically during high-load moments. Had this exact headache on my Vivaro conversion when running a kettle alongside the inverter charger simultaneously.

Also, what's your DC input low shutdown voltage set to? If your battery bank (LiFePO4 or AGM?) dips under that threshold mid-day — perhaps from a poorly balanced Fogstar or Pylontech cell group — the unit will drop to passthrough as a protective measure even with solar coming in.

Pull the VRM logs if you're on a GX device; the passthrough events will show up clearly with timestamps so you can correlate exactly what triggered it. Much easier than guessing.

EcoFlow_Master | 312 posts | ☀️ Static Caravan Off-Grid

Had something similar on my static a couple of seasons back. Turned out the culprit was temperature — the Multiplus was tucked in a poorly ventilated cupboard, and as afternoon heat built up, it would start behaving erratically near its thermal thresholds, which manifested as what looked like passthrough switching but was actually the unit protecting itself.

Worth pulling the VE.Configure logs if you haven't already. The Alarm & Warning tab will show thermal events alongside the AC input faults — helps separate the two causes before you start chasing the wrong rabbit.

Also worth checking whether it correlates with peak solar output time rather than just daytime generally. Sometimes a charge controller pushing hard can introduce enough noise to upset things upstream.

RustyTinker | 634 posts | 🔧 Cabin & Motorhome

Had this exact headache in my motorhome last winter. Spent two days convinced it was a firmware issue before I finally sat down with VictronConnect and actually watched the AC input voltage in real time.

The culprit was a long, undersized hookup lead — voltage was sagging under load, dipping below the Multiplus's lower threshold and triggering the switch. Shortened the lead, problem gone.

Worth checking your Dynamic ESS settings too if you've enabled those — I've seen them behave unexpectedly when grid voltage wanders.

@VivaroWanderer makes a fair point on the input current limit, but in my experience the voltage sag issue is more common on site hookups than people realise. A cheap plug-in voltmeter on the input tells you everything within about thirty seconds.