Victron Multiplus keeps switching to passthrough at night — anyone else had this?

Had almost exactly this on my narrowboat last winter — drove me absolutely mad for about three weeks before I tracked it down.

Mine was switching to passthrough somewhere around 2–3am every night like clockwork. Turned out the battery voltage was dipping just enough to trigger the low battery cutoff threshold, even though the State of Charge on my Victron BMV-712 looked perfectly reasonable. The culprit was a dodgy cell in one of my old AGM batteries that was masking itself during the day but showing its true colours under overnight load.

A few things worth checking in your situation:

• VEConfigure settings — specifically your low battery cutoff and restart voltages. These can be surprisingly aggressive out of the box depending on battery type
• Are you on lithium or lead acid? If lithium, check whether your BMS is momentarily disconnecting and confusing the Multiplus
• What's your overnight load like? Even a small parasitic draw adds up over 8 hours
• Check the AC input voltage too — if shore power or a generator is fluctuating, sometimes the Multiplus will briefly drop to passthrough then recover

The VRM portal was genuinely brilliant for diagnosing mine — I could see the exact moment everything shifted and correlate it with battery voltage graphs.

What battery bank are you running and how old is it? That's usually where I'd start the investigation. Others on here will have seen more of these than me, would be curious whether anyone's had it caused by something completely different.

@SueParker64 classic one this — worth checking your AC input current limit setting in VEConfig. If it's set too low the Multi will just hand off to shore power rather than inverting.

Also check your dynamic current limiter is off unless you actually need it. Caught me out on my narrowboat setup years ago.

The other thing nobody mentions — battery voltage at that time of night. If your SOC is dropping and you've got a low voltage cutoff set slightly too high, it'll bail out to passthrough before the batteries are actually flat. Check your CCGX or Cerbo logs for the exact voltage when it switches.

What firmware version are you running? Some older versions had quirks with this.

Great shout @Wayne1980 on the AC input current limit — worth adding that if you're on shore power via a marina bollard, those supplies can be genuinely rubbish at 2–3am when heating systems cycle on across the whole pontoon and voltage drops. The Multiplus will switch to passthrough if it detects the incoming AC quality isn't up to scratch.

@SueParker64 what did yours turn out to be in the end? I'm curious whether it was a settings issue or something external.

Also worth checking your battery voltage threshold settings in VEConfig — if your batteries are sitting lower than expected overnight and the SOC estimate is drifting, the unit can get confused about when to prioritise shore power. What battery chemistry are you running?

Really common one on boats — I spent ages chasing this on my narrowboat conversion before realising the culprit was the dynamic current limiter interacting with a dodgy shore power connection.

Worth checking your AC input voltage in the VRM portal overnight. If the marina supply is sagging below your configured minimum input voltage (default 187V on most UK installs), the Multiplus will drop to passthrough to protect itself. Cold nights + high marina demand = voltage sag is surprisingly common.

In VEConfig, check "Lower DC input voltage limit" isn't set too aggressively either — if your battery bank is slightly undersized for overnight loads, it could be hitting that threshold around 2–3am when temps drop and internal resistance rises.

What battery chemistry are you running @SueParker64? Flooded lead acid behaves very differently to LiFePO4 in cold conditions and that changes the whole diagnosis.

Has anyone checked whether this could be tied to a low battery voltage trigger rather than the AC input side? On my static caravan setup I had the Multiplus dropping to passthrough when the batteries dipped below the configured threshold — turned out my Fogstar cells had some variance between them and one group was pulling the overall voltage reading down faster than expected.

Worth connecting VictronConnect and watching the live data around the time it switches — you can see whether it's an AC input event or a battery voltage event. The history logs in the app should also show which condition actually triggered it.

@SueParker64 did yours ever show a pattern with battery state of charge at the time, or was it purely time-based?