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

Had almost exactly this on my shepherd's hut build last summer. Turned out my BMS was sending a false high-voltage signal to the Multiplus via the VE.Bus, which made it drop to passthrough thinking the batteries were full when they absolutely weren't.

Few things worth checking:

• What's your battery voltage actually doing when it switches? Pull up VictronConnect or VRM and look at the exact moment it happens — the pattern usually tells you everything
• Check your VE.Bus BMS settings if you're running a lithium setup. The charge disconnect signal can trigger passthrough behaviour unexpectedly
• Grid/shore input voltage — if it dips even briefly below your input voltage range, the Multiplus will drop to passthrough automatically. Had a dodgy hookup cable cause this for ages before I found it

Also worth checking whether it's actually passthrough or whether it's a brief AC ripple issue making it look like passthrough in the logs.

What batteries are you running? If it's Fogstar Drift cells with a Daly BMS, there's a known quirk with the charge parameters that can cause odd handshaking with Victron kit. Took me a good two weekends to sort mine properly.

What does your VRM history show in terms of state of charge at the moment it switches? That'd help narrow it down considerably.

@TinaHenderson worth checking whether it's actually the BMS signal or the AC input voltage fluctuating. Had something similar in the van — Multiplus kept dropping to passthrough and I was convinced it was a comms issue.

Turned out my hook-up cable had a dodgy connection causing the AC input to briefly dip, and the unit was doing exactly what it should — protecting itself.

Quick test: pull up VictronConnect while it happens and watch the AC input voltage graph in real time. If you see it sagging below your configured threshold right before the switch, that's your culprit rather than any BMS chatter.

Also worth checking your AC input current limit setting in VEConfigure — if it's set too low, any modest load spike can trigger passthrough mode.

Rule out the simple stuff first before diving into VE.Bus diagnostics.

Great thread, this caught me out too last year. One thing neither @TinaHenderson nor @SussexVanLifer have touched on yet — check your AC input current limit setting in VE.Configure. If it's set too low, the Multiplus will drop to passthrough when it decides it can't support both the load and charging simultaneously. I had mine set at 6A from a previous hookup and forgot to update it when I moved to a proper 16A supply. The unit was basically throwing its hands up during peak solar hours when loads spiked. Dead simple fix once you spot it. Worth logging everything through VRM for a day or two as well — the graphs make it really obvious when and why the switchovers are happening rather than just guessing at the cause.

What's the actual trigger showing in the VEConfigure logs when it switches over? Because I've got a similar setup across both my motorhome and static caravan and I've never been able to tell if passthrough is being triggered by the input side or output side without pulling the logs first.

Also — is anyone running a Cerbo GX here? Wondering whether the Venus OS version matters, as I've seen mention of a firmware bug causing spurious assistant triggers on older builds.

@ForestWanderer you said there's something others haven't touched on yet — don't leave us hanging, what is it?

@GrumpyDrifter makes a good point about the logs — that's where I'd start too.

One thing worth checking that nobody's mentioned yet: have you looked at whether your AC input current limit is set too low in VEConfigure? If it's throttling the incoming current and then a large load kicks in (kettle, compressor, whatever), the Multiplus can momentarily flip to passthrough rather than supplement from the battery.

Caught me out in my garden office build — I'd left the input limit at 6A from a previous test and forgot to bump it back up. Looked like a BMS/voltage issue on the surface but was something far simpler.

What's your AC input limit set to currently, and is the passthrough coinciding with any particular loads switching on?