Victron Multiplus keeps switching to passthrough overnight - anyone seen this?

Had this exact issue on my shepherd's hut build last winter. Multiplus II 24/3000 was dropping to passthrough somewhere between 2-3am most nights, then picking back up once the sun hit the panels.

After a fair bit of head-scratching I traced it to the low battery voltage threshold being set too conservatively in VEConfigure. The default settings were cutting out earlier than they needed to, especially with the voltage sag you get under load overnight. Temps were dropping to near freezing which wasn't helping the battery readings either — I've got Fogstar Drift lithium cells and they do behave differently once it gets cold.

A few things worth checking:

• DC input low shut-down voltage — is it set appropriately for your battery chemistry?
• Are you running a BMS? If so, is it possibly triggering the Multiplus via the VE.Bus signal?
• Check VRM overnight logs if you've got a Cerbo GX — the SOC and voltage graphs usually tell the story pretty clearly

Also worth looking at whether you've got any parasitic loads quietly draining the bank more than you expect. I added a simple clamp meter overnight and found my water pump control board was pulling more than I thought.

What battery setup are you running and what does your overnight load look like? Makes a difference to where the fault might sit. Curious whether others have seen the BMS comms triggering this — I've read a few threads suggesting that's a common culprit but never fully confirmed it on my own setup.

@RussScott classic case of your Multiplus throwing a midnight strop — check your DC input low shutdown voltage in VictronConnect, mine was set way too conservatively from factory and kept bottling it right when my Fogstar lithium had plenty left to give! 🔋

@BatteryPaula is on the right track with the voltage settings, but also worth checking your AC input current limit in VE.Config — had this exact scenario on my narrowboat last autumn. Turned out my Multiplus was seeing a brief voltage sag on the shore power feed around 2am when the marina's overnight load shifted, and it was toggling into passthrough rather than inverting properly.

Check the UPS function setting too. If it's enabled and your AC input goes slightly out of spec (even momentarily), it'll do exactly what you're describing.

Pull the VE.Bus System Configurator logs if you haven't already — they'll show you precisely what triggered the switch. Don't just guess at voltage thresholds and start bumping numbers blindly, the logs will tell you whether it's a DC low issue or an AC input problem.

@RussScott had almost identical behaviour on my boat last autumn. Before you go down the settings rabbit hole, pull up your VRM logs if you've got a Cerbo or CCGX — the state-of-charge graph at 2-3am will tell you straight away whether it's a low voltage cutout or something else triggering passthrough.

One thing neither @BatteryPaula nor @GafferTapeKing19 have mentioned — check your battery temperature compensation settings. Cold nights can make cells behave like they're lower capacity than they actually are, especially if you're running lithium and the BMS is being conservative. What battery chemistry are you on? That changes the diagnosis considerably.

Also worth checking if you've got ESS assistant installed or just running standalone — the behaviour can differ quite a bit between the two modes.

@RussScott one thing nobody's mentioned yet — check your battery temperature compensation settings. In winter, cold batteries have a higher internal resistance and sag harder under load. Your Multiplus might be seeing a legitimate low-voltage condition at 2-3am when temps drop lowest, even if your batteries are nominally fine.

If you're running lithium (Fogstar Drift cells or similar) make sure your BMS low-voltage cutoff isn't triggering before the Multiplus even gets a chance to react — sometimes it's the BMS pulling the plug and the Multiplus just follows.

Also — what's your overnight load? My garden office setup had something similar until I realised a small heater on a timer was hammering the batteries right around that window. Sounds obvious but worth ruling out before diving deep into VEConfigure.