Anyone else's Victron MultiPlus throwing a fit when the alternator kicks in on a narrowboat?

Mine's a MultiPlus 12/3000/120 and every time the engine fires up, the inverter throws a low voltage alarm for about 30 seconds before it settles down — even with a Fogstar 200Ah lithium that's sitting at 80% SOC. Feels like it's panicking over nothing.

I've had a poke around in VictronConnect and the AC input current limit is set sensibly, but I'm wondering if the dynamic current limiter is doing something daft during that initial alternator surge. Narrowboat alternators aren't exactly known for their gentle manners.

Anyone had similar on a boat install, or is this just a "live with it" situation? Tempted to adjust the low voltage restart thresholds but don't want to break something I can't fix mid-canal in the middle of nowhere.

@LazyFisher this is almost certainly your charge current limit setting in VictronConnect — the MultiPlus sees the alternator voltage spike, tries to push current into the lithium, the BMS momentarily restricts it, and you get that brief voltage sag causing the alarm.

Few things worth checking:

• AC input current limit — make sure it's not fighting with your alternator-fed DC side
• More critically, have you set a proper charge profile for the Fogstar in the MultiPlus? Lithium absorption/float voltages need to be spot-on
• Is your alternator isolated from the MultiPlus DC bus, or are they sharing the same busbars? That interaction can cause exactly this 30-second chaos

On my narrowboat I had nearly identical behaviour until I wired a Victron Battery Protect between the alternator feed and the main bus — sorted it immediately. Also worth checking your VSR settings if you're running one.

Had this exact issue on my setup. Worth checking your AC input current limit too — if the MultiPlus thinks shore power has appeared when the alternator spikes, it can get confused about what's actually happening.

Also, have you set the battery capacity and charge voltage correctly for lithium in the VE.Configure settings? The default profiles are tuned for AGM and the absorption/float voltages will be wrong for Fogstar cells, which could be making the voltage swings look worse than they are.

One more thing — what's your DC cable run length to the battery? Voltage drop on a narrowboat with undersized cable can make a healthy battery look poorly during inrush.

Hey @LazyFisher — worth looking at your battery monitor settings and specifically whether your MultiPlus knows it's dealing with lithium. If it's still configured for AGM defaults, the charge absorption voltage thresholds will be off, which can cause it to panic during that initial alternator surge before the BMS has had a chance to settle.

Also, have you got a DC-DC charger (like a Victron Orion-Tr Smart) between your alternator and the Fogstar? Running alternator output directly into lithium is quite harsh on the alternator anyway, and a DC-DC will smooth out exactly the kind of voltage spike you're describing. Might solve both problems in one go.

What firmware version are you running on the MultiPlus? Some older versions had quirks with lithium profiles that were patched fairly recently. 🔧