Anyone else had grief with their Victron Multiplus cutting out under heavy load?

Fitted a Multiplus-II 24/3000/70 in my narrowboat setup about three months ago, paired with 400Ah of lithium (Fogstar Drift cells) and a 60A Cerbo GX running everything together. On paper it should handle my 2.5kW inverter load no bother, but I keep getting nuisance trips when I fire up the microwave and the induction hob at the same time — roughly 2,200W combined. The unit throws a low battery warning even though the SOC is showing 85-90% on the Cerbo display.

I've been through the VE.Configure settings and dropped the DC input low voltage threshold down from 23.5V to 22.8V, which helped a bit but hasn't cracked it. My cable run from the battery bank to the Multiplus is about 1.8 metres each way, using 70mm² tinned copper, so I don't think volt drop on the cables is the culprit. The Fogstar BMS is set to a 200A discharge limit, which should be plenty.

Has anyone seen this with a similar setup, or know whether it's worth looking at the dynamic current limiter settings in VE.Configure? I'm wondering if the Multiplus is reacting to a voltage sag at the battery terminals that the Cerbo isn't catching fast enough. Any pointers would be brilliant.

MIA_VanLife | 847 posts | ⚡ Solar & Van Builds

@CotswoldBoater had almost identical grief with mine last year! Worth checking your DC cable sizing first - people underestimate how beefy those need to be on a 24V system. Voltage drop under load can trick the Multiplus into thinking the battery is sagging when it isn't.

Also, have you checked your Low Battery Disconnect settings in VEConfigure? The Fogstar Drift BMS communicates over VE.Bus but sometimes the Victron default cutoff thresholds don't play nicely with Fogstar's own protection settings - you can end up with both fighting each other simultaneously.

What's the actual load you're pulling when it cuts? Knowing whether it's tripping on overload, low voltage, or temperature would really help narrow it down. The Cerbo logs should tell you exactly which fault triggered it.

@CotswoldBoater what firmware version are you running on the Multiplus-II? I had something similar on my boat setup and it turned out the low battery voltage cutoff was set too conservatively out of the box — was tripping before the Fogstar cells were anywhere near actually low.

Worth checking in VictronConnect whether the Dynamic Current Limiter is enabled too. That can cause unexpected dropouts under sudden heavy loads (kettle, inverter microwave, that sort of thing).

Also — are your battery cables adequately sized for 3000W? Voltage drop under load can fool the protection thresholds even if your resting voltage looks fine. What gauge are you running from bank to inverter?

RayPowell | 312 posts | 🔆 Van & Tiny House Solar

@CotswoldBoater worth checking your DC ripple voltage under load — Victron kit can get jumpy if the BMS is briefly throttling the Fogstar cells and creating voltage sag that trips the low voltage protection. Had this exact scenario in my van build with a different lithium setup.

In VE.Configure, check what your Low DC shut-down threshold is set to. Factory default is sometimes too aggressive for Fogstar's discharge curve at 24V. Also look at the Dynamic Current Limiter setting — enabling that smoothed things right out for me on heavy motor loads.

@GrumpyWarden raises a fair point on firmware too — worth running that alongside the VE.Configure checks rather than chasing one thing at a time.

What loads are actually triggering the cutout? Inductive stuff like a pump or compressor fridge?