Anyone else had grief with a Victron Multiplus 2 tripping on a small resistive load?

Bit of a weird one and I'm hoping someone here has seen this before. I've got a Multiplus 2 48/3000/35 installed in my off-grid cabin setup, running off a 48V 200Ah lithium pack (Lifepo4, 16-cell DIY build). The inverter's been solid for about eight months but last week it started tripping out with a low battery warning whenever I switched on a basic 1500W fan heater. No other significant loads running at the time, just the fridge ticking away.

The weird part is the battery pack is sitting at 52.4V under no load, which should be well healthy. The moment that heater kicks in though, it drops to around 47V on the BMS display within a second or two and the Multiplus throws a low battery alarm and shuts down. Doesn't seem right for a 200Ah pack — the heater's only pulling about 30A at 48V, which should be well within its comfort zone.

I've checked all the main cable connections and they look fine to me, but I'm wondering if I've got a dodgy cell dragging the pack down under load, or whether this could be a Victron settings issue. My low battery cutoff is set to 46V in VE.Configure and the absorption/float are standard. Has anyone seen a healthy-looking resting voltage collapse like this under a modest load? Is this a BMS problem, a bad cell, or am I missing something obvious in the Victron settings?

@OddJobBob79 seen this exact thing with mine. Turned out to be the AC input sense settings in VE.Config — had it set too sensitive and it was interpreting the inrush from a small resistive load as a fault.

Worth checking your UPS function setting too. If that's enabled it can cause odd tripping behaviour when switching between sources.

Also — what's your battery cable run like? Even a small voltage drop under load can confuse the BMS and trigger a cutoff that looks like the inverter tripping.

What firmware are you on? There were some quirky builds around the 490-500 range that had protection threshold issues.

@OddJobBob79 had something similar on the narrowboat actually. Turned out my earth-neutral link was dodgy — the MP2 was seeing a floating neutral and getting confused about load detection. Worth checking your AC wiring before diving into VE.Config rabbit holes.

Also worth noting — what's your PowerAssist set to? Mine was set really aggressively and it would trip on anything with a slightly spiky inrush, even small resistive loads that shouldn't cause it.

Check your event logs in VE.Connect first, the fault codes will tell you if it's a protection trip vs a sensing issue. Saves a lot of guessing.

Hey @OddJobBob79, just to add something different to what @GrumpyWanderer and @DaiBennett have already covered — have you checked your DC ripple voltage under load? With a resistive load switching on, even a modest one, if your BMS has any internal resistance or your battery cabling isn't quite up to scratch you can get a momentary voltage dip that the MP2 interprets as a fault condition. Worth chucking a multimeter across the battery terminals when the load kicks in and seeing what you get. Also, what's the "Low DC shut-down" set to in VE.Configure? I've seen folk inadvertently have it set a touch too high and it catches them out on transients. Cheers

Really interesting thread — I had something similar crop up during testing on my van build before I got everything dialled in.

One thing nobody's mentioned yet: have you checked the low battery shutdown voltage in VE.Config? Even with a healthy 48V pack, if that threshold is set conservatively and there's a momentary voltage sag under load (even a small resistive one), the MP2 can trip before the BMS does.

Also worth connecting a laptop via the VE.Bus interface and watching the #65 inverter mode parameter in real time when you apply the load. That'll tell you exactly why it's tripping — overload, low battery, temperature, or something else — rather than guessing from the front LEDs alone.

What does the pack's BMS report during the event? Any logged overcurrent or undervoltage flags there?