Multiplus II continuously resetting

My Multiplus II 12/3000 has been throwing a wobbly every time I crank up the compressor on my roof-mounted Dometic — turns out 300Ah of AGM simply doesn't have the grunt to handle that inrush current without the inverter going into full existential crisis mode.

Anyone else found their battery bank is the actual bottleneck rather than the inverter itself? Victron's VictronConnect logs made it pretty obvious once I stopped ignoring the warnings like a sensible adult — voltage was sagging to about 10.8V on startup spike, which is basically waving a red flag in front of a bull.

A few things I'm considering:

• Swapping to lithium (eyeing up Fogstar Drift 200Ah cells) for the better discharge curve
• Tweaking the AC input current limit to let shore power share the load
• Adding a soft starter to the compressor — apparently these are genuinely magic for this exact problem

Currently living in the van full-time and the British summer is somehow simultaneously too hot and too cold to make a decision, which feels very on-brand for this country.

Has anyone here successfully run a decent-sized air con unit from a Multiplus II on a modest lithium setup without constantly babysitting it? Curious what bank sizes people are actually running day-to-day — the online chat tends to be very American-spec and those 12V AGM setups aren't really translating across the pond.

@FrostyHiker AGM's internal resistance is basically your compressor's nemesis — swap those 300Ah AGM for lithium (Fogstar Drift 12V 100Ah cells in parallel are cracking value) and that inrush current stops being a drama because lithium sags about as much as a concrete trampoline. 🪨

If you're wedded to the AGMs for now, dig into the Multiplus II settings via VictronConnect and lower the AC output voltage slightly — sometimes just softening the inverter's transfer characteristics buys enough headroom to stop the unit throwing a strop. You can also set a pre-alarm low voltage threshold to warn before things get critical. Long-term though, AGM and compressor fridges are a loveless marriage.

@FrostyHiker I had almost the exact same saga on the narrowboat last summer — Multiplus II kept throwing protection faults every time the 12V compressor fridge cycled on. Turned out the AGM bank was fine in isolation, but the combination of already-depleted cells plus the inrush spike was briefly dragging voltage below the Multiplus's low-voltage cutoff threshold.

Before you spend on lithium, check your VictronConnect settings first. I bumped up the DC input low shut-down from the factory default and the restarts stopped almost immediately. Worth five minutes before committing to a battery swap.

That said, @BoatPaddy isn't wrong — once I did eventually move to Fogstar Drift 200Ah cells, the whole system became noticeably more stable. Lithium's low internal resistance genuinely laughs at compressor inrush. But diagnose before you spend.

@FrostyHiker Ooh this is interesting — I've got a similar setup on my boat (Multiplus II 24/3000) and I'm curious whether the issue is purely battery chemistry or if there's something in the Victron settings worth tweaking first?

Before you go full lithium swap, have you had a look at the DC input low shutdown/restart voltages in VEConfigure? I found mine were set way too conservatively from the factory. Also worth checking if your PowerAssist is actually enabled — that can help bridge exactly this kind of inrush spike.

What's your battery bank voltage sitting at when it trips? And are you seeing a specific fault code on the Multiplus, or is it just going dark? Might help narrow down whether it's genuinely a capacity problem or more of a configuration gremlin 🤔

@FrostyHiker worth checking whether the Multiplus II is actually hitting its low voltage cutoff or overcurrent protection — they're quite different faults. Have you looked at what VEConnect is logging at the moment of the trip? Curious whether the battery voltage is collapsing before the fault triggers or after.

Also — is your Dometic wired directly to the inverter output, or through a distribution panel with any additional protection in the circuit? Wondering if there's something else throttling available current before it even reaches the compressor.

I'm planning a similar build and this thread is making me reconsider AGM entirely for any setup with high-inrush appliances.

@FrostyHiker this is actually what pushed me toward lithium for my static caravan setup — AGM just can't deliver those big instantaneous currents without the voltage sagging badly. Had a similar issue with a smaller load and the difference after swapping to a Fogstar Drift 200Ah was night and day.

One thing worth checking before you commit to anything expensive though — have you looked at a soft starter for the Dometic? I believe Victron sell one, or there are third-party options. Massively reduces that inrush spike and might save you a battery upgrade entirely. Anyone here actually tested one with a roof-mounted unit specifically? Curious whether it's enough on its own or just delays the inevitable when your AGMs are already a bit tired.