Anyone else had grief with a Victron MultiPlus cutting out under heavy load on a small battery bank?

Been running a MultiPlus 12/1200/50 in my van for about eight months now and it's generally been brilliant, but I've started getting these frustrating cutouts when I run the microwave (it's a 700W rated Tesco own-brand thing, but pulls closer to 1050W at the wall). The inverter just shuts off, LED flashes amber a few times, then recovers after 30 seconds or so. No fault code I can obviously read without hooking up a laptop.

My battery bank is four 100Ah 12V AGM batteries wired in parallel — so 400Ah in total, but I know AGMs don't like being hammered too hard. I've got 4mm² cable running from the batteries to a 250A fuse and then to the inverter. I'm starting to wonder if the cable run is a bit long (about 1.2 metres each way) and whether I'm getting enough of a voltage drop to trip the low voltage protection, even though the batteries are ostensibly well charged.

Has anyone used VictronConnect to pull the history data and actually seen what's triggering this? I've downloaded the app but I'm not sure I need the Bluetooth dongle or whether the built-in interface on the MultiPlus handles it. Also open to suggestions on whether 4mm² cable is simply undersized for this inverter — I've seen people mention going up to 70mm² on bigger installs but wasn't sure where the sensible cutoff sits.

@RelayAdventure classic low-voltage disconnect scenario. The MultiPlus has a default low-battery shutdown around 10.5V, and a 700W microwave will easily pull 100A+ at 12V when it peaks on startup — that's a brutal surge on a small bank.

A few things worth checking:

• VE.Configure — look at your "DC input low shut-down" voltage; you can tweak it cautiously
• Battery cable gauge and fuse proximity — voltage drop across undersized cables mimics a low-battery condition at the inverter terminals
• What battery chemistry are you running? If it's flooded lead-acid or even AGM, internal resistance climbs sharply under that kind of instantaneous draw
• How many Ah are you actually working with?

My 12/3000 on a 200Ah Fogstar Drift LiFePO4 handles a microwave without flinching, but I went 70mm² cable throughout which made a noticeable difference.

Hey @RelayAdventure, worth checking your battery cabling as well as the voltage settings. Even with decent batteries, undersized or lengthy cable runs create voltage drop under surge loads, and a microwave can pull a nasty spike on startup. I'd check the cable gauge between your batteries and the MultiPlus - anything less than 35mm² on a 12V system that size is asking for trouble. Also, have you configured the PowerAssist feature in VEConfigure? It can help bridge those short surge moments. What batteries are you running - AGM, lithium? That'll make a big difference to how much sag you're seeing under load.

Hey @RelayAdventure, one thing nobody's mentioned yet — have you checked the AC input current limit settings in VE.Configure? Sometimes people overlook the transfer switch settings which can cause odd cutouts that look like low-voltage but aren't.

Also worth considering: microwaves are notorious for pulling significantly more than their rated wattage at startup. A 700W microwave can easily spike to 1,200–1,400W in the first second or two, which on a 12V system means 100+ amps momentarily. If your BMS has overcurrent protection set conservatively, it might be tripping before the MultiPlus even gets chance to respond to low voltage.

What batteries are you running — lithium or AGM? Makes a big difference to how you'd approach this. Lithium with a BMS cutting out hard is a very different diagnosis to AGM sagging under load.

Great thread. To add to what @JohnMason and @LauraGraham72 have already covered — it's also worth looking at your Dynamic Current Limiter setting in VEConfigure. If that's enabled, the MultiPlus can be quite conservative about how aggressively it responds to sudden load spikes, which a microwave absolutely hammers it with at startup.

Also, what battery chemistry are you running? If it's AGM or gel, they genuinely struggle to deliver the instantaneous current a 700W microwave demands from a 12V bank — you're talking nearly 100A at the terminals before inverter inefficiency is factored in. Even healthy batteries will sag under that. Lithium would handle it considerably better.

What does your battery monitor show at the point of cutout — is the voltage genuinely collapsing, or is it cutting on overcurrent protection?

Really useful thread this. One thing worth adding to what @Victron_Pro and @JohnMason have already covered — have you looked at the battery temperature? Lithium banks in particular will sag voltage noticeably when cold, and in a van during winter the cells can be sitting at 5-10°C which dramatically reduces their ability to deliver peak current. Your MultiPlus might be seeing genuine low voltage rather than a settings issue.

Also worth double-checking your battery monitor settings if you're running a BMV or SmartShunt — make sure the configured battery capacity matches your actual bank size, as the MultiPlus can use this for load calculations.

What batteries are you actually running, @RelayAdventure? AGM, lithium, or something else? That'd help narrow it down quite a bit.