MultiPlus-II 4k5 + one Dyness DL5.0C Pro shuts down on grid loss with Low Battery and High DC Ripple

Got a head-scratcher on my narrowboat setup and wondering if anyone's wrestled with this beast before.

Running a MultiPlus-II 4k5 paired with a single Dyness DL5.0C Pro (51.2 V / 100 Ah) and a Victron SmartSolar MPPT 150/45, all configured as an ESS — or at least it was ESS until I decided canal life meant I'd rather go full off-grid and stop relying on marina hookups entirely.

The problem: the moment shore power drops (whether I unplug intentionally or the marina supply wobbles — and blimey, it wobbles), the MultiPlus throws a Low Battery alarm almost instantly, then a High DC Ripple warning follows within seconds, and the whole inverter shuts down. Boat goes dark. Kettle dies mid-brew. Tragedy.

Battery is sitting at a perfectly healthy state of charge — we're talking 80%+ — so it's not actually low. My suspicion is that the single Dyness unit can't handle the instantaneous current demand when the MultiPlus switches from grid-assisted to standalone inverting, causing a voltage sag that the MultiPlus misreads as a dying battery, with ripple spiking because the BMS is momentarily current-limiting.

Questions for the collective wisdom of this forum:

• Is one 100 Ah LiFePO4 just fundamentally undersized for a 4k5 inverter in standalone mode?
• Should I be looking at the charge current settings or the AC input current limit in VEConfigure?
• Anyone had luck with the ESS Assistant vs pure off-grid config making a difference here

@VoltFiona the classic "one battery trying to be a power station" problem — I've been down this rabbit hole.

Your Dyness is rated 100A continuous discharge, but the MultiPlus-II 4k5 can pull ~90A at full load plus inverter inefficiency losses. The moment grid drops and the inverter switches to island mode, that inrush spike is likely tripping the BMS on overcurrent before the low battery alarm even makes sense.

The high DC ripple is your tell — it's screaming insufficient battery impedance capacity for that inverter size. One 5kWh pack is genuinely undersized for a 4500W inverter on a narrowboat, where you'll have compressors, pumps and kettles all kicking simultaneously.

Minimum I'd suggest is two DL5.0C Pros in parallel. Fogstar Drift 48V units are worth a look too if budget allows.

What's your current cable spec between battery and inverter?

Welcome to the forum @VoltFiona! Narrowboat builds are brilliant 🚤

@Jock's spot on about the single battery struggle. The 4k5 can pull serious amps on inverter surge and that one DL5.0C Pro just can't keep DC ripple in check under that load.

Worth checking in VictronConnect what your DC input low shut-down is set to — sometimes it's too aggressive for a single pack. Also look at your AC input current limit if you're occasionally on shoreline, as the transition to inverter mode is where ripple spikes nastily.

Longer term, a second Dyness in parallel makes a massive difference — I run two Fogstar Drift 100Ah units on my motorhome and the ripple behaviour is night and day vs when I trialled just one.

Post your VEConfigure screenshots if you can — someone here will spot it quickly!

The MultiPlus-II 4k5 is rated for up to 70A charge current and your single Dyness is only rated for 100A continuous discharge — on paper fine, but the moment grid drops and the inverter slams into full load, that DC ripple spikes faster than my Victron MPPT can spend my wife's patience.

Worth adding — check your DC cable cross-section between the Dyness and the MultiPlus, because undersized cabling on a narrowboat (especially with any length run) will cause exactly that ripple spike the moment grid drops and the inverter suddenly demands peak current from one lonely battery. Victron's own guidance suggests 95mm² minimum for a 4k5 at full whack. If your cables are thinner, you've basically found your culprit without even touching VEConfigure. 🔌