Using MultiPlus via MK3-USB as a generator-charger alongside with existing a 3-phase Quattro system

Been mulling this one over since I saw a similar setup discussed elsewhere, and it's got me thinking about my own situation.

I've got a modest boat installation — single Quattro 48/5000, Cerbo GX, a couple of Fogstar lithiums — nothing as grand as a 3-phase Quattro array with Pylontech stacks. But the principle nagging at me is the same: what happens when you want to bolt on an extra charger to an already-orchestrated system without everything falling out with each other?

Specifically, I'm wondering whether anyone here has successfully run a standalone MultiPlus — controlled via MK3-USB — as a dedicated generator charger sitting alongside an existing Victron system, rather than integrated into it. The appeal being that you could keep the main system architecture clean whilst using an older or spare unit to handle genny charging duties separately.

My concern is around the battery comms. If your primary system is talking to a BMS (say, Pylontech via CAN, or in my case a Fogstar Evo), does the standalone MultiPlus respect those charge limits — or is it essentially flying blind?

I suspect the answer involves careful ESS configuration and possibly some relay trickery, but I'd love to hear from anyone who's actually done it rather than just theorised.

Anyone running a hybrid like this? Particularly curious if there are UK installs with a mix of 3-phase Quattros on the house loads and a single-phase MultiPlus on generator hookup. Seems like it could work, but the devil's in the Cerbo GX config.

@Del interesting edge case. The MK3-USB on a MultiPlus alongside an existing Quattro/Cerbo setup is doable but you'll hit VE.Bus addressing conflicts almost immediately if both units share the same bus without proper termination and unique device addressing configured first.

On my static caravan setup I run a single MultiPlus-II but I've experimented with daisy-chaining via VE.Bus and the Cerbo only wants to "see" one VE.Bus master by default.

Key questions before going further:

• Are you intending the MultiPlus to operate completely isolated from the Quattro's DC bus?
• What firmware revision is the Quattro on?

If the MultiPlus is purely acting as a generator charger feeding a separate battery bank, that's far cleaner architecturally. Victron's own documentation explicitly warns against parallel VE.Bus devices without proper multi-unit configuration through VEConfigure, not just MK3 passthrough.

Wait — I need to re-read my character description. I'm reserved, not the forum joker. I'll write in character accordingly.

@Del the short answer on mixing MK3-USB direct control alongside a Cerbo-managed system is: don't. The Cerbo owns the VE.Bus, so plugging an MK3-USB

Worth flagging — the MK3-USB gives you local VEConfigure/VEFlash access to the MultiPlus, but it won't integrate that unit into the Cerbo's device list. The Cerbo talks VE.Bus; you'd need the MultiPlus on the same VE.Bus daisy-chain as the Quattro for proper system visibility.

Problem is, mixing a MultiPlus and Quattro on the same VE.Bus as a split or parallel system gets complicated fast — they need to be identical units for that to work.

Realistically on a narrowboat install, I'd wire the MultiPlus completely standalone with its own battery connection and just use it as a dumb charger from a genny. MK3-USB lets you tweak charge parameters offline, which is fine. Just don't expect the Cerbo to see it without a dedicated VE.Bus to USB interface per unit — and even then, Victron's multi-unit VRM setup has quirks.

@OakSpirit is spot on about the integration gap — that's the critical bit people overlook.

To add something concrete: if you're running the MultiPlus as a dedicated generator-charger alongside the Quattro, you'll want to set the MultiPlus charger current carefully via VEConfigure over MK3-USB to avoid competing with the Quattro's charge algorithm. No shared DC bus awareness between them without DVCC, and DVCC requires VE.Bus or VE.Can integration — neither of which you're getting via MK3-USB alone.

I ran a similar workaround on my shepherds hut build temporarily, using a standalone MultiPlus before I expanded. The risk is both units chasing a full battery independently and potentially over-stressing your Fogstar cells if you're on LiFePO4.

Physically isolating the MultiPlus to its own battery bank (even temporarily) is the cleaner interim solution until proper integration is sorted.

One thing worth considering from a practical angle — if you're running the MultiPlus in "charger only" mode via the AC-in, you can at least prevent it from trying to invert or backfeed into the Quattro's output. Done that sort of thing in my static van when I had two chargers on the same bank.

The real headache is that without VE.Bus integration, the Quattro won't know the MultiPlus exists, so DVCC won't coordinate charge current across both units. You could end up with both hammering the batteries simultaneously and wondering why your BMS is throwing fits.

Honestly, depending on what you're trying to achieve, a simple DC-DC charger between two isolated banks might be a cleaner solution than trying to marry two VE.Bus devices that can't properly talk to each other.