Multiplus 2 three phase system and single phase generator

Been mulling over a similar setup to something I saw discussed recently and curious what others here have done.

Currently running 3x Multiplus II 5000s in three-phase with a Cerbo GX (moved away from RPi + Venus OS Large after one too many SD card headaches), 6x Pylontech US3000Cs, and a Fronius Symo on the AC output side. Works brilliantly most of the year.

The bit I'm stuck on is generator integration. I've got a single-phase diesel genny as emergency backup — which is fine for a single-phase system, but with three-phase Victron setups the Multiplus will only charge via whichever phase the generator is connected to. The other two phases just sit there doing nothing useful.

A few questions I'd love input on:

• Has anyone used a transfer switch before the Multiplus inputs to route the generator to all three phases simultaneously? Seems messy.
• Would it be cleaner to just accept the single-phase charging limitation and right-size the genny so L1 charges fast enough to cover the whole battery bank via the Pylontech BMS balancing?
• Anyone tried a three-phase generator instead? Probably overkill for emergency backup but maybe worth it longer term.

The Pylontechs communicate fine via CAN bus and the whole system is rock solid day-to-day — it's purely the generator fallback that feels like a weak point heading into winter.

Would be especially interested if anyone's got a UK-based installer who's dealt with this, since a lot of the advice I find online is aimed at mainland European or Australian setups where regs differ.

@CrafterSolar the OP seems to have been cut off mid-sentence there — "moved a..." moved a what exactly? 😄

That said, I'll take a stab at where you're probably going with this. Single-phase genny feeding a three-phase Multiplus II setup is a known headache. Victron's approach is to assign the generator to L1 only and let the inverters sort themselves out — works, but your charging capacity takes a hit because only L1 is seeing the AC input.

Worth checking your ESS assistant settings carefully. The other two phases will still invert from battery whilst L1 charges, which sounds clever until your genny struggles under the imbalanced load.

What size generator are you looking at? Makes a big difference to the advice here. My single-phase setup is far simpler but I've followed a few three-phase threads and the devil really is in the detail with Victron config.

@CamperSam yeah noticed that too, suspect autocorrect chaos or a fat-finger moment 😄

@CrafterSolar assuming you're asking about single-phase genny compatibility with the three-phase Multiplus setup — yes it works but only one unit charges from the generator, the other two sit idle. Worth knowing if you're sizing your genny for bulk charging, you're not getting the full 15kW potential, just whatever one unit can pull.

Running a similar-ish setup in my shepherd's hut (single phase, single Multiplus II) and even there the generator input settings in VRM need careful attention. Victron's ESS assistant config is where people tend to go wrong. What generator are you actually running with it?

@CrafterSolar classic cliffhanger post, keeping us all in suspense like a Netflix series 😄

On the actual setup though — Victron's AC coupling on a three-phase Multiplus II system with a single-phase genny is a known quirk; the inverter will only sync the generator to L1, leaving L2 and L3 running off battery alone during gen assist. Worth checking your ESS assistant config in VEConfigure, as the frequency shift behaviour can get a bit spicy if your generator's governor isn't tight. A decent inverter-genny (Honda EU series or a good Pramac) behaves far better than a cheap suitcase unit. Also make sure your Cerbo GX firmware is current — there were some three-phase generator logic fixes in recent releases that made a noticeable difference to charge scheduling.

@CrafterSolar absolutely leaving us hanging there like a proper season finale — though I notice @AnneWatson and @BoatPaddy have already beaten me to that joke 😄

Anyway, the question I think you're building towards — single phase genny feeding a three-phase Multiplus II setup — is something I've been genuinely curious about for my static caravan build.

From what I've read in the Victron docs, the Multiplus IIs will only charge/pass through on the phase the generator is connected to, with the other two phases running purely from battery. Does that match anyone's real-world experience? Feels like it could cause some imbalance headaches depending on your loads.

What size genny are you running with it?

@CrafterSolar while we wait for the rest of your post to materialise, worth flagging that single-phase generator input on a three-phase Multiplus II system is genuinely non-trivial. Victron's own documentation notes the generator must be connected to L1 only in most configurations, with L2 and L3 units operating purely as inverters during that period — they won't assist with AC passthrough on those legs.

Practical consequence: your generator rating needs to cover L1's charging contribution alone. I've seen people underspec this badly.

Also worth checking your ESS assistant version — there were firmware quirks around generator detection in three-phase setups that caused phantom grid-code alarms. VRM community forums have a decent thread on it.

What's your battery bank situation? LiFePO4 with a Victron-compatible BMS changes the charging strategy considerably here.