Using Victron Phoenics as separate supply for IT devices in parallel to 3-phase MultiPlus II setup

Bit of a niche one this, but curious if anyone's gone down this route...

I've been thinking about the voltage sag issue when big loads kick in — obviously not running a 3-phase setup myself (static caravan, so single phase Multiplus II 24/3000), but I've read quite a bit about people getting interference on sensitive electronics when something heavy starts up.

The idea of running a separate Victron Phoenix inverter purely for IT gear — laptops, NAS, router, that sort of thing — makes a lot of sense to me in theory. Basically isolating your sensitive loads from the bus entirely so any transient sag during motor starts doesn't affect them.

A few questions if anyone's actually doing this:

• Are you running the Phoenix from the same battery bank, or a completely separate one?
• Does the Victron ESS logic get confused having a Phoenix operating independently alongside the MultiPlus units?
• Any issues with Fogstar or other lithium banks and multiple inverters pulling from them simultaneously through the same BMS?

I imagine with three MP II 48/8000s and something like a 5.5kW 3-phase motor starting up, the inrush must be fairly savage even with a decent battery bank. Seems like protecting your IT loads with a dedicated inverter is a neat workaround rather than trying to spec the whole system to handle it cleanly.

Anyone actually running this kind of hybrid arrangement? Would love to know how you've configured it in VRM/Venus OS and whether the monitoring makes sense across both inverters.

@Julie1991 the voltage sag thing is real and it bit me hard on the boat last winter. Had my NAS drive corrupting files every time the inverter fired up for the kettle.

What I ended up doing was running a dedicated Victron Phoenix 12/375 purely for the sensitive kit — router, NAS, laptop charger — fed directly off a separate 100Ah Fogstar lithium cell that sits upstream of the main draw. Completely isolated from whatever chaos the domestic side is having.

The key thing is keeping the two battery banks genuinely separate with their own charging circuits, otherwise you're just sharing the sag problem through the interconnect anyway.

For a 3-phase MultiPlus II setup the principle is identical — the Phoenix doesn't care what the main inverters are doing. It's on its own little island of calm.

@HalfAJob NAS corruption in winter on a boat is basically the universe's way of saying "you should've bought a UPS instead of that second inflatable kayak."

Separate Phoenix for IT kit is exactly what I did in the van — sensitive stuff on its own dedicated inverter means voltage sag from the induction hob becomes someone else's problem. Key thing people miss: make sure your Victron DVCC settings aren't fighting themselves when both inverters share the same Fogstar battery bank, otherwise you get some creative charging behaviour that'll confuse your BMS into a sulk.

Worth checking the Victron Community forums for the ESS + standalone inverter threads — someone's mapped this out properly with actual wiring diagrams rather than my legendary back-of-a-beer-mat approach.

@Julie1991 this is actually a solid approach for sensitive IT loads. Running a dedicated Phoenix inverter on a separate smaller battery bank — isolated from your main 3-phase MultiPlus bank — gives you two real benefits: clean voltage independent of your heavy loads, and a natural UPS buffer if your main bank drops out momentarily.

Worth knowing the Phoenix range supports Victron's VE.Direct comms, so you can monitor it alongside your MultiPlus II setup in VRM without juggling separate systems. I'd lean toward a decent LiFePO4 for that satellite bank — Fogstar Drift cells have worked well for me in the shepherd's hut for exactly this kind of secondary supply role.

One thing to check: your IT gear's acceptable input voltage range. Most modern equipment tolerates quite a wide band anyway, which changes the cost-benefit equation considerably before you commit to the extra hardware.

@Julie1991 the theory is sound but there's a gotcha nobody's mentioned yet — if both the Phoenix and your MultiPlus II are drawing from the same battery bank, you haven't actually isolated yourself from the voltage sag, you've just added a conversion stage. The sag still happens at battery level.

Proper isolation means either:

• A dedicated small LiFePO4 pack just for the Phoenix (Fogstar Drift cells are cracking for this)
• Or a DC-DC converter between banks to decouple them

I ran a similar setup in my motorhome — separate 50Ah bank feeding a Phoenix 375 for the laptop and router. Rock solid even when the compressor fridge kicked in on the main bank. Victron's VE.Direct comms also let you monitor both inverters cleanly through Cerbo GX which is a genuine bonus.

@LiFePO4Nerd is absolutely right to flag that — the common DC bus issue is the one that catches people out. Worth adding: if you're pulling from the same LiFePO4 bank, your BMS becomes the weakest link. Sudden load spikes on the MultiPlus II side can trigger overcurrent protection on the whole bank, which then also kills your "protected" Phoenix circuit despite the physical separation at AC level.

The cleaner solution I've seen (and run myself for the tiny house setup) is a dedicated small LiFePO4 pack — even a modest Fogstar Drift 100Ah — feeding only the Phoenix. Keeps the IT loads genuinely isolated from battery-level events. Victron's DVCC helps coordinate charging across both banks if you're using a Cerbo, but that's another layer of configuration complexity to budget time for.