Connecting Vebus bms and Cerbo to 800va multiplus with only one vebus port

Had this exact headache last winter fitting out my garden office — the 800va Multiplus only has the one VE.Bus port, and you're basically playing a game of "who gets the chair" between the VE.Bus BMS and the Cerbo GX.

Ended up using the VE.Bus BMS Assistant loaded onto the Multiplus itself, which lets the Cerbo talk to the inverter via VE.Bus while the BMS handles its charge/discharge allow signals through the dedicated Charge Disconnect and Load Disconnect terminals instead. Means you're not trying to daisy-chain two devices onto a single port like some kind of cursed spaghetti junction.

Key bits that caught me out:

• Make sure you're running the VE.Bus BMS v1 wiring properly — the Allow to Charge and Allow to Discharge wires go to the Multiplus ATC/ATD inputs, not the VE.Bus port itself
• Load the correct Assistant via VEConfigure before you connect anything live
• The Cerbo then just sits on VE.Bus solo and plays nicely

Anyone else running the older BAT512 series batteries with this combo? Curious whether others had to fiddle with the low-cell cutoff settings because mine was tripping the BMS at what felt like perfectly reasonable temperatures for a draughty British shed.

Would love to know if there's a cleaner way — Victron's own docs are about as clear as a November morning in Manchester. 🌧️

Hey @LazyRanger, yeah this is a classic gotcha with the smaller Multis! The solution is the VE.Bus BMS itself doesn't actually need to occupy that port directly — you wire the BMS's "Allow to charge" and "Allow to discharge" signals to the Multiplus's assistant inputs instead, then use a VE.Bus to VE.Can adapter or simply run the Cerbo via the VE.Direct connection on the Multiplus rather than VE.Bus. The 800va does have a VE.Direct port which handles monitoring duties perfectly well through the Cerbo. Victron's own wiring unlimited doc covers this setup — worth downloading if you haven't already. Took me a while to clock that VE.Direct is more than sufficient for what most people actually need the Cerbo comms for on a small system like that. 👍

Good question to dig into. Worth clarifying: the VE.Bus BMS connects directly into that single VE.Bus port on the 800VA, and then the Cerbo GX communicates via a VE.Bus to VE.Net interface or — more practically for most of us — through a separate USB to VE.Bus cable into the Cerbo rather than daisy-chaining off the same port.

I ran into this with my static caravan setup. The key thing to confirm:

• What BMS are you actually using? The VE.Bus BMS v2 handles this differently to the original
• The Cerbo can also monitor via VE.Can or USB depending on your battery setup

Does your battery communicate over a different protocol (e.g. CAN bus)? That might sidestep the port conflict entirely and is worth checking before buying extra cables.

Really useful thread this — just to add a question that's been nagging at me since I started planning a similar setup for EV charging overnight...

@BlownFuse when the VE.Bus BMS occupies that single port, does the Cerbo GX then communicate via VE.Can or Bluetooth instead, or are you genuinely losing some monitoring granularity compared to a larger Multi where it sits on VE.Bus properly?

Wondering if anyone's done a side-by-side comparison of what data you lose in the Cerbo dashboard when it's not getting the full VE.Bus comms from the inverter directly? Particularly around charge state control — that matters quite a bit when you're trying to time EV charging around off-peak rates.

Has anyone on here tried the MK3-USB as a workaround to bridge that gap?

@BlownFuse is on the right track, and here's where it clicked for me — the VE.Bus BMS has a pass-through VE.Bus port on it. So the chain goes:

Multiplus VE.Bus port → VE.Bus BMS → Cerbo GX

The BMS daisy-chains, rather than competing for the port. Took me an embarrassingly long time to spot that second RJ45 socket on the BMS unit itself.

One thing worth flagging from my own install: make sure you're running sufficiently short cable runs between those three devices. I had intermittent comms dropouts on my Cerbo until I swapped to a proper shielded cable rather than the generic flat ethernet I'd bodged in initially. Victron's own VE.Bus cables are worth the few extra quid here — learned that the hard way.

@Drift_Geek daisy-chaining through the BMS's pass-through port is exactly how mine's wired in the static van — Cerbo picks up everything through there no bother, took me longer to find a parking spot at Screwfix than it did to actually cable it up.