Common ground between 2 battery banks of different voltage for BMV 712

Running into something similar with my shepherd's hut setup and it's been gnawing at me for a while, so good to see this topic come up.

My situation: I've got a 24V Victron system — Multiplus, SmartSolar MPPT, BMV 712, the works — and then a separate 12V starter battery kicking the diesel generator into life. Two completely separate banks, different voltages, and I want the BMV to keep tabs on what's happening across both in terms of current flow.

The question that keeps me up at night is whether you can safely establish a common negative between the two banks to give the BMV's shunt a proper reference point — without creating some kind of ground loop nightmare or, worse, letting the two voltage systems bleed into each other.

From what I've read, the negative rails can share a common point so long as there's no positive connection between the banks, and the shunt sits correctly in the negative line of the bank you're primarily monitoring. But I haven't pulled the trigger yet because the consequences of getting it wrong in a small metal-clad hut aren't exactly appealing.

Has anyone here actually done this — particularly with a Cerbo GX in the mix? I'm wondering whether the Cerbo's DVCC settings complicate things further when you've got that mixed-voltage arrangement.

Would love to know how others have physically wired it, and whether Victron's own documentation actually covers this edge case or just assumes everyone's running a single-voltage system like sensible people.

Hey @OllieRoss74, sounds like a great setup! Just to clarify something that might help — the BMV 712 measures the voltage across its shunt, so the common ground connection is really just giving it a reference point rather than carrying any significant current.

For your shepherd's hut with two different voltage banks, the key thing is that the shunt itself needs to sit on the negative side of whichever bank you're monitoring. If you're wanting to monitor both banks simultaneously, that's where it gets trickier — you'd be looking at a second BMV or possibly a SmartShunt for the second bank, then linking them both through VE.Direct or Bluetooth into Victron Connect.

What's the second battery bank for — a separate 12V leisure circuit perhaps? That context would help narrow down the best approach for your wiring. 😊

@OllieRoss74 done this on my narrowboat with a similar split — 24V house bank monitored via BMV 712, separate 12V starter. The key thing people miss is that the negative shunt connection must sit between the shunt and battery negative, not on the load side. Common ground works fine as long as you're running the BMV's aux input to the 12V starter positive and sharing the negative rail properly.

Worth double-checking your shunt isn't carrying the 12V starter loads too — that'll throw your SOC calculations right off. Fogstar cells in particular seem to drift badly if the shunt maths is even slightly compromised.

What's your current shunt placement exactly? That'll narrow it down quickly.

@OllieRoss74 I've got almost exactly this config in my shepherd's hut — 24V Fogstar Drift LiFePO4 house bank with a separate 12V auxiliary for lighting circuits.

The common negative is key, but worth double-checking your shunt placement. All current must flow through the shunt on the negative side between battery and loads/charging sources, otherwise your BMV readings go haywire. Took me an embarrassingly long time to figure out why mine was reading nonsense.

One thing I don't see mentioned — if you're tying the negatives together make sure you're accounting for any potential ground loops, especially with the Multiplus chassis earth in the mix. Victron's own wiring unlimited doc covers this reasonably well, worth a read before you commit to anything permanent.

@OllieRoss74 one thing nobody's explicitly spelled out yet — the BMV 712's negative sense wire (the small one, not the main shunt cable) needs to connect to the battery negative side of the shunt, not the load side. Sounds obvious but I've seen people wire it backwards and then wonder why readings are odd.

Also worth noting: the Bluetooth dongle on the 712 shares the aux input with the temperature sensor. You can't have both simultaneously. Caught me out on my boat setup — I wanted battery temp and wireless monitoring, had to rethink the whole thing.

The common negative between your 24V and 12V banks is fine electrically provided neither bank is floating — both need a proper chassis/common ground reference point. If your 12V starter battery negative goes directly to hull/chassis and your 24V doesn't, you'll get measurement grief.