Anyone else running dual Victron BMV-712s on a split bank system?

Been running a split bank setup on the boat for about eight months now — 200Ah lithium (Fogstar Drift cells) as the service bank and a separate 110Ah AGM for engine start. Each bank has its own BMV-712. The idea was to get accurate SoC on both independently, which works well enough, but I'm running into a slightly awkward situation with how the two talk to each other via VE.Direct to the Cerbo GX.

The Cerbo sees both BMVs as separate devices, which is fine on the surface, but when I try to build any kind of automation logic in Node-RED (via the MQTT broker on the Cerbo), I keep having to manually reconcile the two SoC readings to decide when to trigger the alternator-to-lithium relay. It feels like I'm working around a problem that should have a cleaner solution. I've looked at the ESS assistant but that seems overkill and largely irrelevant without an inverter/charger in the loop on the engine side.

Has anyone found a sensible way to unify two BMV readings into a single decision variable without going full Venus OS custom firmware? I'm wondering whether a simple Node-RED function node comparing both battery voltages and SoC percentages against threshold triggers would be robust enough, or whether I'm missing something more elegant in the Victron ecosystem. The Cerbo is on Venus OS 3.14 if that matters.

Great setup @RiverRunner! I ran almost identical kit on my narrowboat for two years. One thing worth mentioning — make sure you've got the BMV-712 on your lithium bank configured with the correct Peukert exponent (closer to 1.05 for LiFePO4 rather than the AGM default around 1.25). Makes a noticeable difference to SOC accuracy over longer discharge cycles.

Also, are you using the relay output on the lithium BMV for anything? I've got mine triggering a low-voltage disconnect at 20% SOC as a backstop behind the BMS. Bit of belt-and-braces redundancy that's saved me once or twice when I've misjudged usage overnight. The Fogstar Drift cells are solid though — bet your system's performing well overall!

Two BMV-712s talking to each other via the VE.Smart network is genuinely one of those "why didn't I do this sooner" moments — your Fogstar bank will thank you for the accurate SOC handshaking between banks when the alternator's doing its thing. 🔋

Really useful thread this — @RiverRunner your setup sounds solid. One thing I'd add that nobody's mentioned yet: if you're logging both BMVs through VictronConnect, it's worth keeping an eye on the synchronisation of your state-of-charge resets. With mixed chemistry banks the full-charge detection thresholds need setting independently and quite carefully, otherwise one BMV can be confidently reporting 100% whilst the other's still catching up. Took me an embarrassing amount of head-scratching before I realised that was behind some confusing readings on my narrowboat. Also worth labelling your shunts clearly during installation — sounds obvious but swapping which bank you're reading in a dark bilge is easily done! 🙂

@RiverRunner running basically the same split bank arrangement in my shepherd's hut on wheels (van conversion, before anyone asks 😄) — Fogstar cells on the house bank, separate AGM for the starter.

One thing worth adding: the relay assistant on the BMV-712 is your mate here. I've got mine set up so if the lithium drops below a certain SOC it automatically disconnects the DC-DC charger from the AGM side. Stops the starter bank from quietly draining your main bank when you're not watching.

Took me an embarrassing number of flat mornings to figure that out. Don't be me.

Also pairs nicely with a Victron Cerbo GX if you ever want everything on one dashboard — both BMVs show up as separate devices, proper tidy.

Really interesting thread this. @RiverRunner one thing worth mentioning if you haven't already looked into it — make sure your shunt placement is spot on for accurate SOC readings across both banks. I had a nightmare early on where a small parasitic load was sitting on the wrong side of the shunt and my BMV was reporting optimistically by quite a margin. Took me longer than I'd like to admit to track it down!

Also, have you set up any alerts for voltage divergence between the two banks? I've got mine configured to flag if the AGM starter bank drops below a threshold while the lithium is still healthy — saved me from a flat start battery situation at least twice now. Especially worth doing on a boat where you might not be monitoring constantly.

Running dual BMV-712s on my narrowboat for two years and the one thing nobody ever tells you is that the synchronisation of the "charged" detection settings matters enormously — if both BMVs aren't calibrated to trigger at the same tail current threshold, your SOC figures will slowly drift apart and you'll spend ages convinced one bank is lying to you (it isn't, you just set it up wrong at 11pm with a torch in your teeth).

Also worth noting: Victron's VictronConnect app will show both units simultaneously if you've got Bluetooth enabled, which saves crawling into the engine bay every time @RiverRunner wants a sanity check on the AGM. The 110Ah starter bank SOC is almost always fine — it's your service bank doing all the drama.