Multi losing bms

Been having a similar head-scratcher with my motorhome setup and wondering if anyone else has run into this.

Running two Fogstar Drift 100Ah batteries in parallel, Victron Multiplus-II, Cerbo GX, SmartShunt, and a Victron BMS. Everything was playing nicely together for about four months, then last week I got a "No BMS" warning on VRM and the Multiplus dropped into a weird low-power state.

Checked the batteries individually — both showing around 13.2V on the multimeter, so not an obvious low-voltage shutdown. One of them had also vanished from Bluetooth completely, which is what's got me puzzled. The Cerbo still sees the SmartShunt fine, just lost comms with the BMS entirely.

A few things I'm trying to work through methodically:

• Cable issue? The BMS comms cable from the battery to the Cerbo looks intact but I haven't swapped it out yet
• Firmware? Both the Multiplus and Cerbo had recent updates — wondering if something broke compatibility
• Temperature? The motorhome was parked up in some pretty cold weather, not sure if that could trigger a false BMS trip

Has anyone seen a Fogstar or similar LiFePO4 battery just drop off Bluetooth like that while still holding voltage? And does the Multiplus loading down when it loses BMS comms sound right to others — is that just the expected safe behaviour kicking in?

Would be good to rule out whether this is a hardware fault before I start replacing cables or reflashing firmware.

@BevJackson64 the SMA cutting off mid-post is doing you no favours here, but I'll take a stab at the likely culprit — with two Fogstar Drifts in parallel, if the BMS comms are running through the Cerbo via the VE.Direct or BMS-Can port, a brief disconnection on either battery's BMS can cause the Multi to drop into passthrough or shutdown entirely. Classic symptom is the Cerbo log showing "BMS pre-alarm" right before the Multi loses its mind.

Worth checking:

• DVCC settings — ensure "BMS controls charge" is actually enabled
• Cable run lengths between both Drifts — unequal resistance causes one BMS to trip first under load
• Fogstar firmware — there was a batch with dodgy comms stability late last year

Pull the Cerbo VRM logs for the event window and post the screenshot. That'll tell us far more than guessing.

@BevJackson64 your post got absolutely mullered mid-sentence there 😄 finish it off mate, half the diagnosis is missing.

That said — Fogstar Drifts in parallel can be a bit spicy if the BMS comms aren't set up right on the Cerbo. Had similar grief on the boat before I sorted the VE.Can topology.

Quick questions while you finish your post:

• Are both batteries showing up individually in VictronConnect?
• What firmware is your Multi on?
• How are you connecting the BMS to the Cerbo — VE.Can, USB, or nothing at all?

"Multi losing BMS" could mean a dozen things. Don't waste everyone's time guessing when the actual error codes will tell us straight away. Check Device List on the Cerbo and screenshot whatever's flagging red.

@BevJackson64 your post cut off like the BMS did 🔋

Ha, @BevJackson64 you've got us all on tenterhooks here! 😄 As the others have said, do finish the post off — but while we're waiting, I'll add that with parallel Fogstar Drifts I'd be looking closely at whether both batteries are properly balanced before you connect them. If one goes into protection before the other, the remaining pack tries to carry the full load and you can get a cascade where the second BMS trips shortly after. What does the Cerbo show in the logs around the time it happens? The history graph in VRM is your friend here — it'll show you exactly when the BMS dropped off and what the voltage was doing beforehand. Also worth checking your inter-battery cabling is equal length and same cross-section, otherwise one pack works harder than the other.