Multiplus internal soc

Been wrestling with something similar on my garden office setup and wondering if others have hit the same wall.

Running a Victron Multiplus-II with a Cerbo GX and a bank of Fogstar Drift cells with a Daly BMS — not quite the same scale as some builds here, but the principle should be the same.

The issue I keep running into is the Multiplus using its own internal SOC estimate rather than trusting the BMS data coming through via the Cerbo. Even with DVCC enabled and the BMS set as the controlling source, I've had moments where the unit's behaviour doesn't quite match what the SOC is actually doing.

A few things that helped me:

• Making sure DVCC is fully enabled and the BMS is selected as the primary SOC source in the Cerbo settings
• Doing a proper battery full synchronisation — the Multiplus needs at least one clean 100% charge cycle to anchor its internal estimate
• Checking the assistant logic is referencing SOC rather than voltage thresholds, which can be sneaky if you've copy-pasted settings from an older config

Still not 100% confident everything is talking cleanly though. The assistant-based AC output switching feels like it should be straightforward but there are clearly some edge cases.

Anyone running a larger system with a smart BMS fully overriding the Multiplus internal SOC successfully? Curious whether the Lynx BMS integration is tighter than what I'm getting with third-party kit.

@HamishMitchell the Multiplus internal SOC is basically useless without proper comms to the BMS. Fogstar Drift supports Victron VE.Can if I remember right — are you actually connecting that up?

If not, the Cerbo is just guessing based on voltage curves which is notoriously dodgy, especially with LFP's flat discharge profile.

What I did on my static caravan setup was make sure the BMS SOC was the only thing the Cerbo trusted — took a bit of fiddling in the DVCC settings but once it clicked it was solid.

Post your VRM screenshots if you've got them, easier to diagnose that way.

@RelayDream is absolutely right, and this is where I see so many people trip up.

The Multiplus-II's internal SOC estimator is essentially doing dead reckoning navigation with a blindfold on — it's counting coulombs but has no idea what the cells are actually doing. Without the Fogstar Drift talking properly over VE.Can

@HamishMitchell had almost identical grief with my shepherd's hut setup last winter. The Cerbo was showing 87% while the BMS app was screaming at me that cells were already being pushed.

Once I got the Drift talking properly to the Cerbo via the RS485 cable and set DVCC to let the BMS take charge control, everything just... clicked into place. The Multiplus stopped doing its own thing and actually listened.

Key thing I'd add that hasn't been mentioned — make sure you've got DVCC → SVS and STS both enabled in VRM. A lot of guides skip over that bit and then people wonder why it's still behaving oddly even after sorting the comms.

Check your firmware too. Had a version on the Cerbo that was just... wrong about everything.

@HamishMitchell worth checking one thing nobody's mentioned yet — even with the CANBUS link sorted between the Fogstar Drift and Cerbo, you need to make sure the Multiplus-II is set to "Managed Battery" in VEConfig rather than relying on the internal charger algorithm. Otherwise the Multiplus ignores what the BMS is actually telling it and does its own thing anyway.

Also, have you set the DVCC settings correctly in Venus OS? Specifically:

• SVS (Shared Voltage Sense) enabled
• SCS (Shared Current Sense) enabled
• Limit charge current set to whatever your Fogstar bank can handle

Had similar confusion on my motorhome build before I realised DVCC was half-configured. The Cerbo looked happy but the Multiplus was basically freelancing. Once that's all consistent the SOC tracking becomes much more reliable.

What firmware version is your Cerbo running?