BMS SOC is Different from CerboGX Display

Anyone else's Cerbo GX and BMS playing a game of "who's right?" with the SOC figures?

Running a Fogstar Drift 16kWh on the boat and my BMS reckons I'm at 78% whilst the Cerbo is confidently displaying 64% — both can't be correct, and frankly neither inspires confidence.

The core issue as I understand it (pedant hat firmly on):

• The Cerbo/CCGX calculates SOC via coulomb counting from a shunt, or trusts the BMS over the VE.Bus depending on your settings
• Without a Lynx Shunt (or SmartShunt) in the loop, you're essentially letting the Multiplus II make educated guesses
• The BMS SOC is calculated internally and may not account for the same charge/discharge cycles your Victron kit sees

My shepherd's hut setup has a SmartShunt fitted and the two figures stay within 2-3% of each other — probably not coincidental.

Worth checking Device List → System → DVCC settings and whether "Use solar charger current to improve VE.Bus SOC" is ticked, as that can cause drift if your MPPT isn't reporting cleanly.

Also: has the BMS done a proper full charge cycle to calibrate itself? A fresh installation SOC is basically fiction until it's seen at least one full top-up to absorption.

Curious whether anyone running BasenGreen packs specifically has cracked this — their BMS comms over CAN seem a bit inconsistent from what I've read.

@LochLover classic issue this. The Cerbo's SOC is calculated independently via coulomb counting unless you've told it to use the BMS as the "battery monitor" source.

Go to Settings > System Setup > Battery Monitor on the Cerbo and set it to your BMS (should appear as a connected device if you're on DVCC). Once synced to a single source the discrepancy disappears.

Worth noting — after any full charge cycle, both figures should self-correct anyway as they both reset to 100% at absorption/float transition. If yours aren't doing that, check your tail current settings match between devices.

Had the same confusion when I first set up my static caravan system with a Victron SmartShunt. Two devices both "monitoring" independently will always drift apart over time.

@RetiredPlumber is on the right track. Worth checking whether your Cerbo is set to sync SOC from the BMS via the VE.Can or VE.Direct connection, or whether it's doing its own thing with coulomb counting from the shunt.

If you've got a Victron SmartShunt in the mix as well, you've got three devices all with opinions about your battery state. Been there with my cabin setup — drove me mad until I just picked one source of truth and stuck with it.

For a managed lithium like the Fogstar Drift, I'd trust the BMS SOC and tell the Cerbo to use that as the reference. Go into Settings > System Setup > Battery monitor on the Cerbo and select the BMS as the source rather than the shunt.

The drift between the two usually gets worse the longer it runs without a full sync at 100%.

@LochLover worth being precise here — the Cerbo isn't just "calculating independently," it's resetting its coulomb count based on whatever synchronisation trigger you've configured (full charge detection, BMS signal, etc.).

Key thing to check: in VRM / GX settings, is your DVCC switched on? If the Fogstar Drift is talking over VE.Can or a supported protocol, DVCC should let the BMS SOC take priority.

Also check your charged voltage and tail current thresholds under the battery settings — if those are misconfigured the Cerbo will never think it's hit 100% and the drift compounds over time.

For my cabin setup I had a solid 12% discrepancy until I corrected the tail current setting. It's almost always a config issue rather than a faulty unit.

@LochLover had exactly this with my setup last year. The fix that sorted it for me was going into the Cerbo's battery monitor settings and selecting the BMS as the trusted SOC source rather than letting it do its own thing. Once you hand control to the Fogstar's BMS, both figures snap into agreement pretty sharpish.

Device List → your battery → Battery Monitor

Worth double-checking your VE.Can cable is seated properly too — a dodgy connection means the Cerbo stops receiving BMS data and falls back to its own counting, which drifts noticeably over a few days. Seen it happen on mine after a bumpy trip rattled things loose!

@LochLover one thing nobody's mentioned yet — check whether your Fogstar Drift's BMS is actually publishing SOC over VE.Can or just voltage/current data. Some units claim CAN integration but only expose partial data, leaving the Cerbo to coulomb-count from scratch with whatever charge efficiency factor it's assumed.

Go to Remote Console → Device List and see what the BMS entry actually shows. If you're only seeing voltage rather than a proper SOC figure being received, that's your culprit.

Also worth verifying your battery capacity is entered correctly in VEConfig — I've seen Cerbos default to 200Ah after a firmware update, which absolutely wrecks the coulomb counting on a 16kWh bank.