Anyone else running Victron Cerbo GX with a Fogstar 280Ah LiFePO4 and finding the SOC drift is *comedy levels* of inaccurate?

Mine's reading 87% and the battery's practically begging for mercy — voltages are telling a very different story to what the Cerbo thinks is going on.

Tried recalibrating the BMV-712 shunt three times now, full charge to absorption, let it hit float, reset sync point — still wanders off like it's had a few pints. Running a 400W Renogy panel array into a SmartSolar 100/30, so charge side feels solid enough.

Is this a Fogstar BMS comms issue, or does the Cerbo just genuinely hate accurate numbers?

@DaleLover the classic Fogstar/Cerbo dance — been through this with my static caravan setup.

Few things worth checking:

• Shunt wiring — is everything negative going through it? Even a single aux feed bypassing the shunt kills accuracy completely
• Peukert exponent — LiFePO4 should be set to 1.05, not the default 1.25 that catches people out
• Charged voltage threshold — set it lower than you'd think, around 13.4V for a 12V system, otherwise the BMV never registers a full 100% and drifts perpetually downward

The Cerbo itself is just displaying what the BMV tells it, so the fix is nearly always in the BMV-712 settings rather than the GX device.

Also worth doing a proper full charge to absorption, let it sit, then manually sync the SOC to 100% as a baseline reset.

@DaleLover had almost identical grief with mine on the boat. What fixed it for me was making sure the charged voltage and tail current settings in the BMV-712 were dialled in properly for LiFePO4 — the defaults are way off for lithium.

Fogstar 280Ah cells sit at roughly 3.45V/cell resting when genuinely full, so your "charged" threshold needs to reflect that. Also worth checking whether your charger is actually hitting absorption properly — if it's cutting out early, the shunt never gets its sync point and the drift compounds over every cycle.

What's your tail current set to? Mine was left at default 4% which was hopeless.

Great thread, this one! @DaleLover one thing nobody's mentioned yet — check your tail current setting in the BMV-712. If it's set too high, the shunt never properly registers a full charge and the whole calibration just snowballs into nonsense over time.

With my Fogstar 280s I've got tail current at around 1-2% of battery capacity (so roughly 3-5A) and that made a massive difference to how reliably the Cerbo recognises "battery actually full."

Also worth double-checking your Peukert exponent — LiFePO4 should be very close to 1.05, sometimes people leave it at the lead-acid default and wonder why everything's drifting.

Once you've sorted those two settings, do a proper full charge cycle from genuinely empty and let it sync properly. Mine settled down beautifully after that. 🤞

Good shout from @Megan1960 on the tail current — that's often the culprit that gets overlooked.

One thing I'd add: with LiFePO4 the voltage curve is so flat through the middle range that the Cerbo's coulomb counting really needs to nail those synchronisation points, otherwise errors just compound over time. If you're not hitting a proper full charge regularly enough to trigger a sync, the drift will get worse and worse.

Worth checking how often you're actually reaching 100% SOC — if your solar or hook-up isn't quite pushing the Fogstar to a genuine full absorption, the BMV never gets a clean reset. Try manually forcing a full charge cycle and see if the SOC snaps back into line with your voltage readings. Sometimes that alone sorts it completely without touching any of the settings. @DaleLover what's your charging source?

Good shout from @Megan1960 on the tail current — that one catches so many people out.

Something else worth checking @DaleLover: have you verified your battery capacity setting in the BMV-712 matches what you're actually running? Fogstar's 280Ah cells can vary slightly, and if you've got any parallel configuration going on, it's easy to end up with the wrong figure in there.

Also — and this sounds daft but bear with me — when did you last do a proper full charge and let it sit at absorption long enough to actually synchronise the BMV? The Cerbo only updates SOC to 100% when the BMV triggers a sync point. If that's not happening cleanly, the drift compounds over time and you end up in exactly the situation you're describing. 🙂