Victron BMV-712 showing different SOC to my BMS — which one do I trust?

Had exactly this on my narrowboat build last year — drove me absolutely mental for about three weeks before I figured out what was going on.

Short answer: trust the BMS for protection, trust the BMV for trend and history.

The BMV-712 works on coulomb counting — it tracks every amp going in and out and does the maths. Sounds foolproof but it drifts over time, especially if your tail current settings aren't dialled in properly for your specific battery chemistry. Lithium behaves very differently to AGM here, and the defaults are basically nonsense for LiFePO4.

The BMS on the other hand is reading actual cell voltages and applying its own SOC curve — which may or may not be accurate depending on how well the manufacturer has characterised the cells. Fogstar and Victron's own Smart batteries tend to have decent curves baked in, but cheaper Chinese units can be wildly optimistic at the top end.

What actually fixed mine:

• Set the BMV tail current to around 1-2% of capacity for LiFePO4
• Enabled the synchronisation on full charge so it resets to 100% reliably
• Compared both readings only after a proper full absorption cycle

Once I did that, they were within 3-4% of each other pretty consistently, which is about as good as you'll realistically get.

The bigger question is — what BMS are you running? Some of the JBD units communicate over Bluetooth and you can pull proper cell-level data, which changes everything. Would be worth knowing your full setup before anyone gives you more specific advice.

What discrepancy are you actually seeing — is it consistent or does it vary depending on charge state?

@Boycie has nailed the core distinction already, so I'll add the practical layer.

The BMV-712 drifts because it's coulomb counting — tiny measurement errors compound over time. The key fix most people miss is nailing your Peukert exponent and charge efficiency factor in the settings. Victron defaults are conservative and rarely match real-world LiFePO4 behaviour.

My setup on the Vivaro: I set charge efficiency to 99% (LiFePO4 is incredibly efficient) and let the BMV sync to 100% on a proper absorption tail current rather than just voltage. Made a massive difference.

Also worth checking your shunt wiring — any load bypassing the shunt will make the BMV progressively lose track. Classic culprit is a direct chassis earth somewhere in the install.

@Boycie @VivaroWanderer — both spot on, but there's one sneaky culprit nobody's mentioned yet: tail current calibration.

If your BMV's "charged" detection threshold is set too high, it never registers a proper full charge and the coulomb counting drifts further with every cycle. Classic symptom is the SOC slowly creeping downward over weeks even when the bank is genuinely healthy.

On my narrowboat setup I had the tail current set at the Victron default (4% of capacity), which was completely wrong for my Fogstar Drift cells — they finish much lower than that. Once I dropped it to around 1%, the BMV and BMS figures came within 2-3% of each other and stayed there.

Worth checking Settings > Charged Detection before assuming anything more dramatic is wrong. Took me an embarrassingly long time to find that particular rabbit hole.

@VictronPro tail current is the one that gets everyone — mine was set for lead-acid out of the box and my Fogstar Drift cells were sitting there thinking they were full at 85% like a battery with impostor syndrome 🔋

@KatePrice that Fogstar Drift point is crucial — LiFePO4 absorption tail currents are so much lower than lead-acid that the BMV never "sees" a proper full charge and the SOC counter never synchronises.

One thing worth adding: charged voltage threshold is equally guilty. If your charge voltage is set even 0.1V below what your charger actually terminates at, the BMV won't register a full cycle and drift compounds over time.

I run a Victron SmartSolar into a Fogstar 200Ah, and I've got:

• Charged voltage: 27.2V (24V system)
• Tail current: 1% of capacity
• Charge detection time: 3 minutes

That combination almost eliminated my drift entirely. Check these three parameters together rather than individually — they all need to agree with your actual charge profile before the BMV will synchronise reliably.

@MistyTinker absolutely right, and here's the thing that'll really bake your noodle — I spent an embarrassing amount of time convinced my Victron BMV-712 was faulty, convinced my Daly BMS was faulty, and nearly convinced myself I was faulty before someone pointed me at the tail current setting