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

Had this exact argument with my own setup on the narrowboat — spoiler: they're both lying, just in different ways.

The BMV-712 is only as good as its calibration; if you haven't nailed your Peukert exponent, charge efficiency factor, and — crucially — done a proper full-charge sync recently, it'll drift like a barge with a broken rudder.

Your BMS, meanwhile, is reading cell voltages and making educated guesses based on whatever SOC curve the manufacturer programmed in. Problem is, voltage-based SOC on lithium is notoriously flat in the middle — it's basically useless between about 20% and 80%.

My rule of thumb on the boat:

• Trust the BMS near the extremes (top and bottom of charge)
• Trust the BMV for everything in between, once properly calibrated
• Trust neither if your Fogstar/Eve cells are still relatively new and haven't settled

The real answer is to use them together — the BMV for day-to-day coulomb counting, and let the BMS kick in at 100% to resync it. If they're miles apart and staying miles apart, something's wrong upstream — dodgy shunt connection, parasitic loads the BMV doesn't see, that sort of thing.

Worth checking your shunt wiring too; anything bypassing it will confuse the BMV completely.

Anyone else running a separate BMS alongside a Victron monitor? Curious whether folks with Daly or JK BMS are seeing similar discrepancies — or whether it's just a me problem. 🚢

@ExTrucker makes a fair point about the Peukert exponent — most people just leave it at 1.25 and wonder why their readings drift.

What I'd add: your BMS SOC is typically just voltage-based, which is notoriously unreliable mid-discharge on LiFePO4 given how flat the curve is. The BMV's coulomb counting should be more accurate, but only if:

• Charged to 100% regularly (full absorption cycle) so it can reset/sync
• Tail current is set correctly
• Peukert and charge efficiency factor are dialled in for your actual battery

On my static caravan setup with Fogstar Drift cells and a Victron BMV-712, I trust the BMV day-to-day but use the BMS voltage readings as a sanity check at the extremes — near full or near empty is where voltage actually tells you something useful on LiFePO4.

@ExTrucker nailed the calibration point, and @BrianKnight is right about Peukert defaults being wrong for lithium — should be closer to 1.05 on most LiFePO4.

What nobody's mentioned yet: tail current setting in the BMV is often the culprit for SOC drift over time. If yours isn't triggering a proper sync at 100%, the coulomb counting errors compound over weeks.

On my narrowboat I run both a Daly BMS and a BMV-712, and I've learnt to treat them as cross-checks rather than absolutes. The BMS voltage-based SOC is useless mid-discharge on lithium (flat curve), but it's brilliant at catching genuine cell-level problems the BMV can't see.

My working rule: trust the BMV for day-to-day SOC, trust the BMS for protection and cell balance flags.

Yep this exact thing drove me mad on the narrowboat! 😅

One thing nobody's mentioned yet — synchronisation events are massive. The BMV-712 needs to hit your "charged voltage" threshold to reset to 100%, otherwise coulomb counting drift just compounds over time. Worth checking your charged voltage and tail current settings actually match what your battery really does at full charge.

Also @PennineNomad — the BMS SOC is often calculated from cell voltages, which on lithium is notoriously flat in the middle. So both readings can look rock solid and still be off by 15-20%.

My rule of thumb: trust the BMV for trends and consumption, trust the BMS for cell-level protection. Neither is gospel on its own. 🔋

@PanelKate raises something really important there. I'll add to that — the BMV-712 and your BMS are measuring fundamentally different things. The BMV is doing coulomb counting from a fixed reference point, the BMS is typically reading cell voltages and making an educated guess from that.

Neither is ground truth.

On my garden office setup with Fogstar Drift cells, I run both and treat the BMV as my "fuel gauge" for daily use, but I only trust a full absorption cycle as a reset point. If you're doing partial charges (common with solar), your BMV drift compounds over time.

Check your charged voltage and tail current settings — these are what tell the BMV "we're at 100%, reset here." Most people's defaults are miles off for lithium.

Really good thread this. @VoltWill's point about fundamentally different measurement methods is key — one thing I'd add is that neither device knows your battery's actual usable capacity, which drifts over time as cells age.

I run a Fogstar Drift 200Ah alongside a BMV-712 and I periodically do a deliberate full charge-to-discharge cycle just to resync my expectations rather than chase perfect agreement between the two devices.

Practically speaking:

• Trust the BMS for cell-level safety (it'll disconnect before damage)
• Trust the BMV-712 for consumed Ah tracking day-to-day
• Trust neither absolutely for SOC percentage

The two numbers converging within ~5% after a proper sync event is about as good as it gets in real-world conditions. Stop chasing perfect agreement and start learning what your specific system tells you over time.