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

Been through exactly this with my motorhome setup. Short answer: trust the BMV-712, but understand why they disagree first.

The Daly BMS reports SOC based on cell voltage curves, which sounds reasonable until you realise how flat LiFePO4 voltage curves are between roughly 20–80% — it's essentially guessing in that middle range. The BMV-712 uses coulomb counting (measuring actual charge flowing in and out), which is far more accurate over time provided you've set it up correctly.

Key things to check on your BMV-712:

• Battery capacity — set this accurately, not the marketing figure. Fogstar and similar suppliers sometimes overstate slightly.
• Peukert exponent — for lithium, set this to 1.05 or even 1.00
• Charge efficiency factor — 99% for LiFePO4
• Tail current and charged voltage — these trigger the BMV's sync point, critical for long-term accuracy

The BMV needs to see a proper full charge to sync properly. If you're boondocking and rarely hitting 100%, drift accumulates and both readings become suspect.

The Daly BMS SOC is really just there to protect the cells — it's not designed to give you accurate state of charge information for daily planning. Treat it as a safety layer, not a fuel gauge.

That said, if the two are wildly different (20%+ gap), I'd check whether your Daly has any significant self-discharge or parasitic drain the BMV isn't seeing.

What's your current BMV configuration — have you manually set the charged parameters, or left them at defaults?

@RustyRoamer makes a solid point about voltage-curve SOC being unreliable, but worth adding why the BMV wins long-term: it's coulomb-counting from a properly synchronised full charge, so it's tracking actual electrons in and out rather than guessing from voltage — which is notoriously flat across the middle 20–80% of LiFePO4's discharge curve.

Key thing people miss: the BMV needs correct setup to be trustworthy. Your charged voltage threshold, tail current percentage, and Peukert exponent all need dialling in for your specific cells. Fogstar cells behave slightly differently to EVE, for instance.

My 200Ah Fogstar Drift setup: BMV sits within 2–3% of actual capacity tested. The Daly was sometimes 15% optimistic at 50% SOC — exactly where you don't want nasty surprises on a remote site.

Set both up properly, use the BMV as primary, Daly as your low-voltage safety net.

Great thread this. One thing I'd add that hasn't been mentioned yet — make sure your BMV-712 is properly synchronised before you start trusting it blindly. If the charged voltage threshold and tail current settings aren't dialled in correctly for your specific LiFePO4 bank, it'll never get a clean full-charge sync and the SOC drift will drive you mad over time.

I spent weeks convinced mine was faulty until I realised my tail current was set way too low for my battery capacity. Once I sorted that, it synced properly at 100% and has been rock solid since.

@RustyRoamer what capacity is your bank? That'll affect the ideal tail current setting quite a bit. Victron's defaults are very much a starting point rather than a final answer. 🙂

Great points all round. One thing worth adding from my own experience running a 200Ah Fogstar Drift LiFePO4 in the van — the Daly's SOC will drift badly after partial cycles if it never sees a proper full charge to reset its reference point.

The BMV-712 wins because it's measuring actual coulombs in and out, not inferring from voltage. But it's only as good as your shunt installation. Every amp must pass through that shunt — I've seen setups where a secondary fuse bypass was quietly stealing current outside the measurement loop, causing the BMV to read consistently optimistic.

Worth checking:

• All negative loads route through the shunt
• Tail current and charged voltage thresholds are set correctly for LiFePO4 (not the default AGM values)

Get those parameters right and your BMV will track remarkably accurately over hundreds of cycles.

@MarinePhil nailed the Fogstar point — those Drift cells have a frustratingly flat voltage curve mid-range, so your Daly is basically just guessing between 20% and 80% SOC like it's on a pub quiz.

One thing nobody's touched on yet: make sure your BMV shunt is the only path for current — any rogue earth paths bypassing it will send your coulomb counting completely haywire, and you'll be chasing phantom discrepancies forever.

Learned that the hard way on the narrowboat when I couldn't work out why my BMV-712 kept drifting — turned out a secondary earth was sneaking past the shunt entirely. Victron's own wiring guide is actually decent on this.