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

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

The Daly BMS reports SOC based on cell voltage, which sounds reasonable until you remember how flat the LiFePO4 discharge curve is. Voltage tells you almost nothing useful between about 20% and 90% — it's essentially a horizontal line. The Daly is basically guessing.

The BMV-712 uses coulomb counting — it's literally measuring every amp in and out and doing the maths. Far more accurate across that flat middle section, provided you've set it up correctly. That's the catch.

A few things worth checking on your BMV config:

• Charged voltage — needs to match your actual absorption voltage
• Tail current — I run mine at 4% for LiFePO4
• Peukert exponent — set to 1.05 for lithium, not the lead-acid default

The BMV only resets to 100% when it sees a genuine full charge, so if you've been partial charging (guilty as charged on my solar-only weeks), drift creeps in over time.

My Fogstar 200Ah cells and the Daly BMS would regularly disagree by 15-20% with mine. Once I dialled in the BMV properly and let it witness a few proper full charges, it settled down beautifully.

What does your charging profile look like? That's usually where the mystery starts.

@LiFePO4Nerd this is really timely — I've been scratching my head over almost exactly this on my narrowboat setup!

Quick question though: does the BMV-712's accuracy depend heavily on having the correct battery capacity programmed in? I've got a 200Ah Fogstar Drift and I'm second-guessing whether I entered the right Peukert exponent for LiFePO4 vs the default (which I think assumes lead-acid?).

Also — does the BMV need a full charge cycle to synchronise properly first? Mine's only been installed a week and I'm wondering if it just hasn't had chance to calibrate itself yet against the actual capacity.

Basically trying to work out if my readings are wrong because of a settings issue or because the Daly is genuinely confusing things upstream. Anyone dealt with this specifically on lithium rather than AGM?

@LiFePO4Nerd my Fogstar Drift cells laughed at my Daly's SOC readings until I let the BMV-712 do a proper full-charge synchronisation — after that it was like the two finally agreed to be in the same relationship.

@RustySkipper narrowboat here too — had this exact headache when I first fitted my 200Ah Fogstar bank.

One thing worth checking that nobody's mentioned: make sure your BMV-712 has the correct charged voltage and tail current thresholds set for LiFePO4. Out of the box the defaults are often aimed at lead-acid, which means the synchronisation point never triggers properly and the BMV drifts over time.

On mine I run:

• Charged voltage: 13.5V
• Tail current: 4%
• Peukert exponent: 1.05

Once those were dialled in the BMV stayed accurate for weeks without needing a manual sync. The Daly is still useful for cell-level protection, but I wouldn't rely on its SOC figure for day-to-day decisions — the voltage-based calculation it uses is far too flat across most of the LiFePO4 discharge curve to be meaningful.

Interesting thread — just picked this up while troubleshooting something similar on my narrowboat.

One thing nobody's mentioned yet: have you checked your shunt wiring? On mine, the BMV-712 shunt wasn't the only path current was taking — a second negative cable was bypassing it entirely to the engine alternator. Took me weeks to figure out why my SOC kept drifting optimistically high after any motoring time.

Every single load and charge source must pass through that shunt, otherwise the Victron's coulomb counting is working with incomplete data and you'll see exactly the kind of mismatch you're describing.

@RustySkipper worth checking on your narrowboat especially — alternator charging is an easy one to miss if the previous owner wired it up.