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

Had this exact headache last summer in my shepherd's hut build. Short answer — trust neither blindly, but lean on the BMS for lithium.

The BMV-712 is brilliant but it's coulomb counting, so any calibration drift or peukert settings being slightly off will compound over time. Mine was reading 67% when the BMS was screaming low voltage protection at around 45%. Took me ages to figure out the BMV just hadn't seen a proper full charge cycle to resync itself.

Few things worth checking:

• Charged voltage setting in the BMV — needs to match what your BMS considers "full"
• Tail current — if it's too high it'll call the battery full too early
• Peukert exponent — for LiFePO4 this should be really close to 1.05, not the default lead-acid value

Once I sorted those the two readings came within about 3% of each other and have stayed there.

The BMS has actual cell-level data so it knows when things are genuinely low. The BMV is more of an educated estimate. That said, the BMV is far better for day-to-day trend watching — the history screen is gold for spotting problems early.

What BMS are you running? Some of the cheaper ones have dodgy SOC algorithms themselves which complicates things further. Would be useful to know your battery brand too — Fogstar cells tend to behave quite differently to some of the grey import packs.

Anyone else had persistent drift between the two even after recalibration?

Good shout from @Fiona1974 on the coulomb counting point.

The real culprit is usually the tail current setting on your BMV-712. If it's not calibrated correctly for your specific lithium cells, the BMV never gets a clean "full" reference point, so drift compounds over days.

On my narrowboat I run a Fogstar Drift 200Ah with its own BMS alongside a BMV-712. What I found is the BMS SOC is based on cell voltage thresholds — crude but self-correcting every cycle. The BMV is far more granular mid-discharge but needs that synchronisation point.

My workflow:

• Set tail current to ~2% of battery capacity
• Let the BMS trigger the "full" sync via a Victron relay signal if possible
• Cross-reference both after a full charge cycle

The two together tell a better story than either alone. Divergence of more than ~5% usually points to a calibration or parasitic draw issue worth investigating.

Good points already made on tail current and coulomb counting. One thing worth adding — the BMV-712 will drift over time if your shunt isn't the only path for current. Classic mistake in static caravan installs is having a secondary earth path bypassing the shunt entirely, often through a chassis bond. Your BMV then consistently under-reads discharge, so SOC looks rosier than reality.

Worth grabbing a clamp meter and checking current on both sides of your shunt under load. If the figures don't match, you've found your problem.

Also, your BMS SOC is only as trustworthy as its own calibration — most budget units use voltage-based estimation rather than true coulomb counting, which is ironic given the debate. A Daly or JK BMS will be more reliable than generic white-label units from certain well-known auction sites.

Great thread. Had this exact battle on my boat last year — Victron BMV-712 saying 87%, Fogstar lithium's BMS saying 71%, and I'm trying to decide whether to risk running the inverter for an EV top-up charge. Not ideal when you're on a mooring with no shore power!

What sorted it for me was doing a proper full charge cycle and letting the BMV-712 sync to 100% correctly. The BMS had actually been spot-on all along — the BMV had never had a clean synchronisation point since I rewired the battery bank.

Worth checking: is your BMV seeing a genuine 100% charged state periodically? On lithium the absorption tail is short, so if the charger cuts off before the BMV registers it, you'll accumulate drift across every cycle without realising.

Really useful thread — this is exactly the kind of thing I'm trying to get my head around before I finalise my own setup.

Quick question for the group: does the cell balancing activity from the BMS affect the BMV-712's coulomb counting in any meaningful way? I'm wondering if that's another source of drift beyond what @MandyRoss mentioned.

Also — has anyone paired the BMV-712 with a Fogstar Drift specifically? Curious whether the BMS comms on that unit are accurate enough to use as the primary reference, or whether it's still worth running both and cross-referencing.

My use case is a tiny house with occasional EV charging loads, so the swings can be pretty dramatic — presumably that makes accurate SOC even more critical than a steady domestic setup?

Good question from @OakSoul about finalising a setup — get this sorted before commissioning, not after.

One thing nobody's mentioned: your synchronisation events matter enormously. The BMV-712 resets its SOC when it detects a full charge, but if your charge parameters don't match reality — tail current threshold set too high, absorption voltage slightly off — it'll "synchronise" at the wrong moment and every reading after that is built on a lie.

On my van build I spent a frustrating weekend chasing a 15% discrepancy before realising my Victron MPPT charge profile was slightly mismatched to the Fogstar cells. Fixed the absorption voltage, tightened the tail current setting, and both devices came into agreement within a cycle or two.

The BMS protects the cells. The BMV monitors the system. Different jobs. Stop asking them to replace each other.