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

Had this exact headache last year with my 280Ah Fogstar Drift cells. Both were reading differently and I didn't know which to believe for weeks.

Short answer: neither is gospel, but the BMV-712 is usually more reliable day-to-day — provided you've nailed your peukert exponent and charged efficiency settings. Most BMS units (especially cheaper Chinese ones) are rubbish at coulomb counting and just reset at full charge then drift badly.

The BMV wins because:

• It measures actual current flow precisely
• You can tune the settings to your specific battery chemistry
• Synchronisation at 100% (tail current) keeps it honest

That said, the BMS protects the cells — so even if its SOC display is nonsense, don't ignore its voltage-based cutoffs.

The real fix is getting your BMV calibrated properly. Set your battery capacity accurately, dial in the charge efficiency factor (I run 99% on LiFePO4), and make sure it's syncing at the top of every full charge. Once I sorted that mine rarely drifts more than 2-3% over a week of cycling.

What BMS are you running? Some of the Daly units are notoriously optimistic with their SOC. Others like the JK BMS are a fair bit better at tracking.

Anyone else had both readings wander miles apart? Curious whether it's a common setup issue or just specific to certain cell batches.

@ShortCircuit oh mate, I've been down this exact rabbit hole in my van build — nearly drove myself round the bend staring at two different numbers on two different screens.

Here's what I eventually landed on: treat the BMV-712 as your long-term accountant and the BMS as your bouncer at the door — the BMS is protecting cells from immediate disaster, but it's rubbish at cumulative tracking.

The key thing nobody tells you: the BMV needs a proper synchronisation point — a genuine 100% full charge held long enough for the tail current to trigger. Until that happens, it's basically guessing.

Once I sorted my absorption settings on my Victron MPPT to actually complete the charge cycle, both readings came within 3% of each other and stayed there. 🎯

Calibration is everything.

@ShortCircuit interesting timing — I've just been wrestling with this exact thing on my static caravan setup with a 200Ah Fogstar pack.

Quick question though: how often are you doing a proper full charge cycle to let the BMV-712 resync? I noticed mine drifts badly if I don't hit the tail current threshold regularly — especially through winter when solar's rubbish and I'm not getting a proper absorption phase.

Also wondering — does your BMS report SOC via Bluetooth or is it wired into the Victron ecosystem somehow? Mine's standalone which I reckon is half the problem. If the two systems aren't talking to each other, they're essentially just making educated guesses independently aren't they?

Have either of you tried calibrating the BMV against a known full charge and resetting from there? Curious whether that closed the gap at all for you @KangooDream.

@OldSparky the real trick is syncing your BMV-712 to 100% only when your BMS confirms full charge and absorption current has dropped to the tail current threshold — get that wrong and you're just arguing between two liars.

My shepherd's hut Fogstar/Victron setup disagreed by nearly 12% until I set the charged voltage to 0.1V below actual float, pegged tail current at 2% of capacity, and let it run a full cycle before trusting either reading. The BMS is reacting to cell voltages in real-time; the BMV is integrating coulombs over time — they're measuring different things, so occasional disagreement is physics, not a fault.

Also calibrate your shunt wiring — a loose connection will make the BMV drift faster than a politician's promises.

@ExFarmer79 nails it on the sync point — that's the bit everyone skips because reading the Victron manual feels like homework.

Mine was out by nearly 18% when I first wired up the garden office setup. Spent a weekend convinced my cells were dying. Turned out I'd just never let it see a proper full charge cycle before the

@BoxerProject ha, the Victron manual — 47 pages to tell you "charge it up and press a button."

One thing worth adding: your BMS SOC is often just voltage-based under the hood, which makes it about as accurate as a wet finger in the wind. The BMV-712 does proper coulomb counting so it's tracking actual electrons