Victron SmartShunt showing different SOC to my BMS — which one do I trust?

Running into exactly this on my shepherd's hut build at the moment. Fogstar Drift 200Ah with its own BMS, and the SmartShunt is regularly reading 6-8% higher SOC than what the BMS app shows. Getting a bit sick of not knowing which one to believe before I head off-grid for a week.

From what I can gather, the SmartShunt is doing coulomb counting — so it's tracking every amp in and out — but it's only as accurate as your initial setup. Did you nail your Peukert exponent and charge efficiency factor in VictronConnect? I'll be honest, I just left mine on defaults and that might be my problem.

The BMS reading is arguably more "honest" because it's measuring at cell level, but they tend to be conservative and will report low to protect the pack.

Few things I'm wondering:

• Does either unit reset/sync its SOC when you hit full charge? My SmartShunt seems to drift unless I regularly hit 100% absorption
• Which reading are you actually acting on day-to-day?
• Is anyone using the BMS comms to feed data directly into Victron's ecosystem to get a single source of truth?

Considering this for the boat build too so I'd rather sort my head around it now. Suspect the answer is "trust the BMS for low-SOC warnings and the SmartShunt for overall tracking" but that feels a bit wishy-washy.

Anyone actually solved this properly rather than just living with the discrepancy?

@BatteryPaddy had almost identical situation in my van conversion with a Fogstar and SmartShunt combo.

Short answer: trust the BMS for lithium, it's measuring cell voltages directly. The SmartShunt is doing coulomb counting which drifts over time, especially if it hasn't had a proper sync point recently.

Few things worth checking:

• Has the SmartShunt actually seen a full charge cycle to reset its 100% reference? If not, it's just guessing from wherever it started
• Check your charged voltage and tail current settings in the Victron app — these need to match your actual charge profile
• Also double-check your Peukert exponent setting

Once I got the sync settings dialled in properly mine stayed within 2% of the BMS reading. Still glance at both tbh.

@BatteryPaddy the SmartShunt is only as good as its calibration — if your charged voltage threshold or tail current aren't set correctly, it'll never properly synchronise and the drift compounds over time.

In my shepherds hut setup I found the SmartShunt was optimistic because it hadn't seen a proper full charge cycle to reset itself. Check your charged voltage and tail current settings in VictronConnect. I'd set charged voltage about 0.2V below your actual absorption finish voltage.

That said, I'd lean on the BMS figures for protection decisions — it's reading cell-level data. The SmartShunt is better for tracking consumption trends over time. Neither is gospel on its own, frankly.

Classic calibration drift problem — been down this rabbit hole myself.

The real culprit is almost always the tail current setting in the SmartShunt. Mine was defaulting to 4% of battery capacity, which is miles off for LiFePO4. Drop it to around 1-2% and suddenly it syncs up properly at the top.

Also worth checking: is your SmartShunt seeing every amp in and out? If your BMS is between the shunt and the battery, you've got a measurement gap right there. The shunt needs to be the single point all current passes through, directly on the negative battery terminal.

The BMS SOC reading is honestly less reliable — it's primarily there for cell protection, not accurate coulomb counting. I'd trust a properly calibrated SmartShunt over a Fogstar BMS app any day, @BatteryPaddy.

@BatteryPaddy worth adding something nobody's mentioned yet — synchronisation events are your friend here.

The SmartShunt only resets to 100% when three conditions are simultaneously met: voltage at or above your charged voltage threshold, tail current below your set value, and the absorption time has elapsed. If any one of those is slightly off for your setup, it'll drift progressively between full charges.

In my shepherd's hut with a Fogstar Drift 200Ah (same battery, funnily enough), I found the default 4% tail current was far too high. Dropping it to 1.5% and setting charged voltage to 13.9V brought the SmartShunt bang in line with the BMS readings within a week of normal cycling.

The BMS SOC is also pretty rough — it's largely voltage-based. Neither is gospel, but a well-calibrated SmartShunt via Coulomb counting is generally more reliable day-to-day.

Good points all round, but nobody's addressed the Peukert exponent angle yet.

The SmartShunt uses coulomb counting adjusted for Peukert losses — if that's misconfigured for LiFePO4 it'll quietly accumulate error across every cycle. LiFePO4 is essentially flat at 1.05, but Victron defaults closer to lead-acid values out of the box. Check your VictronConnect settings under Battery → Peukert Exponent.

Also worth noting: your Fogstar Drift BMS is measuring cell-level voltage and making SOC inferences from that, which on LiFePO4's notoriously flat discharge curve is frankly a bit optimistic itself. Neither instrument is necessarily correct — they're measuring different things entirely.

My money's on the SmartShunt being more accurate long-term once properly calibrated, but it needs those sync events @Trigger mentioned to stop drift compounding. Two imperfect instruments pointing at the same battery doesn't mean either one's telling the truth.