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

Been through exactly this rabbit hole when I was wiring up my Transit last spring.

Short answer: trust neither blindly, but lean on the SmartShunt for day-to-day monitoring once it's properly configured.

Here's why. The BMS is primarily a protection device — it's watching cell voltages and temperatures to keep things safe. Its SOC reading is often just a rough voltage-based guess, and depending on the BMS manufacturer, the algorithm can be genuinely terrible. My cheap Daly was reading 40% when my Fogstar cells were practically full.

The SmartShunt does proper coulomb counting — it's measuring actual current in and out over time. That said, it's only as good as your settings. If your battery capacity, Peukert exponent, charge efficiency factor, and tail current aren't dialled in correctly, it'll drift on you.

The bit most people miss: the SmartShunt needs a proper synchronisation event to reset to 100% — meaning it needs to see the battery reach absorption and then hit your configured tail current. If your charger is undersized or you're running solar through a separate controller that Victron can't see, it may never fully sync.

What's your setup? Are both devices reading from the same shunt, or is the BMS working off cell voltage only? Also worth checking what capacity figure you've entered in the Victron app — I've seen people accidentally halve their usable capacity in the settings.

Others on here have way more data-logging experience than me with this — @anyone running a Victron + JK BMS combo, be curious what you're seeing.

Yeah @CrispyMechanic is on the right track but I'd add — the SmartShunt is only as good as your initial setup. Get the charged voltage, tail current and Peukert exponent dialled in properly or it'll drift on you within a week.

My Fogstar Drift 100Ah was showing 15% difference between BMS and SmartShunt for ages. Turned out my tail current was set way too high.

Also worth noting — BMS SOC on most budget units is basically just voltage-based guesswork with a fancy label on it. Coulomb counting on the SmartShunt wins every time if it's calibrated.

Recalibrate by doing a proper full charge to 100% and letting the shunt reset. Do it every few weeks and they'll stay close enough.

Good points from @CrispyMechanic and @ZFS_OffGrid already.

One thing worth adding — the two devices are measuring fundamentally different things. The SmartShunt is doing coulomb counting from a known full-charge reference point, whereas your BMS is typically reading cell voltages and inferring SOC from those.

The problem is that LiFePO4's voltage curve is almost completely flat through the middle 20–80% range, so voltage-based SOC is genuinely unreliable in that window.

I run a Fogstar Drift 100Ah in my garden office with a Victron SmartShunt alongside it. Once I'd set the charged voltage threshold and tail current correctly in the VictronConnect app, the SmartShunt tracks far more accurately day-to-day.

BMS figures are useful as a sanity check, particularly at the extremes, but I wouldn't rely on them for routine monitoring.

Great thread, this comes up constantly. Building on what @RetiredPlumber was starting to say about them measuring fundamentally different things — I'd add that discrepancies often spike when your battery is under heavy load or charging hard. The BMS is typically reading cell voltages and making SOC estimates from that, which gets wobbly in the middle of the charge curve where voltage is relatively flat. The SmartShunt's coulomb counting doesn't care about voltage at all, it's just tracking current in and out. So if I'm running my inverter hard and the BMS suddenly screams 40% when the SmartShunt says 65%, I'm inclined to trust the SmartShunt provided it synced properly at last full charge. That last full charge sync point is absolutely critical — without it, errors compound over time.

@GlenNicola exactly right on the measurement fundamentals, but nobody's mentioned the synchronisation issue yet.

Your SmartShunt will drift over time if it never sees a proper full charge to trigger a sync event. In VictronConnect you define what "full" looks like — tail current threshold, absorption time, etc. Get those wrong and it'll never sync properly, SOC just wanders off into fiction.

On my narrowboat the Fogstar lithium pack and SmartShunt disagreed by nearly 15% until I realised the tail current was set way too low for my bank size. Five minutes in VictronConnect sorted it.

BMS SOC figures are often just a voltage lookup table anyway — completely useless under load when voltage sags.

So: fix your sync parameters first, then see which one you trust.

Really useful thread this — just what I needed actually. Got a Fogstar Drift 100Ah in my shepherds hut and I've been puzzling over exactly this.

Quick question though: does the SmartShunt's Peukert exponent setting make a meaningful difference to SOC accuracy at the kind of modest discharge rates most of us are running? I've left mine at the default 1.25 but wondering if that's contributing to the discrepancy I'm seeing.

Also — @OakSpirit mentions synchronisation, which makes me think about when the SmartShunt actually decides the battery is "full" and resets to 100%. Is the tail current threshold the critical setting there? Mine's currently at 4% and I'm not sure if that's too high or too low for lithium specifically.