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

Had exactly this with my static caravan setup last summer. Short answer: trust the BMS for protection, trust the SmartShunt for actual SOC accuracy — they're doing different jobs really.

The BMS is watching cell voltages and will cut off if anything goes dangerously high or low. It's not really designed to be a fuel gauge. The SmartShunt on the other hand is counting coulombs in and out, so over time it builds up a much more accurate picture of what's actually in the bank — as long as you've set it up properly.

Key things to check on the SmartShunt config:

• Charged voltage threshold — needs to match your actual absorption voltage
• Tail current — set this too high and it'll declare "full" before you actually are
• Peukert exponent — often overlooked but matters under heavy loads
• Battery capacity — obvious but worth double-checking

The two will drift apart if the shunt never gets a proper sync point. Make sure your batteries are genuinely reaching a full charge regularly so the shunt can reset itself. If you're mostly doing partial cycles (pretty common off-grid), the SOC can wander off quite a bit.

What BMS are you running? Some of the Fogstar Drift cells have decent BMS comms and you can sometimes pull SOC data directly into VRM which helps compare the two. Worth checking if yours supports that.

Anyone else had success getting these two to agree? Curious whether others have found particular Peukert settings that work well with LiFePO4.

@WonkySkipper nailed the core point. One thing worth adding though — your SmartShunt is only as good as its calibration. If you haven't done a proper full charge synchronisation (let it hit absorption, hold it, then bulk finishes), the shunt's percentage drifts over time and becomes meaningless.

Had this exact issue on my narrowboat with a Fogstar Drift 200Ah. Shunt was reading 47% when the BMS said 31%. Turned out I'd never let it fully synchronise after install — once I did a proper topped-up reset, they tracked within 2-3% of each other.

Also check your tail current setting in VictronConnect. Factory default is often wrong for lithium. Should typically be around 1-2% of your battery capacity.

@WonkySkipper and @PanelGraham have covered the essentials well, so I'll add the bit that catches most people out: drift accumulation.

The SmartShunt calculates SOC by counting coulombs in and out — if it never gets a proper full-charge synchronisation event, errors compound over weeks. Victron's algorithm resets to 100% when voltage, tail current, and absorption time conditions are all simultaneously satisfied. If your charge controller terminates early (common with cheaper MPPT units), that reset never triggers.

Worth checking in the VictronConnect app under SmartShunt settings — specifically the "Charged voltage" and "Tail current" parameters. I had mine misconfigured on the narrowboat for months, SmartShunt reading 15% optimistic against my Fogstar Drift cells' actual capacity.

Also: if your BMS is regularly disconnecting before the SmartShunt synchronises, you'll never get accurate readings regardless of how good the hardware is.

@DevonDweller finish your sentence mate, looks like it got cut off.

Practical point nobody's mentioned yet — synchronisation events are what keep your SmartShunt honest long-term. If your batteries rarely hit 100% SOC (common with solar in winter or undersized arrays), the shunt never gets a proper sync point and the drift compounds over time.

I had this exact issue on my motorhome with a 200Ah Fogstar Drift LiFePO4. SmartShunt was reading 67%, BMS app showing 54% — turned out I'd not had a proper full charge cycle in three weeks of cloudy weather.

Forced a full absorption charge via the shore power hookup, both readings snapped back into agreement within a couple of percent.

Set your charged voltage and tail current thresholds correctly in the SmartShunt settings — most people leave them at defaults which are often wrong for LiFePO4.

Great points from everyone so far. To finish what @DevonDweller was likely getting at and pick up my own thread — synchronisation events are everything with coulomb counting shunts. Your SmartShunt resets its SOC reference when it detects a full charge, so if your system never quite reaches the absorption voltage tail current threshold (common with undersized solar in winter), it'll drift progressively further off over weeks.

I'd recommend checking your SmartShunt's "time to go" history and seeing how often it's actually logging a full synchronisation. If it's been weeks, that's your culprit right there. You can manually sync it once you've done a proper full charge from mains or a generator. After that, keep your charge parameters consistent between the SmartShunt and your chargers — mismatched absorption voltages between devices cause no end of confusion.