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

Ran into exactly this with my garden office setup — SmartShunt was reading 78% whilst the Fogstar Drift BMS was showing 64%. Took me a good few weeks to figure out what was going on.

Short answer: trust neither blindly, but lean on the BMS for lithium chemistry.

The SmartShunt is essentially a coulomb counter — it tracks charge in and out, but it drifts over time if you haven't set the tail current and charged-full detection correctly in VictronConnect. If it doesn't see a proper full charge cycle to reset its 100% anchor point, the error compounds. Mine was set up with the default parameters which are frankly optimised for lead-acid.

The BMS is reading actual cell voltages and calculating SOC from the discharge curve. For LiFePO4 that curve is notoriously flat between about 20–80%, so there's genuine ambiguity there too — but at least it's working from real-time data rather than accumulated counting errors.

What actually helped me:

• Set the SmartShunt's "charged voltage" to match what your BMS considers a full charge
• Tighten the tail current threshold (I dropped mine to 2% of capacity)
• Let it do a full charge cycle to resync

After that mine converged to within 3–4%. They'll never be identical because they're measuring fundamentally different things.

Would be curious whether others have found the Victron parameters that work best with Fogstar packs specifically — the BMS documentation is a bit sparse on what voltage it actually uses as its SOC reference points.

Had exactly this with my static caravan setup. The SmartShunt is only as good as its calibration — if you haven't done a proper full charge cycle to let it sync its 100% reference point, it'll drift increasingly out of whack over time.

The BMS is measuring cell voltages directly, so it's arguably more honest about actual state, especially at the extremes. SmartShunt is doing coulomb counting which accumulates errors.

My workflow now:

• Trust the BMS for low-SOC warnings (it sees individual cells)
• Trust the SmartShunt for the middle range trend
• Force a full absorption charge periodically to resync the shunt

Also worth checking your Peukert exponent and charge efficiency factor in the SmartShunt settings — Victron's defaults aren't always ideal for LiFePO4. Fogstar's own docs sometimes give better figures for their specific cells.

Great thread @SuffolkCruiser — worth adding that the two devices are measuring fundamentally different things. The SmartShunt is coulomb counting from a reference point, so any drift in your charged full threshold or a slightly wrong battery capacity setting will compound over time. The BMS, meanwhile, is often using cell voltage curves to estimate SOC, which has its own accuracy issues particularly in the flat middle section of an LFP discharge curve.

What I'd actually do is trust neither in isolation — use the SmartShunt for short-term cycle tracking once properly calibrated, but let a genuine top-balance and full charge reset it regularly. Set your "charged voltage" and "tail current" thresholds in the SmartShunt settings carefully to match your actual charge profile. That synchronisation point is where most people go wrong in my experience.

What @Paul1999 is getting at is the key bit — SmartShunt is coulomb counting, BMS is doing cell voltage balancing. They'll drift apart over time regardless.

My rule: trust the BMS for cell-level protection, trust the SmartShunt for actual capacity tracking day-to-day. Neither is "right" in isolation.

Biggest fix I've found — nail your Peukert exponent and charge efficiency factor in the Victron settings. Most people leave them at default and wonder why the SOC wanders. Also make sure your SmartShunt gets a proper 100% sync point regularly (full charge, tail current drops out). Mine's rock solid on Fogstar cells once I sorted that.

Good point from @HarbourHermit on the coulomb counting side of things.

One thing nobody's mentioned yet — peukert exponent settings on the SmartShunt can cause significant drift if you've left it at the default lithium profile but your actual cells behave slightly differently. Worth checking in the VictronConnect app.

Also, SmartShunt needs a proper synchronisation event to reset its count — usually a full charge cycle where the battery hits absorption voltage and tail current drops below your set threshold. If that threshold is misconfigured, it'll never sync and the drift compounds over time.

On my narrowboat setup I had a similar 10-15% discrepancy for months. Turned out my charged voltage parameter was set 0.2V too high — SmartShunt never thought the battery was truly full, so it never zeroed itself.

Both readings can be "right" in their own context.