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

Had this exact headache last year with my motorhome build. Short answer: trust neither blindly, but lean on the SmartShunt once it's properly calibrated.

The BMS SOC is usually just voltage-based, which is notoriously rubbish for LiFePO4 given how flat the discharge curve is. You could be sitting at 80% or 30% and the voltage barely moves. Mine was reading 67% when the SmartShunt had it at 54% — turned out the BMS was way off.

The SmartShunt does proper coulomb counting, so it's tracking actual current in/out. That's far more meaningful. However, it's only as good as its calibration. Key things to check:

• Tail current set correctly (I use 1% of capacity)
• Charged voltage threshold matches your actual absorption voltage
• You're getting genuine full charges regularly so it can resync

The SmartShunt also resyncs to 100% when it detects a full charge, which helps keep drift in check over time.

The BMS SOC isn't totally useless — it's a reasonable sanity check and will still protect your cells regardless. But for day-to-day capacity planning, the SmartShunt is the one I watch.

Anyone else running Fogstar Drift cells notice the BMS readings are particularly optimistic? Mine seemed to consistently flatter the SOC compared to what the SmartShunt was seeing.

@ExChippie this is really useful, cheers. I've been wrestling with exactly this on my narrowboat setup.

Quick question though — does the SmartShunt's accuracy degrade if the battery rarely hits a proper full charge? My LiFePO4 bank sometimes sits at around 80% because I'm limiting charge cycles, so I'm wondering if the shunt ever gets a chance to synchronise properly.

I've read that Victron's algorithm relies on hitting the absorption/float threshold to reset the SOC counter. Is that right? And if so, what's the recommended workaround — manually triggering a full charge periodically, or adjusting the synchronisation voltage settings in VictronConnect?

The BMS (a Daly, nothing fancy) is all over the place, so I definitely can't rely on that.

@ExChippie this is so timely — I'm dealing with this exact thing on my narrowboat right now! My Fogstar Drift 200Ah is showing 87% on the BMS but my SmartShunt is saying 79%. Quite a gap!

Can I ask — how long did your SmartShunt take to "learn" the battery properly after you first set it up? I've only had mine a few weeks and I'm wondering if it just hasn't had enough full charge cycles to calibrate properly yet.

Also, did you have to tweak the Peukert exponent or charge efficiency settings in VictronConnect, or just leave everything at default? I'm a complete beginner with those settings and slightly scared to touch them!

@ExChippie really interesting thread — just wondering, does the SmartShunt's calibration drift over time if you don't hit a full 100% charge regularly?

I ask because with my EV charging setup I'm often pulling the battery down and topping back up to around 80% to protect cycle life. Does the SmartShunt need a proper full charge periodically to "reset" its reference point, or does it just accumulate error if you never reach 100% SOC?

Also @LindaClark90 — narrowboat is a great use case for this question, presumably you're sometimes running days without hitting full charge if the solar's weak?

Keen to understand whether I should be scheduling occasional full charges just to keep the SmartShunt honest, rather than purely for battery health reasons.

@Cleggy yes, drift is a real thing — particularly if your battery never reaches a proper full charge (common with solar in a British winter, as I know painfully well from my garden office setup).

The SmartShunt recalibrates its 100% SOC point each time it detects a synchronisation event — essentially when voltage hits the tail current threshold you've configured. If your system never quite gets there, the shunt starts flying blind.

What sorted mine was tweaking the charged voltage and tail current settings in VictronConnect to match what my Fogstar cells actually do at full. Spent an afternoon on it last October and the accuracy improved noticeably.

@LindaClark90 on a narrowboat with alternator charging this gets more complicated — the charge profile is choppier, so you might need to set your synchronisation parameters more conservatively.