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

Had this exact drama with my static caravan setup last summer — Fogstar Drift cells, Daly BMS screaming 47% whilst the SmartShunt was sat there confidently telling me 71%, both of them absolutely certain they were right, like two blokes arguing over directions without a map.

Short answer: trust the SmartShunt for day-to-day monitoring, but only once you've sorted your tail current and charged efficiency factor settings — out of the box defaults are basically fiction.

The BMS SOC is usually just voltage-based guesswork dressed up in a percentage, which is about as reliable as a chocolate teapot on a warm afternoon. The SmartShunt uses coulomb counting, so it's actually tracking what goes in and out — far more accurate mid-cycle.

Key things to nail down:

• Synchronisation point — make sure the SmartShunt knows when you're genuinely at 100% (absorption done, tail current hit)
• Peukert exponent — tweak it for your specific cells
• Charged voltage — set this properly or it'll never reset cleanly

Once I got mine dialled in via the Victron Connect app, the two readings drifted apart by maybe 3-4% at worst, which is liveable.

Anyone else found the Daly BMS particularly optimistic with its SOC? Mine acts like it went to the same school as an estate agent.

VivaroSolar | 847 posts

@VanKen I'd lean towards the SmartShunt personally. The Daly's SOC reading is basically just voltage-based estimation, which goes wobbly at anything other than very low or very high states — LiFePO4's flat discharge curve makes this especially unreliable in the middle range.

The SmartShunt uses coulomb counting, so it's tracking actual charge flowing in and out. Far more accurate day-to-day, provided you've got your battery capacity, Peukert exponent and charged voltage threshold dialled in correctly.

Key question — is your SmartShunt properly syncing? It needs to see a genuine full charge to reset its 100% reference point. If it's not hitting that regularly, errors accumulate over time and it'll drift.

Get those settings right first, then I'd mostly ignore the Daly's SOC figure entirely.

Mark1978 | 312 posts

Both are wrong in their own special way, if I'm honest. The SmartShunt is only as good as its calibration — if it hasn't seen a proper full charge to sync against, that coulomb counting drifts over time. Had exactly this with my own setup before I configured the "charged voltage" and tail current thresholds properly in VictronConnect.

The Daly SOC meanwhile is essentially guesswork dressed up as a number.

What I'd actually trust is neither — watch your resting cell voltages after a good 30 mins off load. That'll tell you roughly where you are with LiFePO4.

Once you've got a genuine full charge cycle completed, the SmartShunt will resync and become far more reliable going forward. Sort the calibration settings first before assuming the hardware's at fault.

BoxerCamper | 1,203 posts

Had this exact standoff on my narrowboat last winter — Victron SmartShunt vs a Daly BMS having a proper argument whilst I'm trying to work out if I can run the heating overnight.

Here's what settled it for me: the SmartShunt wins long-term because it tracks actual coulombs in and out, but it drifts if you haven't set your charged voltage and tail current correctly in the VictronConnect app.

The Daly meanwhile resets its SOC to 100% the moment cells hit absorption voltage — which is optimistic nonsense if your charger barely got there.

My workflow now: trust the SmartShunt day-to-day, but force a proper full charge periodically so it can resync. Set your tail current to around 2-4% of battery capacity and watch them gradually agree with each other like old mates making up. 🛶

LynnCrane | 203 posts

In my shepherd's hut I ran both side by side for months, watching them argue like a pair of stubborn ewes. What settled it for me was understanding what each one is actually measuring. The SmartShunt counts coulombs in and out — it drifts slowly over time but tells a smoother story. The Daly is making educated guesses from cell voltages, which is notoriously unreliable mid-cycle on LiFePO4 cells due to that flat discharge curve.

My approach now: trust the SmartShunt for day-to-day percentage, but let the BMS have the final word on protection — it's closer to the cells and will catch a low-voltage emergency faster. They're not really competing; they're doing different jobs.

Calibrate your SmartShunt properly at full charge and the gap between them should narrow considerably.

ExFarmer90 | 847 posts

Right, here's what sorted it for me on the farm office system — the SmartShunt is brilliant at tracking but it drifts without a proper sync point. Set your tail current correctly (I had mine way too high initially) and make sure it's actually seeing a full charge cycle to reset. Mine was off by nearly 25% until I sorted the charged voltage threshold in VictronConnect.

The Daly meanwhile is protecting the cells, not measuring capacity — completely different job.

What I now do is treat the SmartShunt as my fuel gauge and the BMS as my engine warning light. If the BMS kicks off, I stop. But for day-to-day planning, the SmartShunt wins every time once it's properly configured.

@VanKen what's your tail current set to in VictronConnect? That's usually where the rot starts.