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

Running into exactly this on my motorhome build at the moment. My Fogstar Drift 100Ah has its own BMS reporting 78% whilst the SmartShunt is sat there saying 91%. That's a fair gap and it's been bugging me for weeks.

From what I've gathered so far, the SmartShunt is doing coulomb counting — tracking what goes in and out — so it's only as accurate as its last sync point. If it hasn't hit a proper full charge recently to reset itself, drift creeps in. The BMS on the other hand is working off cell voltages, which has its own quirks depending on temperature and load.

My suspicion is the SmartShunt hasn't had a clean 100% sync in a while because my solar isn't quite keeping up during these grey winter months. I've started manually telling it to sync when I know the battery is genuinely full after a shore power session.

Few things I'm still unsure about though:

• Which should I actually trust for day-to-day decisions about loads?
• Is there a correct way to set the SmartShunt's charged voltage and tail current so it syncs properly with a LiFePO4 profile?
• Does anyone run both readings side by side on a Victron display and just accept the discrepancy?

Would be interested to hear from anyone running a similar setup — particularly if you've dialled in the SmartShunt parameters specifically for Fogstar or similar lithium cells. Feel like I'm missing something obvious in the configuration.

@OldSailor75 classic headache this one. Short answer: trust the SmartShunt for day-to-day SOC, but respect the BMS for protection limits.

The Fogstar BMS is almost certainly reporting voltage-based SOC, which is notoriously inaccurate mid-charge — LiFePO4 has such a flat voltage curve that voltage-to-SOC conversion is basically guesswork between about 20-80%.

The SmartShunt does coulomb counting, so it's actually measuring electrons in and out. Far more accurate provided you've set it up correctly — correct battery capacity, Peukert exponent, and crucially nailed the charged voltage/tail current thresholds so it synchronises properly at full charge.

Check your synchronisation settings in the VictronConnect app. If it's never hitting the sync point, errors accumulate. Mine drifted badly on my narrowboat until I tightened the tail current setting from 4% down to 2%.

@OldSailor75 the root cause is almost certainly a synchronisation drift issue. The SmartShunt calculates SOC via coulomb counting from its last "100% full" sync point — if it hasn't seen a proper absorption/float tail current recently, it'll read optimistically high.

Check your SmartShunt settings in the VictorConnect app:

• Charged voltage — should be around 27.2V for a 24V Fogstar Drift, or 13.6V for 12V
• Tail current — typically 4% of capacity (4A for 100Ah)
• Charge detection time — 3 minutes works well

The BMS SOC is often calculated differently, frequently using voltage lookup tables, which has its own inaccuracies especially mid-range.

Run both batteries to a low state, do a proper full charge cycle to float, and let the SmartShunt re-sync. They should converge considerably after that. Done this exact process on my own setup and closed a similar gap down to under 3%.

@OldSailor75 worth checking your SmartShunt's charged voltage threshold and tail current settings in VictronConnect. If these aren't dialled in correctly for your Fogstar cells, the shunt won't know when to reset to 100% and the drift compounds over time.

On my shepherd's hut setup I had almost exactly this gap until I set the charged voltage to 14.2V and tail current to 4% — after a proper full charge cycle it snapped back into sync with the BMS overnight.

Also make sure the shunt is measuring everything — if any loads or charge sources bypass it the coulomb counting goes out the window immediately. Common one people miss is a direct solar controller connection that doesn't route through the shunt.

Had this exact mismatch with my garden office setup last winter. What sorted it for me was doing a proper full charge cycle to let the SmartShunt resync — it resets its coulomb counting at 100% when it detects the charged voltage threshold being hit.

Worth noting: Fogstar Drift BMS tends to be quite conservative with its SOC readings. It's deliberately cautious to protect cell longevity, so it'll often read lower than reality.

@GazPrice is right about the settings — but also check your Peukert exponent in VictronConnect. Default value sometimes doesn't suit LiFePO4 chemistry properly, which throws the maths off over time.

The 13% gap you're seeing is bigger than normal drift though. I'd suspect the SmartShunt hasn't had a clean full-charge sync recently. Do a complete charge to absorption, let it sit, see if they converge.

@OldSailor75 one thing nobody's mentioned — check your peukert exponent and charge efficiency factor in the SmartShunt settings. Out of the box Victron uses fairly generic defaults that don't suit LiFePO4 well at all.

For lithium I run:

• Peukert: 1.05
• Charge efficiency: 99%

Default settings assume lead-acid behaviour which throws coulomb counting off progressively over time.

Also worth noting — the Fogstar Drift BMS is reading cell-level data directly, so it's arguably got better ground truth on actual capacity. The SmartShunt is estimating from current flow. After correcting my settings on the boat build the two stayed within 2-3% of each other consistently.