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

Had this exact headache last winter up at the cabin. Both my SmartShunt and the BMS on my Fogstar Drift 200Ah were giving me completely different readings — sometimes 8-10% apart at the worst of it.

Short answer: neither is perfectly right, but they're measuring different things.

The SmartShunt is doing coulomb counting — tracking electrons in and out. It's brilliant when calibrated properly, but it drifts over time if you're not hitting 100% regularly to let it resync. Mine was way off after a few weeks of partial cycling through a grey Scottish winter, barely seeing full sun.

The BMS is working from cell voltages, which isn't always a reliable SOC indicator either — especially mid-range on LiFePO4 where the voltage curve is almost flat for ages. It can look like 65% for hours.

What I ended up doing:

• Set the SmartShunt's charged voltage and tail current settings properly (Victron's defaults needed tweaking for my specific cells)
• Let the bank hit genuine 100% every week or so to resync
• Used the BMS reading mainly as a sanity check for cell balance rather than SOC

The SmartShunt is now my primary SOC reference and it's rock solid. The BMS is there to protect the cells, not to be my fuel gauge.

Curious whether anyone's had better results trusting their BMS over the SmartShunt long-term? Feels like the right answer probably depends on which BMS you're running.

@MountainBarry classic calibration drift scenario — seen this plenty on my own Vivaro setup running a Fogstar Drift 200Ah (same battery, interestingly).

The short answer: trust the SmartShunt for day-to-day SOC, but use the BMS as your hard floor protection.

The SmartShunt measures coulombs in/out with proper temperature compensation, whereas most budget BMS units use voltage-based estimation which gets wobbly under load. That 8-10% discrepancy usually comes down to:

• Tail current setting on the SmartShunt — if it's too high, it never hits the "full" sync point
• Peukert exponent needing adjustment for LiFePO4 (should be ~1.05)
• BMS voltage thresholds not accounting for surface charge

Set your SmartShunt tail current to around 1-2% of capacity and let it do a proper full-charge sync cycle. The gap should close dramatically.

@MountainBarry the root cause is almost always one of two things: either your SmartShunt hasn't been allowed to reach a proper 100% sync point (it needs a genuine full charge with tail current dropping below your configured threshold), or your battery capacity figure in the SmartShunt settings doesn't account for Peukert losses at your typical discharge rate.

On my motorhome setup with a 280Ah Fogstar Drift, I found the BMS SOC was actually the less reliable of the two long-term — it resets aggressively on voltage alone, which flatters the reading after a partial charge.

The SmartShunt, once properly calibrated and given a genuine full charge cycle to resync, is generally more trustworthy for day-to-day tracking. Check your charged voltage threshold and tail current settings in the Victron Connect app — those two parameters alone sorted my 12% discrepancy overnight.

@MountainBarry had this exact situation in my motorhome last February — three days of solid rain, panels doing nothing, and suddenly my SmartShunt was reading 47% while the Fogstar's BMS was screaming blue murder at 31%.

Turns out the SmartShunt had never properly synced to 100% because I'd been topping up in stages rather than letting it hit a proper full charge and hold there. The BMS was closer to reality that particular week.

Here's the blunt answer though: neither is gospel on its own.

What I do now is treat the BMS as my safety net — it'll protect the cells regardless — but trust the SmartShunt for day-to-day SOC once it's been properly calibrated with a genuine full charge cycle. Get that sync right first before chasing the discrepancy.

@MountainBarry worth checking whether your SmartShunt's charged voltage threshold is actually set correctly for LiFePO4. Out of the box it often defaults to settings more suited to AGM, and if it never registers a proper "100% sync point" it'll drift further and further from reality over repeated cycles.

On my boat I run a Victron SmartShunt alongside a Daly BMS on a 280Ah bank — I set the charged voltage to around 14.2V (just below my absorption ceiling) and the tail current to about 4% of capacity. Once those two conditions are met simultaneously, the SmartShunt resets to 100% and everything stays remarkably consistent.

The BMS coulomb counter is generally less reliable long-term because it doesn't self-correct — the SmartShunt wins on accuracy provided it's calibrated properly and gets a full charge regularly.