SOG shunt vs batteries bms

Been there with my Fogstar Drift cells and a Victron SmartShunt — the BMS was cheerfully reporting 98% whilst the shunt was essentially screaming "mate, you've got about enough juice left to boil a kettle... once."

The root issue is that most BMS percentage readouts are voltage-based estimates, whereas a proper Coulomb-counting shunt (like the SmartShunt or a BMV-712) is actually tracking electrons in and out. When the two disagree wildly, trust the shunt — the BMS is basically just eyeballing it.

Classic scenario where this bites you:

• BMS says 100% ✅
• Shunt says 31% 😬
• Multiplus wakes up on shore power and absolutely launches 1,400W at your bank trying to top up what it correctly knows is missing
• You panic, post on forums at midnight (we've all been there)

The fix that sorted mine was properly synchronising the shunt — make sure it's set to register a full charge only when the battery hits absorption voltage and tail current drops appropriately. Otherwise it just... drifts. Hence the name, presumably.

Also worth checking your charge efficiency factor in VRM if you're Victron-based — lithium should be around 99%, not the 95% default which is more AGM territory.

Anyone else running a mix of BMS Bluetooth readouts alongside VRM and finding them permanently at odds? Curious whether this is a Victron/Fogstar quirk or if folks with Pylontech stacks or Liontron batteries see the same thing — particularly those of you with garden offices drawing overnight loads where the drift

@QuietHiker oh this is such a classic "who do you trust?" scenario and honestly the answer is neither of them fully.

The BMS SoC is essentially vibes-based after a few charge cycles — it drifts like mad without a proper full-cycle calibration reset. Meanwhile the SmartShunt is doing proper coulomb counting but its accuracy lives or dies by how precisely you've set the Peukert exponent and charge efficiency factor in VictronConnect.

My setup: Fogstar Drift 280Ah bank, SmartShunt, and I've found the shunt wins every time once you've let it synchronise through a proper full absorption cycle. Key thing — make sure your SmartShunt knows your actual charged voltage, not just whatever the BMS is reporting upstream.

The BMS percentage display is basically decorative at that point. Pretty number. Meaningless number. 😄

@QuietHiker the shunt's only as honest as its last proper calibration cycle, and the BMS is basically a toddler counting on its fingers — both are just confidently wrong in different directions.

@QuietHiker the shunt wins for coulomb counting accuracy over time, but only if you've actually let it hit a proper full charge to reset. If you're doing partial cycles on the boat like I do, drift accumulates and suddenly both readings are fiction.

The BMS SOC is essentially just a voltage lookup with extra steps — fine for a rough sanity check, useless for anything precise.

What I'd actually trust: neither in isolation. Cross-reference both and watch your resting voltage after a few hours off load. That'll tell you more than either percentage figure.

Also worth checking your shunt's charged voltage threshold is set correctly in the Victron app — factory default is often wrong for LiFePO4 and throws the whole calibration off from the start.

@QuietHiker the bit nobody mentions is that these two are actually measuring different things — the BMS is making educated guesses based on cell voltage curves, which get wildly inaccurate in the middle SOC range where LiFePO4 is essentially flat. The SmartShunt meanwhile is just doing maths on electrons in and out, which sounds more reliable until you've had a dodgy connection skewing its current reading for a week without noticing.

What I'd actually do: use the shunt as your primary day-to-day reference but let the BMS low-voltage cutoff act as your safety net rather than your fuel gauge. And @HollyBaker is clearly heading somewhere useful about full charge resets — that's genuinely the key to keeping the shunt honest long-term. Without regular top-balance reference points, the drift accumulates and suddenly you're both lost.