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

My Fogstar 200Ah laughs at both of them simultaneously, so I've learned to split the difference and just assume I'm about to run out of power at all times — works a treat for my anxiety levels.

Genuinely though, the SmartShunt is only as good as its calibration; if you've not told it the correct capacity and Peukert exponent, it's just making educated guesses like the rest of us. The BMS meanwhile will cut you off without so much as a polite warning, so I'd say trust the BMS for protection but trust the SmartShunt for accuracy — assuming you've set it up properly.

Key things to check on the Victron side:

• Charged voltage threshold — is it actually seeing a full charge to reset its 100%?
• Tail current — set too high and it calls "full" far too early
• Battery capacity — obvious, but easily wrong if you've upgraded cells

The two will drift apart over time regardless. Mine used to agree beautifully, now they're 12% apart after six months of static caravan abuse. Victron Connect lets you sync the SmartShunt back to 100% when you know you're genuinely full — do that regularly and you'll keep them reasonably friendly with each other.

What BMS are you running? Some of the cheaper ones report SOC via a method that's essentially witchcraft, which doesn't help matters.

Anyone else here found one consistently more reliable than the other? Curious whether it's just my setup or a universal headache.

@RetiredNurse61 😂 honestly that anxiety-driven approach probably keeps more batteries healthy than any fancy monitoring setup

For what it's worth — I trust my Victron SmartShunt over the BMS SOC display for day-to-day use. The BMS is primarily there for protection, not precision metering. Its SOC is often just a rough voltage-based estimate.

The SmartShunt does proper coulomb counting, so it's more accurate if you've got your settings right:

• Correct Peukert exponent
• Charge efficiency factor dialled in (LiFePO4 ≈ 99%)
• Syncing properly at 100% charged

Main thing — make sure your SmartShunt actually sees a genuine full charge periodically so it can re-sync. That's usually where drift creeps in.

@RetiredNurse61 the "assume you're broke" method is genuinely underrated haha

To actually answer the thread question though — SmartShunt wins for me in normal operation, but only if you've got the capacity and Peukert settings dialled in properly. BMS SOC tends to drift because most of them recalibrate on full charge and then just... wander.

The real issue is they're measuring different things. SmartShunt is counting coulombs in/out. BMS is usually voltage-based with some integration on top. Neither is gospel.

What I do: let the SmartShunt be your daily driver, trust the BMS for low-voltage cutoff protection, and make sure the shunt gets a proper sync at 100% SOC regularly. My Fogstar 280Ah setup settled down a lot once I set absorption properly in Victron so it actually hits full charge rather than just nearly full.

@RetiredNurse61 your anxiety method is genuinely solid engineering philosophy, but let me actually answer the question properly.

Trust the SmartShunt long-term, but respect the BMS short-term.

Here's why: your BMS reads cell voltages and makes instantaneous decisions — it has to, because it's protecting the cells right now. The SmartShunt does coulomb counting, which is far more accurate over a charge cycle but drifts if it never sees a proper full charge to reset against.

The classic mismatch scenario: SmartShunt thinks you're at 75% because it's been weeks since a genuine top-balance, but your BMS sees a weak cell voltage and disagrees.

Fix — make sure your Victron absorption stage actually completes fully. Check your charge parameters in VictronConnect. Once the SmartShunt syncs at 100% regularly, the two readings converge beautifully. Mine did anyway, running a 280Ah Fogstar setup in the motorhome.

@RetiredNurse61 😄 brilliant system, genuinely.

To actually add something useful here — the SmartShunt and BMS are measuring fundamentally different things. The SmartShunt tracks coulomb counting from a known reference point, so it drifts over time if it hasn't had a proper full charge cycle to reset itself. The BMS is usually going off cell voltages, which are notoriously unreliable mid-discharge on LiFePO4 given that flat voltage curve.

In practice I'd trust the SmartShunt more if it's regularly seeing full charges and you've got your charged voltage threshold set correctly in the Victron app. If your system rarely hits 100%, that's probably where the discrepancy creeps in.

Worth checking what voltage you've set as your "charged" trigger — a lot of people have it slightly off and wonder why the SOC never syncs properly.