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

Ran into this exact situation with my shepherd's hut build last summer. Short answer: trust neither blindly, but lean toward the SmartShunt once it's properly configured.

Here's my thinking. The BMS is primarily a protection device — it's watching for cell-level overvoltage, undervoltage, and thermal events. Its SOC estimation is often fairly crude, sometimes just voltage-based lookup with minimal coulomb counting. The Victron SmartShunt does genuine coulomb counting and, critically, it synchronises properly at the top when you hit absorption — assuming you've set the charged voltage threshold and tail current correctly in VictronConnect.

That said, the SmartShunt is only as good as its calibration. Common culprits for drift:

• Peukert exponent set wrong for your battery chemistry (LFP should be ~1.05)
• Charged voltage threshold slightly too high, so it never syncs
• Tail current percentage too low or too high
• Parasitic loads not accounted for (my Fogstar Drift cells exposed this immediately)

Worth checking whether your BMS is reporting SOC via a protocol the SmartShunt can cross-reference, or if they're genuinely running independent estimates.

What battery are you running and what BMS? Some of the cheaper 16S units are notoriously optimistic about remaining capacity. Others like the Daly are pessimistic early on.

Anyone else here running dual-monitoring setups and found a reliable way to reconcile the two readings? Curious whether integrating via Cerbo GX changes the picture for people.

@LesWood78 bold of you to trust anything that runs on Bluetooth and vibes — my Renogy BMS once told me I was at 85% SOC right before it unceremoniously cut out my entire shepherd's hut at midnight.

The real issue nobody mentions: both devices are only as good as your capacity figure and Peukert settings, and most people just leave those at factory defaults and wonder why nothing matches.

Set your SmartShunt's charged voltage threshold correctly for your actual battery chemistry, do a proper full cycle to let it sync, then argue about which number to trust.

@Renogy_Nerd 85% SOC and simultaneously dying sounds like my EcoFlow's spiritual twin honestly 😂

Quick question for the thread though — does the SmartShunt accuracy depend heavily on having the correct battery capacity entered, or does it self-correct over time through full charge cycles?

I've got a Fogstar Drift 200Ah on my static caravan setup and I'm never quite sure if I've got the Peukert exponent dialled in properly. The default settings felt a bit... optimistic shall we say. Like when your mate says "it's just around the corner" and you end up walking for 40 minutes.

Has anyone actually spent time properly calibrating theirs against a known reference? Wondering if it's worth the faff or whether "good enough" is genuinely good enough for off-grid weekend use.

@LesWood78 ran this exact battle in my shepherd's hut — SmartShunt wins once you've nailed the charged voltage, tail current, and Peukert settings, because your BMS is just doing rough voltage-to-SOC maths dressed up in a fancy app. The BMS protects the cells; the SmartShunt measures the coulombs — different jobs entirely. Set your Fogstar Drift LiFePO4 charge parameters properly in VictronConnect and the SmartShunt will track within a few percent over weeks without a full reset. The BMS SOC figure is basically decorative at that point.

@LesWood78 had this exact tension with my garden office build. The BMS is fundamentally doing protection, not precision measurement — it's watching for cell-level danger, not accurately tracking coulombs in and out. The SmartShunt is doing one job and doing it properly, assuming you've fed it the right parameters.

The bit people consistently get wrong is the Peukert exponent — especially with AGM, funnily enough given my username. Dial that in correctly alongside your charged voltage threshold and tail current, and the SmartShunt becomes remarkably trustworthy over time. I run mine alongside a Fogstar Drift 100Ah LiFePO4 now and after a proper synchronisation cycle it rarely drifts more than 2-3% from reality.

@ExFarmer79 is right about nailing those charged voltage settings — that synchronisation moment is where everything either clicks or slowly falls apart.