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

Been through exactly this with my cabin setup last winter. The short answer: trust neither blindly, but lean on your BMS for protection and your SmartShunt for trend-watching.

Here's what was happening in my case — the SmartShunt had drifted because I'd never given it a proper full charge to let it re-sync. Victron's coulomb counting is brilliant but it needs that 100% anchor point regularly, otherwise errors compound over weeks. Once I sorted my absorption/float settings in VictronConnect, the two figures came within 3-4% of each other pretty quickly.

The BMS (I'm running a Fogstar Drift with built-in BMS) reads cell voltages directly, so it's got a more honest picture of what's actually happening at the chemistry level. But it can also be pessimistic — it'll scream at 20% SOC based on one low cell even if the pack average is fine.

My workflow now:

• SmartShunt tells me the story over time — consumption trends, overnight draw, whether my solar is keeping up
• BMS is the hard floor — if it says stop, I stop, no argument

One thing worth checking: have you calibrated your battery voltage curve in the SmartShunt settings? Getting the charged/discharged voltage thresholds right for your specific cell chemistry made a noticeable difference for me.

What BMS are you running? Makes a big difference to how chatty it is with accurate data. Some of the budget units are genuinely optimistic with their figures.

@Brummie interesting you say lean on the SmartShunt for trends — I've been going back and forth on this with my static caravan setup using Fogstar Drift cells.

One thing I haven't seen mentioned: does the chemistry of your cells affect which to trust more? My understanding is that LFP's flat discharge curve makes voltage-based SOC (which the BMS often relies on) pretty unreliable mid-range anyway. So in theory the SmartShunt's coulomb counting should win there — but only if you've got your charged voltage and tail current configured correctly in VictronConnect.

Has anyone actually done a proper calibration run to verify which is closer to reality? Curious whether fully cycling the pack and letting both reset would bring them back into agreement, or whether the drift just keeps compounding over time.

@Brummie @TelHall — worth adding that the SmartShunt's accuracy lives or dies on your charged detection settings. If the synchronisation voltage is even slightly off, coulomb counting drift compounds over weeks and you end up with readings that are essentially fiction.

On my narrowboat I had a persistent 8-12% discrepancy between the Fogstar Drift 100Ah's internal BMS and my SmartShunt for ages. Turned out my charged voltage threshold was set 0.2V too low, so it was never properly resetting the 100% reference point.

Key parameters to check:

• Charged voltage (typically ~0.2V below absorption setpoint)
• Tail current percentage
• Charged detection time

Once those are dialled in properly the two readings converge quite nicely in my experience. The BMS protection thresholds still take absolute priority obviously — the SmartShunt has no relay outputs, it's purely informational.

@PanelEwan nailed it on charged detection — I'd add that peukert exponent is another one people overlook in VictronConnect. Get that wrong and your SmartShunt drifts badly under heavy discharge.

My garden office setup runs a Fogstar Drift 100Ah with its own BMS, and I treat the two readings like this:

• BMS = hard limits, I don't override it
• SmartShunt = day-to-day monitoring, capacity trending over time

The SmartShunt's historical data is genuinely useful — if your charged capacity is slowly dropping month on month, that's your first warning the cells are degrading. Your BMS won't tell you that.

One practical tip: after a proper full charge, manually hit "sync to 100%" on the SmartShunt if they're still diverging. Sometimes that's all it takes.

@IslandExplorer good shout on Peukert — something I had to tweak on my shepherd's hut setup after noticing the SmartShunt consistently underreporting capacity during high-draw mornings (kettle + inverter startup).

One thing nobody's mentioned: synchronisation frequency. If your BMS isn't regularly triggering a full-charge sync event on the SmartShunt, coulomb-counting drift compounds over weeks. With my Fogstar Drift LiFePO4, the BMS hits 100% quite sharply at absorption end, which should sync the shunt — but only if your tail current threshold in VictronConnect matches actual battery behaviour. Mine needed dropping to 1% of capacity rather than the default 4% before syncs happened reliably.

Run both for a month, log the divergence pattern. If SmartShunt is drifting low progressively, it's a sync problem. If it's erratic, suspect a poor shunt connection or wiring issue upstream.

@HighlandExplorer @IslandExplorer — both solid points, and I'd add one more that bit me hard on the boat last autumn: tail current setting.

If your tail current threshold is set too high, the SmartShunt declares "fully charged" prematurely, resets to 100%, and from that point your SOC figures are fiction. On LiFePO4 particularly, the absorption tail can be quite small — I was running 4% tail current which is more appropriate for AGM. Dropped it to 1% on my 280Ah Fogstar cells and suddenly the SmartShunt and BMS were reading within 2-3% of each other consistently.

The three settings that genuinely matter in combination:

• Charged voltage threshold
• Tail current %
• Charged detection time

Get all three dialled in for your specific battery chemistry rather than leaving Victron defaults, and the SmartShunt becomes a genuinely reliable instrument rather than an expensive guess.