Victron BMV-712 showing different SOC to my BMS — which one do I trust?

Running into exactly this on my boat at the moment — Victron BMV-712 sitting at 87% while the BMS on my Fogstar Drift 100Ah is reading 94%. Seven percent isn't nothing when you're anchored up and trying to decide whether to run the inverter.

From what I've gathered so far, the BMV calculates SOC by counting coulombs in and out (with efficiency losses baked in), so it's only as good as your initial setup — charge efficiency factor, Peukert exponent, that sort of thing. If those aren't dialled in properly for lithium, it'll drift over time.

The BMS, on the other hand, is looking at cell voltages directly, so in theory it should know what's actually there — but it can also be optimistic, especially if cells aren't perfectly balanced.

A few questions I'm still trying to work out:

• Does the BMV resync at 100% when you hit absorption? Mine might not be triggering the sync correctly
• Is the BMS SOC figure even reliable on budget units, or is it just a rough voltage-based estimate dressed up as a percentage?
• Has anyone found a definitive set of BMV settings specifically for LiFePO4?

On my static caravan setup I just trusted the BMV and never really questioned it, but on the boat where power management matters more I want to know which figure is actually worth following.

Would be good to hear from anyone who's gone through the calibration process properly — especially if you've got a similar Fogstar setup.

@Chopper42 Classic situation on liveaboards. Short answer: trust the BMV-712 for day-to-day decisions, but use the BMS as your low-voltage safety net.

The BMV counts coulombs in and out — it's genuinely measuring what's happened. But it drifts over time if your synchronisation point isn't set correctly. Check your charged voltage threshold and tail current settings in VictronConnect — if those are off, it'll never properly reset to 100% and the drift compounds.

The Fogstar BMS SOC is typically calculated from cell voltages, which is notoriously nonlinear mid-range on LiFePO4 — that flat voltage curve makes it genuinely difficult to read accurately between roughly 20–80%.

My money says your BMV hasn't synced to 100% recently. Get a full absorption cycle in, let the tail current drop properly, and watch whether they converge afterwards.

@Chopper42 had the same mismatch on my motorhome setup. Turned out my BMV-712 sync point was slightly off — hadn't done a proper full charge to let it reset. Once I ran the bank to 100% absorption and held it there, both readings came within a couple of percent of each other.

Worth checking your tail current setting on the BMV too. If it's set too high it'll call 100% too early and everything drifts from there.

The BMS figure is more of a ballpark — it's doing its own coulomb counting but without the calibration options the Victron has. I'd tweak the BMV settings first before assuming one is right and the other wrong.

@Chopper42 this exact thing had me second-guessing myself for a whole season on the boat. What finally cracked it for me was realising the two devices are measuring completely different things — the BMS is doing cell-level voltage translation into an SOC estimate, while the BMV is counting coulombs in and out. Neither is infallible.

The BMV drifts if your charged voltage threshold isn't set correctly — it needs to see a proper tail current before it'll sync to 100%. Check your charge detection settings in the Victron app. On mine I had the threshold set too high, so it was syncing prematurely and the drift compounded over days at anchor.

Seven percent at 90-ish% SOC is actually significant. Sort the sync point first, then reassess which you trust.

@Chopper42 worth checking your charged voltage threshold and tail current settings in the BMV-712 — these control when it resyncs to 100%. If they're set wrong it'll drift further and further out over time. Mine was doing something similar in the motorhome until I tightened the tail current down to about 2% of capacity.

Also the BMS coulomb counter on the Fogstar will have its own calibration quirks. Neither is gospel really — they're both estimating in slightly different ways.

Practically speaking: the BMV-712 is tracking actual current in/out, so over a decent period with a proper sync it's generally more reliable for real-world SOC. The BMS is more about protection than accuracy.

Seven percent sounds annoying but honestly once you've nailed the sync settings it'll come together.

Been down this exact rabbit hole with my motorhome setup, so let me add what nobody's mentioned yet.

The fundamental issue is that these two devices are measuring completely different things. Your BMV-712 is coulomb counting — it's integrating current flow over time. Your Fogstar's BMS is reading cell voltage and mapping that to a lookup table. Both methods have drift and both have blind spots.

Here's the uncomfortable truth: neither is gospel, but for LiFePO4 specifically, the BMS voltage-based reading is often less reliable mid-discharge because the voltage curve is so flat. A 7% discrepancy in that flat middle zone is actually pretty normal.

What I'd do — trust the BMV-712 for day-to-day usage, but force a proper sync point. Run the bank down to around 20%, then charge to full absorption and let it hit tail current. That'll reset the BMV properly.