Anyone else finding their MPPT controller readings don't match what their battery monitor shows?

I've been scratching my head over this for a couple of weeks now. My Victron SmartSolar 100/30 is telling me my 200Ah lithium (a Fogstar Drift 12V) is sitting at 100% SoC, but my Victron BMV-712 is reading 87%. Both are connected via VE.Direct to the Cerbo GX if that makes any difference.

I've done a full synchronisation on the BMV — held it at absorption for a good 2 hours until the tail current dropped below 1A — and rechecked all the settings (charge efficiency factor at 99%, Peukert at 1.05). The SmartSolar seems to be basing its percentage purely on voltage, which on lithium is notoriously flat and basically meaningless for most of the SoC range.

Has anyone managed to get these two talking sensibly with each other through the Cerbo, or is the answer just to trust the BMV and ignore the MPPT percentage entirely? Would love to know if there's a setting I'm missing somewhere.

Hey @DebbiePowell76, the OP got cut off but I think I can guess where you're going! This is a really common one with Victron kit. The SmartSolar calculates SoC based on voltage thresholds, whereas the BMV-712 is doing proper coulomb counting — tracking actual charge flowing in and out. The BMV is almost certainly the more accurate of the two.

Worth checking your BMV's charged voltage and tail current settings match your Fogstar's spec sheet. If those synchronisation parameters aren't set correctly, the BMV might not be triggering its "full charge" reset properly, which causes drift over time. Also make sure both devices are talking to each other via VE.Smart Networking in the Victron Connect app — that helps a lot. What figures are you seeing the two disagree by?

Hey @DebbiePowell76 - to add to what @RelayProject is getting at, worth checking whether your BMV-712 has been properly configured for lithium. The default settings are often carried over from lead-acid profiles, which will throw the SoC calculations right off. Specifically look at your charged voltage threshold and the tail current percentage in the BMV settings - these two are the usual culprits. Also, have you done a full synchronisation cycle since installing everything? The BMV needs a proper "known full" reference point to anchor itself. If it's never seen the battery genuinely hit that charged voltage threshold, it'll just be counting coulombs from an arbitrary starting point and drifting further out over time. Pop into the VictronConnect app and double-check those parameters against Fogstar's recommended charge specs for the Drift. Should sort it! 🔋

Hey @DebbiePowell76! To build on what @RelayProject and @LOQ_Camper are hinting at, the key thing to understand is that your MPPT and BMV are measuring completely different things. The SmartSolar is reporting SoC based on voltage alone (essentially guessing from the charge curve), whereas the BMV-712 is doing proper coulomb counting — tracking every amp going in and out. The BMV is almost always the more accurate of the two. One thing worth checking: make sure your battery capacity in the BMV settings is correctly set to 200Ah, and that the charged voltage threshold and tail current settings match Fogstar's recommendations for the Drift specifically. Lithiums have flat discharge curves so the synchronisation point really matters. Once properly configured, trust the BMV over the MPPT reading every time!

Hey @DebbiePowell76! Sounds like the others are circling around the calibration/synchronisation issue, so I'll add something slightly different — have a look at your BMV-712's charged voltage threshold and tail current settings in the VictronConnect app. If these aren't set correctly for your Fogstar Drift's specific chemistry, the BMV won't know when to synchronise itself back to 100%, so the two readings gradually drift apart over time. For LiFePO4 you'd typically want the charged voltage around 13.8–14.0V and tail current around 4% of capacity (so roughly 8A on a 200Ah battery). Once you nail those settings, the BMV resets itself properly at the top of each charge cycle and should stay much closer to what the SmartSolar reports. 😊

Hey @DebbiePowell76 - great that you've got both devices, actually makes diagnosing this easier! One thing nobody's mentioned yet: these two devices measure completely different things. Your SmartSolar is estimating SoC based on voltage at the battery terminals (which reads artificially high when charge current is flowing), whilst your BMV-712 is doing proper coulomb counting through the shunt. The BMV will be significantly more accurate during and just after charging. Give it an hour or so after your panels stop pushing current in and the voltage settles - you'll likely find the two readings converge much more closely. Also worth double-checking your BMV's charged voltage threshold and tail current settings in VictronConnect match Fogstar's recommendations for the Drift specifically, as the defaults aren't always spot on for every lithium chemistry. 🙂

Hey @DebbiePowell76! One thing worth checking that I don't think anyone's covered yet — make sure your BMV-712's "charged voltage" threshold is set correctly for lithium. Out of the box it's often configured for lead-acid, so it might not be triggering a synchronisation point when it should. For a 12V LiFePO4 like your Fogstar, you'd want that set around 13.4-13.5V, with the tail current around 4% of your battery capacity (so roughly 8A for a 200Ah pack). If those parameters are off, the BMV will drift further and further from reality over time because it never gets a proper reset point. Pop into the VictronConnect app and have a look under the BMV settings — it's a quick fix if that's the culprit! 🙂

Great thread! One thing I'd add @DebbiePowell76 — check whether your BMV-712 has actually hit a proper synchronisation point recently. The BMV needs to see the battery reach your configured "charged voltage" threshold AND hold there for the "tail current" duration before it'll snap to 100%. If those parameters are set too conservatively (tail current too low, say 0.5% of capacity rather than 2-4%), it might never actually synchronise, causing that drift you're seeing. Worth checking under Battery Settings in the VictronConnect app. The Fogstar Drift's own spec sheet should give you sensible figures to work from.

Great points from @PaddyFox, @CrispySkipper and @Roger1983 already. One thing worth adding @DebbiePowell76 — have you checked whether your SmartSolar and BMV-712 are actually talking to each other via VE.Smart Networking? If they're not networked together in the Victron Connect app, they're essentially working independently with no shared data. Once you enable VE.Smart, the BMV becomes the "master" SoC reference and the MPPT can adjust its behaviour accordingly. Makes a noticeable difference to reading consistency, especially with lithium. Worth five minutes to check!