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

I've been scratching my head over this for a few days now. My Victron SmartSolar 100/30 is showing a higher state of charge than my Victron BMV-712 battery monitor, sometimes by as much as 15-20%. Both are set up with the same battery capacity (200Ah lithium), and I've checked the wiring more than once. The solar controller seems overly optimistic about how full the batteries actually are.

From what I can gather, the MPPT estimates SoC based on voltage, which is notoriously unreliable with lithium — especially LiFePO4 because the discharge curve is so flat. The BMV on the other hand is coulomb counting, so it's actually tracking current in and out. I think the BMV is giving me the more accurate picture, but I'd love to hear from people who've been through this.

Has anyone found the sweet spot for calibrating both so they roughly agree, or do you just pick one to trust and ignore the other? I've tried syncing the BMV's charged voltage threshold and tail current settings but I'm not convinced I've got them right. Currently running 14.2V absorption and 13.6V float if that helps.

Hey @HarbourKate, this is really common and usually comes down to the MPPT estimating SoC purely from voltage, whilst the BMV-712 is doing proper coulomb counting - actually tracking charge in and out. The BMV will almost always be more accurate, especially once you've got it properly configured with the correct battery capacity, Peukert exponent and charge efficiency factor.

Worth double-checking your BMV settings match your actual battery bank. Also, has it had a proper full charge cycle since you set it up? The BMV needs to hit the tail current threshold to "synchronise" its 100% reference point. If it's never done that, the readings can drift quite badly over time.

Trust the BMV over the MPPT for SoC - the SmartSolar display is really just a rough guide. 🙂

RobHenderson | 847 posts

Worth adding to what @T5Solar said - check your BMV-712's Peukert exponent and charge efficiency factor settings. If these aren't dialled in properly for your specific battery chemistry, the coulomb counting can drift over time and give you misleading figures.

Also, when did you last have a proper full absorption/float cycle that allowed the BMV to resynchronise? Mine used to show a similar discrepancy until I realised my system was rarely reaching the synchronisation SoC threshold I'd set. Once it syncs properly at 100%, both readings should align much more closely.

What battery chemistry are you running, @HarbourKate? AGM, lithium, or something else? That'll help narrow down which settings might need tweaking.

Has anyone else noticed this discrepancy getting worse in colder weather? I'm curious whether temperature compensation is playing into it — my Fogstar Drift LiFePO4 cells behave very differently at 5°C versus 15°C, and I wonder if the MPPT's voltage-based estimate is even less reliable once ambient temps drop.

Also @HarbourKate — what battery chemistry are you running? The gap between voltage and actual SoC is notoriously wide with LiFePO4 across that mid-range plateau. A BMV-712 with a properly calibrated shunt should be far more trustworthy than any voltage-derived figure.

Have you set the correct battery capacity in the BMV, and are you confident your shunt is wired to capture all loads and charge sources? Missed loads are a common culprit for drift over time.

Good point from @WonkySparky on temperature — absolutely a factor. The BMV-712 has a temperature input if you're using the optional sensor, and if it's not configured or connected, it's calculating capacity based on an assumed 25°C. At 5-10°C (fairly typical in a UK van in winter) you can lose 15-20% usable capacity from LiFePO4 and more from AGM/gel. That alone explains a chunk of the discrepancy you're seeing @HarbourKate.

Worth checking in the BMV settings:

• Battery capacity — is it set correctly for your actual bank?
• Charged voltage threshold — needs to match your charger's absorption voltage
• Tail current — this triggers the "fully charged" sync point

The BMV learns over time once those are dialled in properly. Mine took a few full cycles before it settled down.

BurnJim | 312 posts

Something worth checking that nobody's mentioned yet — are your battery capacity settings actually correct in the BMV-712? It sounds obvious but I had mine set to the nominal 100Ah when my actual usable capacity was closer to 85Ah after accounting for aging. That alone was throwing my SOC readings off considerably.

Also, the MPPT calculates SOC purely from voltage, which is notoriously unreliable especially mid-charge. The BMV uses coulomb counting which is far more accurate long-term, so I'd generally trust the 712 over the SmartSolar reading. @RobHenderson's point about Peukert settings is spot on for making sure the coulomb counting stays accurate too.

Have you done a full absorption and float cycle recently to let the BMV resync itself? That usually irons out drift that's built up over time.

Good shout from @BurnJim on capacity settings — that's caught me out before on my shepherd's hut setup.

One thing nobody's touched on yet: Peukert's exponent. Your BMV-712 applies Peukert correction to account for the non-linear capacity loss under higher discharge rates, whereas the SmartSolar is simply calculating based on voltage and charge current flowing in. So if you're pulling heavy loads intermittently, the BMV's figure will be more pessimistic (and more accurate, frankly).

Also worth checking your tail current setting in the BMV. If it's set too high, the monitor thinks the battery is full before it actually is, which can skew subsequent readings considerably. Default is often 4% of capacity — I run mine at 2% on my 200Ah Fogstar lithiums and it's much tighter now.

The BMV is almost always the one to trust over the MPPT reading.

Ran into this exact mismatch last winter with the shepherd's hut. Turned out my synchronisation trigger was the culprit — the SmartSolar was calling "full" before the BMV had seen a proper tail current. Worth checking your charged voltage and tail current settings in the BMV are realistic for your battery chemistry. Fogstar Drift LiFePO4 here, and the default absorption voltage Victron ships with is often slightly optimistic. Once I tightened those parameters up, both units landed within 2-3% of each other consistently. @BurnJim's point on capacity is solid too — the two issues compound each other badly if both are slightly off simultaneously.