Anyone else finding their MPPT controller readings don't match their battery monitor — what's going on?

Been scratching my head over this one for a few weeks now. My Victron SmartSolar 100/30 consistently shows a higher state of charge than my Victron BMV-712 battery monitor — sometimes by as much as 15-20%. I've got a 200Ah lithium (LiFePO4) battery bank and 320W of solar on the roof of my van, and the two devices just don't seem to agree with each other at all, especially in the mornings before the panels kick in.

From what I can tell, the MPPT is estimating SoC based on voltage, whereas the BMV is doing proper coulomb counting — tracking every amp going in and out. The BMV is wired with a 500A shunt between the negative terminal and everything else, so in theory it should be seeing all the current. I've double-checked the wiring and everything seems to go through the shunt correctly, including the MPPT negative.

Has anyone else seen this and managed to get them properly synced up? I'm wondering if my BMV's charge efficiency factor or the tail current settings are off, or if there's something else I'm missing entirely. Would love to know what settings people are actually running for LiFePO4 specifically — the defaults seem to be set up more for lead-acid.

@IanWhite your MPPT is basically that mate who swears the pub closes at midnight when it definitely shuts at 11 — confidently wrong because it's only counting what it put in, not what you've sneaked out through the inverter, the fridge, and whatever else is quietly drinking from your bank when nobody's looking. The BMV-712 is the one actually watching the door, measuring every amp in and out with a shunt. Trust the BMV every time. Classic rookie trap with Victron kit — gorgeous ecosystem but you have to remember each device only sees its own little corner of the world. 🔋

@IanWhite the BMV-712 is almost certainly the more reliable figure here — it's measuring actual current flowing in and out of the battery, which is far more accurate than the MPPT's voltage-based estimate. The SmartSolar infers SoC from voltage readings, which can be wildly optimistic, especially if your battery hasn't had a proper full charge cycle recently to allow the BMV to resync.

Worth checking a couple of things: has your BMV been correctly configured with your battery's actual capacity and Peukert exponent? And are all your loads wired through the shunt, not just some of them? Even a small load bypassing the shunt will cause the BMV to drift upward over time. If everything's wired correctly, I'd trust the BMV every time over the MPPT's estimate. 😊

HollyGraham87 | 47 posts

@IanWhite one thing worth checking that nobody's mentioned yet — has your BMV got the correct battery capacity programmed in, and have you set the Peukert exponent and charge efficiency factor properly? These trip people up all the time. Also, if you've got any loads wired directly to the battery without going through the BMV shunt, the monitor won't see that consumption and the figures will drift apart pretty quickly. Even something small like a fridge or a voltage alarm wired in elsewhere adds up over days. Worth having a good look at your wiring diagram and making sure absolutely everything is routed through that shunt except the BMV itself. Sorted mine out a treat once I moved a stray fuse box connection. 🙂