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

I've been scratching my head over this for a couple of weeks now. My Victron SmartSolar 100/30 is showing charge figures that are consistently about 8-10% higher than what my Victron BMV-712 is recording going into the same 200Ah lithium battery. Both devices are on the same VE.Smart network so they're talking to each other, but the numbers just won't align.

From what I can gather, the MPPT measures what's coming out of the controller, whereas the BMV shunt sits on the negative of the battery and catches everything actually entering (and leaving) it. So there's presumably some loss in the wiring between the two — I've got about 1.2 metres of 16mm² cable running between them. At 10A charge that 8-10% gap feels way too big to just be cable resistance though, doesn't it?

Has anyone else seen this kind of discrepancy with a similar setup? I'm wondering whether my shunt is positioned correctly or if there's a parasitic load somewhere sneaking in between the controller and the battery terminal that's skewing things. Would love to know if this is just normal behaviour or if I should be digging deeper.

LochFinn | 847 posts | ⚡ Solar Enthusiast

@Finn1975 Classic scenario this one! The discrepancy you're seeing is almost certainly down to wiring losses between your controller and battery, plus the BMV measuring actual charge acceptance rather than what the MPPT thinks it's delivering.

Worth checking a few things: how long are your cables between the MPPT and battery, and what gauge are you running? Even modest resistance adds up, especially at higher currents.

Also, are your battery parameters configured identically on both devices? If the BMV has slightly different absorption/float voltages programmed, the state-of-charge calculation drifts over time.

The 8-10% figure sounds about right for a less-than-ideal cable run. Trust the BMV — it's measuring directly at the battery terminals, so it's giving you the more accurate picture of what's actually going in.

@Finn1975 is the 8-10% gap consistent regardless of charge level, or does it drift more at certain points?

Wondering if it's a wiring issue — specifically whether your BMV shunt is positioned correctly to capture all loads and charge sources. If anything bypasses the shunt, the battery monitor will never have accurate figures.

Also worth checking — what cable runs between your MPPT and the shunt? I had a similar head-scratcher on my static caravan setup and it turned out to be resistance losses across a dodgy crimp causing the discrepancy. The MPPT measures at its output terminals, the BMV measures at the shunt — any resistance between those two points will show up as a difference.

Have you synced both units via VictronConnect to make sure neither needs a firmware update?

ThistlePaul | 312 posts | 🔋 Off-Grid Convert

@Finn1975 Worth checking your BMV-712's shunt wiring too - specifically whether everything on the negative side is routed through that shunt. If even one load or charge source bypasses it, the BMV never sees those amps and will consistently read lower than reality. Had exactly this catch me out when I added a secondary charging feed. Also, what have you got your Peukert exponent set to on the BMV? An incorrect value there can compound the discrepancy over time, particularly if you're regularly doing partial cycles rather than full charge/discharge. The BMV's SOC calculation drifts if it's not getting regular synchronisation points either - check your "charged voltage" threshold is actually being reached regularly enough to trigger a sync.

GazBrown72 | 312 posts | 🔋 Off-Grid Convert

@Finn1975 Had almost identical with my setup last year. One thing worth checking that nobody's mentioned yet - have you calibrated your BMV-712 with the correct battery capacity and Peukert exponent? Out of the box the defaults are fairly generic and can throw readings off noticeably, especially if you're running lithium rather than lead-acid. Also double-check your shunt is wired so that absolutely everything drawing current passes through it - even a sneaky trickle to a fridge or alarm that bypasses it will cause the two devices to gradually disagree. The MPPT only knows what it's putting in, the BMV tracks net flow, so any leakage point outside the shunt essentially becomes invisible to it.

Jock | 847 posts | ⚡ Battery Obsessive

@Finn1975 Something nobody's mentioned yet — check your Peukert exponent setting on the BMV-712. Out of the box it's often set for lead-acid, and if you're running lithium (Fogstar Drift or similar) it'll skew the BMV's calculations noticeably over time.

Also worth looking at your charged voltage and tail current thresholds in the BMV settings. If those aren't calibrated right, the monitor never properly resets its 100% reference point, so the gap compounds with every partial cycle.

I chased almost exactly this discrepancy across two winters before realising my BMV had never had a clean synchronisation since I'd switched battery chemistry. Five minutes in the VictronConnect app sorted it completely.

The MPPT figures are generally the more accurate of the two in my experience — it's measuring real current, not inferring it.

CliffGazer | 203 posts | 🚤 Boat/Backup Hybrid

@Finn1975 One thing worth flagging — the MPPT measures input to the battery, the BMV measures net flow through the shunt. If anything bypasses that shunt (even briefly, even a small load wired incorrectly on the battery side of the shunt rather than the load side) your BMV will under-read consistently.

On my boat setup I had exactly this issue for months before I found a bilge pump wired directly to the positive terminal post-shunt. Drove me absolutely mad.

Also — are your VE.Direct cables syncing both devices in the app? Sometimes the SmartSolar and BMV-712 need to be properly networked via VE.Smart Networking before the synchronised readings make any sense. Worth double-checking that's actually active rather than just... appearing to be.

NotAnElectrician | 47 posts | 🔌 Still Learning

@Finn1975 Following this closely — I've got almost the same discrepancy on my setup (Victron SmartSolar 75/15 into a Fogstar 100Ah lithium). One thing nobody's touched on yet: have you checked your cable losses between the MPPT output and the battery? Even fairly short runs of undersized cable can drop enough voltage to account for that kind of difference. My electrician friend reckoned I was losing nearly 0.3V across a dodgy connection on my negative busbar. Might be worth checking all your terminals are properly torqued down too.