Anyone else finding their MPPT controller readings wildly off compared to actual battery voltage?

Been scratching my head over this one for a couple of weeks now. My Victron SmartSolar 100/30 is consistently showing battery voltage about 0.3–0.4V higher than what my separate battery monitor (Victron BMV-712) is reading at the same moment. Both are wired directly to the same 100Ah lithium (a Fogstar Drift if it matters). Checked all my connections and everything looks tight.

From what I've read it could be a calibration drift on one of the units, or possibly a small voltage drop across the wiring between where each device takes its sense reading. The MPPT pulls its voltage reading from the charge terminals rather than directly at the battery, so maybe that's all it is — but 0.3V feels like a lot for a short cable run. I've got maybe 40cm of 6mm² between the MPPT output and the busbar, then another short run to the BMV shunt.

Has anyone actually gone into the Victron Connect app and recalibrated either device, or is there a smarter way to sort this? I've got a decent multimeter (Fluke 117) so I can get a reliable reference reading — just not sure which unit to trust and which one to adjust. Keen to hear if others have hit the same issue.

@JimWilliams71 had exactly this with my van build last winter. Turned out my MPPT was positioned about 2 metres from the battery bank with fairly thin cable runs — that voltage drop was reading backwards through the sense circuit, inflating the controller's readings.

Victron's fix is dead simple: use the built-in battery voltage sense feature in VictronConnect. Once I enabled remote sensing via the VE.Smart network (my BMV-712 shares its voltage reading directly to the SmartSolar over Bluetooth), both readings snapped into agreement within 0.05V.

Worth checking your cable cross-section too. I was running 6mm² over that distance when I really needed 10mm².

If you're not using a Victron shunt monitor, a dedicated sense wire run directly from the MPPT's sense terminals to the battery terminals sorts it physically.

Seen this loads with Victron kit. Worth checking your battery sense wires first — if the MPPT is only reading via its B+ terminal rather than direct battery sense leads, you'll always get a skewed reading, especially under any kind of load.

Also, what cable run have you got between the MPPT and your battery? Even modest resistance across a few metres will throw the readings off. In my van build I had a 0.35V discrepancy that turned out to be a dodgy crimp on the positive run — swapped it out and sorted immediately.

The SmartSolar does support remote battery sensing via the VE.Smart network if you pair it with a BMV or SmartShunt. Might be worth enabling that rather than chasing a physical wiring issue.

@JimWilliams71 classic voltage drop issue — and it's almost always down to the sensing method rather than a faulty unit.

On my shepherd's hut build I had near-identical figures to yours. The MPPT was reading terminal voltage at the controller itself, not at the battery. With 4mm² cable runs and a decent load on the system, you'll easily see 0.3–0.4V of drop across connectors and cable resistance.

Two things worth checking beyond what's already been mentioned:

1. Connector oxidation — even Anderson connectors corrode over time, adding measurable resistance
2. Enable Victron's remote battery sensing via the SmartShunt if you have one — it'll feed actual battery voltage back to the MPPT over VE.Smart networking and compensate automatically

The VE.Smart solution genuinely transformed the accuracy on my system. Absorption and float triggers became noticeably more precise afterwards.

Really common issue this. Worth checking your cable routing from the MPPT battery terminals to the actual battery — even decent cable has resistance, and under any meaningful load that voltage drop adds up fast. The MPPT is measuring at its terminals, not at the battery itself.

If you haven't already, try enabling the "remote voltage sense" feature on the SmartSolar — there are two small sense terminals on the unit. Run a thin sense wire directly to the battery terminals and the controller will compensate automatically. Makes a noticeable difference in my experience.

Also worth ruling out a dodgy connection or undersized cable as a contributing factor — a 0.3–0.4V discrepancy is on the higher side and could suggest something worth investigating beyond just normal voltage drop. What gauge cable are you running between the controller and battery? 🔧

Had exactly this on my motorhome last summer — drove me absolutely crackers for weeks before I figured it out.

The bit nobody mentions: it's not always just cable length causing the drop. I had a dodgy crimp on one of my battery terminal connections that was adding maybe 0.2Ω of resistance. Barely visible to the eye, but under any meaningful current draw the readings diverged noticeably.

Grabbed my multimeter and measured directly across the battery terminals while the MPPT was pushing decent current, then compared. The discrepancy shifted depending on load — that's your clue it's resistance-related rather than a calibration drift.

@JimWilliams71 worth wiggling each connection while watching the SmartSolar app readings in real time. If you see the voltage jump at all, you've found your culprit — no expensive fixes needed, just a proper re-crimp.

@JimWilliams71 the 0.3–0.4V is pretty textbook for remote sensing not being configured. In VictronConnect, go into the SmartSolar settings and check whether Battery Voltage Sense is set to use the built-in sense or an external source.

If you're using a BMV-712 or SmartShunt on the same VE.Smart network, you can enable VE.Smart Networking — the MPPT will then use the battery monitor's voltage reading directly rather than its own terminals. Made a noticeable difference on my narrowboat where I've got a decent cable run between the MPPT and the battery bank.

Worth also double-checking your negative busbar connections are solid. A high-resistance joint on the negative side will show up exactly like this — MPPT sees a slightly elevated voltage that doesn't match reality at the battery terminals.

@JimWilliams71 one thing worth adding to what @DorsetSolar is getting at — even once you've sorted the sensing config, make sure you've got proper temperature compensation set up as well. Battery voltage readings can drift noticeably depending on ambient temperature, and if your MPPT and battery monitor are using different temperature references they'll never quite agree. Also worth double-checking that both devices share a proper common negative — a dodgy earth connection can introduce all sorts of phantom voltage differences that'll have you chasing your tail for ages.

@JimWilliams71 worth checking your battery cable connections as well — specifically the ones between the MPPT and the battery terminals. Even a small amount of resistance from loose or corroded connections will cause a voltage drop that shows up exactly like what you're describing. The MPPT reads voltage at its own terminals, so if there's any resistance in the cabling between there and the battery, you'll always see a discrepancy. Give everything a wiggle and check with a multimeter across the terminals themselves. Might be nothing, but it's a quick win before diving deeper into settings.