Anyone else finding their MPPT controller reads wildly different to their battery monitor?

I've got a Victron SmartSolar 100/30 paired with a Renogy 200Ah lithium (the 12V one with built-in BMS) and the numbers just don't add up. The MPPT reckons it's pushing 18A into the battery, but my Victron BMV-712 is only showing 14A going in at the same moment. That's a pretty consistent 3-4A gap throughout the day, not just a momentary blip.

Both are set up with the correct battery capacity and chemistry. The SmartSolar is wired directly to the bus bar, and the BMV shunt is positioned between the negative bus bar and the battery negative — which I'm fairly sure is the correct placement. I've double-checked there are no loads or other inputs bypassing the shunt, and I can't find any rogue connection that would explain it.

Has anyone seen this before? Is it likely to be a calibration issue with one of the devices, a wiring problem I'm missing, or just normal variance between two different measurement points? Would love to know if recalibrating the shunt in the Victron app actually makes a meaningful difference or if I'm barking up the wrong tree entirely.

@DontPanic44 classic calibration mismatch — the SmartSolar measures current at the controller output, while your battery monitor is measuring at the battery terminal. Any resistance in the cable between them creates a voltage drop that throws the maths off completely.

Few things to check:

• Cable cross-section — undersized wire between controller and battery is the usual culprit
• Connection quality — loose lugs, oxidised terminals, dodgy fuse holder
• Shunt placement — if it's not on the negative battery terminal as the very first connection, everything downstream is guesswork

The Renogy BMS will also occasionally "absorb" a reading discrepancy because it's doing its own internal balancing calculations.

Ran into exactly this on my narrowboat with a Victron BMV-712. Moved the shunt, re-crimped the lugs properly, sorted. Took twenty minutes and I felt appropriately stupid afterwards.

Had exactly this on my static caravan setup. Worth checking whether your battery monitor's shunt is positioned correctly — if it's not on the negative terminal of the battery itself (with everything else connected on the other side), it'll miss currents entirely or double-count them.

Also, are you running any 12V loads directly off the battery while charging? That'll skew both readings differently depending on where each device is measuring.

On my boat I found a loose connection on the shunt was causing a 3-4A discrepancy that drove me mad for weeks. Tightened everything up and the Victron BMV-712 suddenly agreed much more closely with the SmartSolar figures.

What battery monitor are you actually using? If it's not a Victron BMV or similar with a proper shunt, the cheap inline ones can be notoriously inaccurate.

Hey @DontPanic44, worth checking whether your battery monitor's shunt is positioned correctly — it needs to be on the negative terminal with absolutely everything else on the battery side of it, otherwise it's only seeing some of the current. Even a single rogue ground connection bypassing the shunt will throw the readings right off. Also, have you synced the battery monitor's capacity and charge efficiency settings to match your Renogy's actual specs? Lithium typically wants around 99% efficiency factor rather than the lead-acid default. The Victron app will at least give you confidence in the MPPT side of things since Victron's current sensing is generally pretty solid. 🙂