Anyone else had their MPPT controller read wildly different voltages to their BMS?

Been scratching my head over this one for a couple of weeks now on the narrowboat. My Victron SmartSolar 100/30 is consistently reading about 0.4–0.6V higher than my Fogstar Drift 100Ah battery's own BMS via the Fogstar app. We're talking 13.8V on the Victron versus 13.2V showing on the battery — that's not a rounding error, that's genuinely confusing when you're trying to figure out your actual state of charge.

I've got two 200W Renogy panels wired in series feeding into the controller, running a fairly modest 12V system for the boat's lighting, a compressor fridge, and the usual 12V outlets. Wiring's all pretty tidy — 6mm² cable, decent crimps, short runs. I did wonder if it's a voltage drop issue somewhere between the controller and the battery terminals, but I've checked the connections and they look fine.

Has anyone actually sat down with a multimeter and traced exactly where the discrepancy is coming from? I know the Victron has that remote voltage sense feature but I've never set it up — wondering if that's the fix here or if there's something else going on entirely.

@Jess1989 wire resistance between the controller and the battery is doing exactly what Ohm's Law told it to — your MPPT is measuring voltage at its terminals, not at the battery, so every milliohm of dodgy cable or loose connection between the two is costing you volts and confusing the maths.

@Jess1989 @FellGraham is right about the resistance drop, but there's another factor worth checking: where exactly your Victron's voltage sense point is. The SmartSolar 100/30 measures voltage at its own battery terminals, not at the battery itself. If you haven't run a separate sense wire, you're seeing the voltage before the cabling losses.

Victron's VE.Smart networking is the proper fix here — pair the MPPT with a SmartShunt or even another Bluetooth device on the battery, and it'll use that remote voltage for charge decisions rather than its own terminal measurement. Made a noticeable difference on my cabin setup where I had a similar discrepancy across 4m of 16mm² cable. The Drift's BMS reading is almost certainly the accurate one.

JackieCrane | ⚓ Narrowboat & Off-Grid Enthusiast | Posts: 847

@Jess1989 @FellGraham is spot on about the voltage drop, but worth adding — the Victron SmartSolar has a nifty fix for this built in. If you're using a SmartShunt or a BMV battery monitor alongside it, you can pair them via VE.Smart Networking in the Victron Connect app. Once linked, your MPPT will use the battery's actual measured voltage for charge control rather than its own terminals. Sorted it completely on my own boat setup. Even without that, just check your cable runs and connections — a corroded terminal or undersized wire can cause surprisingly large discrepancies over longer runs. What gauge cable are you running between the controller and your batteries, and how far apart are they?

HollyWatson | ☀️ Off-Grid Nerd | Posts: 312

@Jess1989 One thing nobody's mentioned yet — double-check your MPPT's battery voltage sense setting. On the Victron SmartSolar you can enable remote voltage sensing via the VE.Direct or by connecting the dedicated sense terminals directly at the battery. Without this, the controller is measuring voltage at its own terminals rather than at the battery itself, which compounds the wire resistance issue @FellGraham raised. Once I enabled remote sensing on mine the discrepancy dropped from around 0.5V to virtually nothing. In VictronConnect it's under the battery settings tab. Worth a five-minute check before you start rewiring anything! Also make sure your Fogstar BMS display is actually calibrated correctly — some budget units ship slightly out of the box.