Anyone else finding their MPPT controller readings are way off compared to actual battery voltage this winter?

Been scratching my head over this one for a few weeks now. I've got a Victron SmartSolar 100/30 paired with two 200Ah 12V lithium (LiFePO4) batteries wired in parallel, and since the temperatures dropped the MPPT is consistently showing about 0.3–0.4V higher than what my separate Victron BMV-712 battery monitor reads. In summer they were almost spot on — maybe 0.05V out at most.

I've read a bit about temperature compensation but my understanding was that's mainly relevant for lead-acid. The SmartSolar is mounted on an exterior wall of my workshop (unheated), so it's sitting at maybe 3–4°C most mornings, while the batteries are inside at around 12–13°C. Wondering if the sensor in the controller is reading its own local temperature and that's throwing the voltage measurement out somehow, or if there's a wiring issue I've not spotted. The cables between the controller and battery are about 4 metres of 10mm², so voltage drop under load could explain some of it, but this discrepancy is showing up even at night with zero current flowing.

Has anyone else noticed this with Victron kit specifically, or is this a more general MPPT quirk in cold weather? Would the VE.Direct temperature sensor cable make any difference here, or is that purely for lead-acid charge curve adjustments?

@BrianLewis79 had exactly this on the Persistent Dream last January moored up in Ellesmere Port. Brutal cold snap, and my SmartSolar was reading nearly 0.4V higher than what my BMV-712 was showing at the battery terminals.

Turned out to be two things working against me simultaneously — the temperature compensation was pulling the charge voltage upward (correct behaviour, actually) but I also had a dodgy connection on the sense wire that was masking the real picture.

Worth checking whether you've got the voltage sense wires properly connected directly at the battery terminals rather than relying on the controller's internal sensing. On a narrowboat the cable runs are rarely short enough to trust that.

Also double-check your battery temperature sensor is actually seated properly against the cell casing — mine had worked loose over a lock landing.

Hey @BrianLewis79, worth checking your temperature sensor connection if your SmartSolar has one fitted - a dodgy or missing temp sensor can cause the controller to apply incorrect compensation and give you skewed readings. Also, are you monitoring via the VictronConnect app or a separate BMV battery monitor? In my experience the MPPT's voltage reading isn't always the most reliable reference point, especially with any cable resistance in the mix. A decent BMV-712 or similar connected directly at the battery terminals will give you a much truer picture. Cold temperatures don't actually affect LiFePO4 voltage readings as much as lead-acid, so if there's a significant discrepancy I'd be looking at connection quality and sensor setup first. What sort of difference are you actually seeing between the controller reading and measured battery voltage?

Hey @BrianLewis79, worth bearing in mind that LiFePO4s have a much flatter discharge curve than lead-acid, so even small voltage measurement errors can translate to wildly inaccurate state-of-charge readings in cold weather. Victron's default charge profile assumptions may also not be perfectly dialled in for your specific battery brand. I'd recommend connecting via the VictronConnect app and double-checking your battery voltage compensation settings, and if your batteries have a BMS with Bluetooth, cross-reference the SoC reading from there against what the SmartSolar is reporting. That'll quickly tell you whether it's a measurement issue or a settings issue. What batteries are you running specifically? Some brands have dedicated Victron profiles available. 🔋

Good shout from @DalesSolar on the flat discharge curve — that catches a lot of people out when they first switch to lithium.

What I'd add from my own experience running a Victron SmartSolar on the garden office: make sure your absorption and float voltage settings are actually calibrated for LiFePO4, not left on a lead-acid profile. Victron's defaults can drift readings noticeably in cold weather if the charge algorithm isn't matched to the battery chemistry.

Also worth checking — are you using the VictronConnect app to cross-reference the MPPT readings against what a proper dedicated battery monitor (like a BMV-712) is showing? The MPPT voltage reading is measured at the controller terminals, not at the battery itself. Any resistance in your cabling will create a gap, and cold weather tightens that margin further.

My garden office Victron reads bang-on once I actually ran proper cable to the battery terminals instead of tapping off some random point halfway down the loom — voltage drop across dodgy connections is basically lying to you in real time, especially when current demand spikes in cold weather and your resistance goes up.