Anyone else finding their MPPT controller underreads battery SOC in cold weather?

Running a Victron SmartSolar 100/30 with a 200Ah lithium (a Fogstar Drift 12V) in my van, and over the last few weeks as temperatures have dropped I've noticed the SOC reading on the VictronConnect app has been all over the shop. We had a few nights down to about 3°C inside the van, and the controller was showing 67% when the battery voltage was sitting at 13.1V resting — that's basically full for a LiFePO4.

I've had a dig through the Victron settings and I'm wondering if the charge absorption voltage needs tweaking for lower temps, or whether the SOC is just drifting because the battery isn't hitting full absorption properly before the solar tails off in the afternoon. Days are obviously short now — I'm getting maybe 4–5 hours of useful generation at best here in the East Midlands, and the panels (2x 175W on the roof) aren't always finishing the job before the sun drops.

Has anyone dialled in their Victron settings specifically for winter use with lithium? Curious whether a scheduled absorption extension helps, or if I should just be plugging into hookup more regularly to let it do a proper full charge and recalibrate. Also open to hearing if others have found the Fogstar BMS throws any curve balls in the cold — mine seems fine electrically but I don't fully trust the SOC figure right now.

@BatteryDoug had almost exactly this with my Fogstar Drift last winter in the van. The MPPT is reading voltage to estimate SOC, and cold lithium cells sag harder under load — so the controller sees a lower voltage and assumes the battery is more depleted than it actually is.

The fix that actually worked for me was adding a Victron SmartShunt into the circuit. Coulomb counting is far more accurate than voltage-based estimation, especially once temperatures drop below 5°C. The MPPT and SmartShunt talk to each other over Bluetooth via VE.Smart Networking, so the SOC figure gets properly corrected.

Worth checking your battery's low-temperature discharge cutoff too — Fogstar Drifts handle cold reasonably well but they're not invincible. If yours is parked overnight in sub-zero temperatures regularly, that's a separate conversation worth having.

Yeah this is a known quirk with lithium in cold weather. The internal resistance of LiFePO4 cells increases noticeably when temperatures drop, which causes voltage to sag more under load — so the MPPT interprets that as a lower SOC than it actually is. It's not really a fault as such, just a limitation of voltage-based estimation.

Worth checking if your Fogstar has a built-in temperature sensor — some of the Drift models do. If you're running a BMV-712 or SmartShunt alongside it, that'll give you proper coulomb-counting SOC which is far more reliable in these conditions. @BatteryDoug honestly that's the route I'd go if you're doing serious van life through winter. The MPPT SOC readout was always a bit of a rough guide anyway, even in summer!

What @Thommo53 said. Had the same on my narrowboat last winter - the Drift was showing 40% when it was actually fine.

Worth adding a BMV-712 or SmartShunt to the setup if you haven't already. Coulomb counting is way more reliable in cold than voltage-based estimation. The MPPT SOC is really just a rough guide anyway.

Once I added the SmartShunt and let Victron's VE.Smart networking tie everything together, the readings settled down massively. Night and day difference.

What @Thommo53 is getting at is something I learned the hard way with my static caravan setup last January. Even with the SmartSolar doing its best, I found the real game-changer was adding a Victron SmartShunt inline — it tracks actual current flow rather than leaning on voltage estimation, so cold-weather resistance changes stop throwing the SOC reading off.

Once I paired it with VictronConnect and let it sync across devices via Bluetooth, the readings became remarkably stable even on bitter nights.

Worth noting: if you're using the Fogstar's own BMS app alongside VictronConnect, the two will sometimes disagree in cold conditions. I just treat the SmartShunt figure as the authoritative one and ignore the rest until temperatures recover.

Good shout from @Thommo53 and @Fogstar_Guy. One thing worth adding that nobody's mentioned yet — have you checked your battery temperature sensor is properly attached and reading correctly in VictronConnect? If the SmartSolar isn't getting an accurate temp reading, it can't properly compensate when calculating SOC. Also worth going into the app and checking your charge voltage settings — some people don't realise you can fine-tune the absorption and float voltages slightly for cold conditions, which can help the controller get a cleaner picture of actual state of charge. The Fogstar Drift is a solid battery and shouldn't be giving you grief, so I'd rule out any configuration issues first before assuming it's purely a chemistry/temperature thing. What temps are you actually seeing inside the van overnight?