Anyone else had issues with MPPT controllers underreporting battery SOC in cold weather?

Been scratching my head with my setup for a couple of weeks now. Running a Victron SmartSolar 100/30 paired with two 120Ah AGM batteries (wired in parallel, so 240Ah total) in my Transit conversion. During this recent cold snap we've had, the controller keeps showing the batteries at around 60-70% SOC even straight after a full charge cycle — but when I slap a multimeter on the terminals I'm reading 12.7-12.8V at rest, which should be closer to 100%.

I've got the battery temperature sensor fitted and the absorption/float voltages set correctly for AGM (14.7V absorption, 13.8V float), so I don't think it's a settings problem. Wondering if the SOC algorithm just gets confused when the charging profile shifts in the cold. Temps here in the van have been dropping to around 3-4°C overnight.

Has anyone else noticed this with Victron kit specifically, or is it more of a general MPPT thing? I'm also curious whether switching to lithium down the line would give me more reliable SOC readings — I know LiFePO4 has a flatter discharge curve which makes it harder to read by voltage alone, but I've heard the smart BMS units handle it better. Any thoughts welcome.

CamperAndrea · 47 posts

@SamWhite this is a classic winter headache! Worth checking whether you've got your battery temperature compensation set correctly in the Victron app. AGMs have a negative temperature coefficient, so if the controller thinks it's warmer than it actually is, it'll cut charging voltage short and the SOC reading goes haywire as a result.

If you haven't got a temperature sensor attached directly to the battery bank, that's likely your culprit right there. The SmartSolar can use the built-in Bluetooth sensor as a rough workaround, but it's measuring ambient rather than actual battery temp. A proper VE.Smart network with a BMV-712 battery monitor will give you far more accurate SOC readings than relying on the MPPT's voltage-based estimates alone — especially in a Transit where temperatures can swing massively overnight.

MaxShaw · 203 posts

@SamWhite the cold temperature issue is real, but with Victron there's actually a decent fix — if you haven't already, grab a Smart Battery Sense (or use a BMV-712 if you want full monitoring). It feeds live temperature data directly to the SmartSolar via Bluetooth, and the controller automatically adjusts its charge voltages accordingly. AGMs are particularly sensitive to this; at 0°C your effective capacity drops noticeably and the charging parameters need compensating upward slightly. Without temperature compensation, the controller's essentially guessing. Also worth double-checking your battery capacity setting in the VictronConnect app matches your actual 240Ah — I've seen vans where that was never properly configured after initial setup, which throws the SOC calculation right off from the start.

StuCross · 312 posts

@SamWhite AGMs are particularly prone to this in cold weather — the internal resistance increases significantly as temperatures drop, which throws off the voltage-based SOC calculations your controller relies on. Your Victron is essentially seeing a slightly elevated resting voltage and misreading where the battery actually sits.

One thing neither @CamperAndrea nor @MaxShaw has mentioned yet — if you're parked up overnight in sub-zero temps, your batteries are also losing usable capacity (not just misreporting it). A 240Ah bank can behave more like 180Ah at 0°C. So you might actually be dealing with two separate issues simultaneously.

Have you checked what your absorption voltage is currently set to? Worth bumping it up slightly for winter to compensate. What temperatures are you typically seeing overnight?

HollyGaz · 89 posts

@SamWhite had almost the exact same panic on my narrowboat last January — woke up convinced my batteries were dead, turned out the Victron was just being a drama queen about the cold 😅

Quick question though — have you set the correct battery temperature compensation in VictronConnect? There's a mV/°C setting that makes a massive difference for AGMs specifically. Also, are you using a BMV battery monitor alongside the SmartSolar, or relying purely on the MPPT for your SOC readings?

The MPPT controller alone is honestly pretty rubbish at accurate SOC — it's more of a rough guess than a proper reading. A BMV-712 transformed my setup, and with the temperature sensor add-on it handles the cold weather compensation properly. Pricey but worth every penny if you're living aboard or doing serious van life.

What temperatures are you actually seeing overnight?

MarineSam · 447 posts

@SamWhite worth checking your battery temperature compensation settings in the Victron app — the SmartSolar has a dedicated temperature coefficient setting (usually around -4mV/°C/cell for AGMs). If that's not dialled in correctly, the controller's voltage-based SOC calculations go properly haywire in cold conditions.

Also, and I suspect this might be contributing — have you set the correct battery capacity in the app? With parallel wiring, people sometimes forget to enter the combined 240Ah figure, which throws the Peukert calculations off entirely.

On my boat I ended up adding a BMV-712 battery monitor separately, which gives a much more reliable SOC reading based on actual current flow rather than just voltage. Transformed how accurately I could track the banks in winter. Might be worth considering if the software tweaks don't sort it.