Anyone else had issues with MPPT controllers going haywire in cold UK winters?

I've been running a Victron SmartSolar 100/30 with two 200W panels on the roof of my narrowboat since last spring and it's been brilliant all summer. But now we're into the colder months I'm seeing some weird behaviour — the controller is regularly throwing a "high voltage" warning first thing in the morning, especially on clear frosty days. Checked the voltage at the array before it connects and I'm getting readings up around 48-49V open circuit, which is higher than I was seeing in July.

Done a bit of reading and I think I understand why it's happening — Voc goes up as temperature drops — but I'm kicking myself because I didn't account for this properly when I sized the system back in the spring. The two panels are rated at 24V Voc each, wired in series, so in summer I was sitting comfortably around 44-45V. The controller's max input is 100V so I've got headroom there, but it's the bulk/absorption thresholds that seem to be getting triggered oddly. Battery bank is 200Ah of lithium (Fogstar Drift cells) set to a 14.2V absorption target.

Has anyone else seen this with Victron kit specifically, or with lithium banks in particular? I'm wondering whether it's worth tweaking the charge profile settings in VictronConnect, or whether there's something else going on I'm missing. Happy to share screenshots of the app if that helps diagnose it.

JohnChapman | Posts: 847

@TerryWatson I think your post got cut off there mate, but I can probably guess where you're going with this!

Cold weather and MPPT controllers can definitely throw up surprises. One thing worth checking is your Voc calculations - cold temps actually raise panel voltage, sometimes significantly. If your panels are pushing voltage close to your controller's input limit during a frosty morning, the controller can behave oddly or even trigger overvoltage protection.

With two 200W panels in series on a narrowboat, what's your open-circuit voltage looking like at -5°C or below? Victron's VictronConnect app is brilliant for checking those logs and seeing exactly what's happening.

Also worth double-checking your battery temperature compensation settings if you're running lead-acid. What battery type are you on?

BigAl | Posts: 312

@TerryWatson assuming you're seeing higher voltages than expected — this is completely normal with Victron kit. Cold panels have lower internal resistance so Voc shoots up noticeably. Worth double-checking your array's cold Voc against your controller's maximum input (100V on that unit). Two 200W panels in series on a frosty morning could get uncomfortably close to that ceiling.

On my static van setup I run mine in parallel partly for this reason — keeps the voltage predictable year-round.

If you're seeing weird behaviour rather than just high numbers, check the Victron Connect app logs. The SmartSolar records fault history which is genuinely useful for diagnosing what's actually triggering it. Sometimes it's just the controller doing its job correctly and we're not used to seeing it work that hard.

VitoProject | Posts: 1,203

Yeah the high voltage thing is well documented with Victron kit in winter — @BigAl's almost certainly right. Cold panels have lower internal resistance so Voc climbs noticeably above the spec sheet figure (which is measured at 25°C STC, not a freezing November morning on the cut).

What's your actual concern though — is the controller throwing errors, or are you just seeing numbers you didn't expect in the VictronConnect app? Two 200W panels in series could genuinely push close to your 100V input limit on a cold clear day. Worth checking your Voc calculations with a proper temperature coefficient correction before assuming something's broken.

My own SmartSolar 150/35 has been running fine through a couple of winters without incident. These things are generally solid. Check your wiring connections first — cold weather loves exposing a dodgy crimp.

TimGreen | Posts: 156

Worth adding to what @BigAl and @VitoProject have said — on a narrowboat you've also got the added factor of frost on the panels themselves. Once it melts off mid-morning you can get a sudden surge of current on top of already-elevated cold-weather voltages, so keep an eye on your controller's temperature during those moments.

Also check your battery temperature compensation settings if you haven't already. Victron's SmartSolar will adjust charge voltages based on temperature, but only if you've got a temperature sensor connected or you're using a compatible Smart Battery Sense. Without that, it's essentially flying blind in winter conditions.

The VictronConnect app will show you historical data — worth having a browse through your logs to see exactly what voltages you're hitting at peak.

EdHamilton | Posts: 847

One thing nobody's mentioned yet — check your temperature compensation settings in the VictronConnect app. The SmartSolar has a built-in temp coefficient adjustment (typically -5mV/°C/cell for AGM, different for lithium), and if you've got a battery temperature sensor fitted, it'll automatically adjust absorption/float voltages downward as the bank warms up during charging.

Worth double-checking which battery type profile you've selected. I made the mistake on my static caravan setup of leaving it on the default AGM preset when I'd actually fitted Fogstar Drift lithium cells — the winter voltage spikes looked alarming until I sorted the profile properly.

Also, @TimGreen raises a good point about narrowboat specifics — condensation affecting connections can cause false voltage readings at the controller. Clean terminal connections genuinely do make a measurable difference to the MPPT algorithm's behaviour in my experience.

ExTrucker | Posts: 412

Had the same panic on my narrowboat last January — turns out my panels were briefly pushing well over spec in that -3°C frost we had, nearly gave my Victron a heart attack it didn't need. Worth punching your actual panel Voc into a temperature correction calculator before winter properly bites — cold weather and short days are basically a recipe for "maximum voltage, minimum charge time," which is a lovely combination nobody asked for. 🥶

FrostyHiker | Posts: 312

My van setup did exactly the same last January — panels were practically generating electricity out of spite in the cold morning sun and the Victron nearly had a meltdown about it. Cold temps push Voc up beyond what your summer calculations assumed, so if you're right on the edge of your controller's input limit you're asking for trouble. Double-check your worst-case Voc using the actual minimum temp you'll see on the cut — panels rated at 25°C will be putting out noticeably more at -5°C. The Victron MPPT calculator tool online does this properly if you haven't already run your numbers through it.