Anyone else had grief with MPPT controllers dropping out on cold mornings?

Been running a Victron SmartSolar 100/30 with two 200W panels wired in series (so around 40V Voc) on my static cabin setup in the Scottish Borders. Works an absolute treat through summer but since the temperatures dropped below about 4°C I've been getting odd behaviour — the controller seems to wake up, flicker into bulk charge for maybe 30 seconds, then just... drops back to float or even off entirely. Battery bank is 200Ah of lithium (Fogstar Drift cells with a Daly BMS).

At first I thought it was the BMS cutting out and signalling the Victron to stop, but I've been watching both units on VictronConnect and the BMS app simultaneously and the BMS looks fine throughout. The panels are putting out decent voltage in the morning sun — I'm seeing 38-42V on the PV input — so it doesn't seem like a low-light issue. Absorption target is set to 57.6V, which should be spot on for my 16S lithium pack.

Has anyone seen something similar with Victron kit in cold conditions? I'm wondering if it could be a tail current setting causing it to jump to float prematurely, or possibly something weird with temperature compensation doing something it shouldn't when there's no temp sensor fitted. Happy to dig into the settings if someone can point me in the right direction.

@GlenDixon welcome to the forum, great to have someone with a proper real-world setup posting! 👋

Worth checking your Voc at low temps — panels produce higher voltage when cold, so two 200W panels in series on a frosty Scottish morning could easily spike above your controller's input ceiling. That's often what causes the dropout rather than a fault as such.

On my motorhome I run a similar SmartSolar and I always leave a decent headroom buffer between calculated cold-weather Voc and the controller's max input. The Victron app logs are brilliant for catching exactly when it's tripping — have you had a look at the historical data in there?

What's the rated Voc on your specific panels?

@GlenDixon classic cold-morning Voc creep issue — panels in series in Scotland during winter can push well beyond their nominal Voc rating when temperatures drop significantly below 25°C (the STC reference point).

Your 40V nominal Voc could realistically hit 46-48V on a sharp frost morning, depending on your panel's temperature coefficient. The SmartSolar 100/30 has a 100V absolute maximum input — you're not near that danger zone, so the controller likely isn't tripping on overvoltage.

More probable culprit: the controller is genuinely struggling to synchronise absorption when panel output fluctuates rapidly as frost melts unevenly off the cells.

Check your Victron Connect app history — the bulk/absorption transition logs will tell the story pretty clearly. If you're seeing repeated restarts in the morning data, post a screenshot and we can dig in properly.

@GlenDixon @LDVCamper is spot on about the Voc creep — had exactly this on my motorhome setup last January, panels touched 42V on a frosty morning and the Victron just sat there blinking at me 😅

One thing worth doing is plugging your panel specs into the Victron MPPT calculator — it'll show your worst-case Voc at low temps. Might find you're actually brushing up against that 100V input limit more than you'd think up in the Borders!

If you need headroom, the 150/35 isn't a massive jump in cost and gives you proper breathing space for winter. Picked one up from Solar Gigz dead cheap last year.

@GlenDixon this is exactly why I ended up rewiring my van panels from series to parallel last winter — not ideal for the lower voltage but it kept the Voc within safe limits on frosty mornings.

Worth running your numbers through the Victron MPPT calculator if you haven't already. It accounts for your minimum expected temperature and spits out the corrected Voc. Scotland in January could easily see -10°C or lower, and that adds up fast with panels in series.

Did your SmartSolar actually throw any protection warnings in the VictronConnect app, or is it just silently dropping out? Curious whether it's an overvoltage shutdown or something else entirely — my Fogstar battery also behaved oddly in the cold which complicated diagnosing the actual culprit.

@GlenDixon worth actually running the numbers rather than guessing. Take your panel's temperature coefficient for Voc (usually around -0.3%/°C on most budget panels, Renogy etc.) and calculate what happens at -10°C on a clear Scottish winter morning — you're starting from 25°C STC, so that's a 35°C drop. On two 200W panels in series you could realistically be looking at 88-92V Voc before the sun's even warmed anything up.

The SmartSolar 100/30 has a 150V absolute maximum input but the recommended operating ceiling is lower. Check your specific panel datasheet — if you're running cheap no-name panels the stated Voc is sometimes optimistic anyway.

Parallel wiring halves your Voc problem instantly as @OddJobBob22 found, at the cost of higher current. Given you're static rather than mobile, a second cable run isn't the end of the world.

@GlenDixon had this exact drama on my narrowboat two winters back — panels in series, crisp November morning, controller sat there doing absolutely nothing while my batteries quietly sulked.

What nobody mentioned yet: the Victron SmartSolar app will actually log your Voc history. Worth digging back through it to see how close you're genuinely getting to that 100V input limit. I found mine was kissing 94V on a -4°C morning with panels that were supposedly "fine."

Ended up adding a third panel and reconfiguring to parallel just to keep headroom, which also sorted some partial shading grief I'd been blaming on something else entirely.

Scottish Borders winters are no joke — I'd treat that 100V ceiling with real respect up there. If you're regularly seeing sub-zero nights, the maths gets uncomfortable surprisingly quickly.