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

Running a Victron SmartSolar 100/30 on a 400W array (two 200W panels in series) on my static caravan setup in the Scottish Borders. During summer it was faultless, but since the temperatures dropped below about 5°C I've been getting weird behaviour — the controller cuts out for a few minutes around 9-10am, just as the panels are starting to produce properly. Comes back on its own, then runs fine the rest of the day.

I've had a look at the VictronConnect history and the only thing that jumps out is the panel voltage spiking to around 82V on those cold mornings, which is nudging close to the 100V input limit on the controller. My two panels are rated at Voc 46.8V each, so in series that's 93.6V at STC — but I know open circuit voltage climbs as temperature drops, and at -3°C or so I reckon it could be pushing 100V or beyond. Is that what's triggering the protection cutout?

Has anyone dealt with this on a similar setup? I'm wondering whether the sensible fix is to rewire the panels in parallel (which would halve the voltage but also halve the charge current efficiency a bit), or whether I should just accept it and look at a controller with a higher input voltage — the 150/35 would give me plenty of headroom. Keen to hear what others have done before I start spending money.

ShaunKelly87 | 234 posts

@ExJoiner6 Classic cold weather behaviour this. Worth checking your open circuit voltage on those panels in the cold - two 200W panels in series can push Voc up significantly when temperatures drop, sometimes enough to exceed your controller's input limit. The 100/30 has a max input of 100V, and panel Voc ratings are given at 25°C. At -5°C or below in the Borders you could genuinely be hitting that ceiling, causing the controller to protect itself by cutting out. Check your panel spec sheets and run the numbers accounting for the temperature coefficient. Might be worth reconfiguring to parallel if that's the case, assuming your current stays within limits. What's the Voc on each individual panel?

@ExJoiner6 classic cold-morning Voc spike issue. When panels are near freezing, open-circuit voltage climbs significantly above the rated figure — your two 200W panels in series could easily be pushing 90V+ at 0°C depending on the temperature coefficient. The SmartSolar 100/30 has a 100V absolute maximum input, so if your calculated cold Voc is nudging that limit, the controller's protection circuit will trip out.

Check your panel datasheets for the temperature coefficient of Voc (usually around -0.3%/°C) and run the numbers for your coldest expected ambient. Victron's MPPT Excel calculator tool handles this properly.

If you're right on the edge, you've got two options: rewire to parallel (halves the Voc but doubles current — check your controller's 30A limit) or swap to a higher-voltage controller like the 150/35. Had similar grief on my shepherd's hut build before I recalculated properly.

FormerMariner36 | 891 posts

Spent three winters liveaboard before I moved the setup to a garden office, so cold-weather Voc spikes nearly killed two controllers on my boat before I properly understood what was happening.

The bit nobody mentions: it's not just how cold the panels get, but the combination of overnight frost plus a crystal-clear dawn sky. That's when you get the nastiest spikes — panels below ambient because they've been radiating heat all night, then suddenly hammered with full irradiance.

@ExJoiner6 Scottish Borders winters will absolutely punish you with that scenario regularly.

What saved my setup was switching to Victron's own sizing tool and treating -10°C as my baseline calculation temperature rather than the typical annual minimum. Gave me proper headroom. Also worth enabling the Over-voltage protection alarm in VictronConnect so you can at least see when it's happening rather than just finding the controller sulking.

@ExJoiner6 this is almost certainly your panels producing voltage that's spiking above the controller's 100V input limit on cold mornings. PV open-circuit voltage increases significantly as temperature drops — manufacturers quote Voc at 25°C (STC), but on a crisp Scottish morning at say -5°C you can easily add 15-20% on top of that.

Two 200W panels in series means your combined Voc could be pushing 85-90V at STC. Add a cold-morning temperature coefficient into the mix and you're potentially hitting 100V or beyond, triggering the over-voltage protection.

Worth checking:

• Your panel datasheets for temperature coefficient (Voc)
• Use Victron's MPPT calculator to model worst-case voltage at your minimum recorded temperature
• Consider rewiring to parallel if your current ratings allow it

The Victron will protect itself automatically, which is the good news — but you're losing harvest every clear cold morning until the panels warm up.

LiamWalker | 347 posts

@ExJoiner6 worth double-checking your panels' temperature coefficient specs too. Most 200W panels have a Voc around 24-25V at STC (25°C), but that figure climbs roughly 0.3-0.35% per degree below that baseline. With two panels in series in the Scottish Borders on a clear frosty morning, you could realistically be nudging 55-58V combined before the sun's had chance to warm them up.

As @VivaroAdventure says, that's dangerously close to your 100V limit once you factor in actual cold temperatures rather than just room temp figures. Victron's own sizing tool lets you plug in your lowest expected temperature to calculate worst-case Voc — I'd run your numbers through that before assuming the controller is faulty. You might find rewiring to parallel rather than series solves it entirely.

MeadowDweller | 1,204 posts

@VivaroAdventure has nailed the likely culprit. Worth adding: on my narrowboat I run a similar series string and I specifically derated to keep a 15–20% headroom below the controller's Voc limit. The Victron SmartSolar documentation actually warns about this in the installation notes — cold-weather Voc can exceed the panel's STC rating by a meaningful margin.

The practical fix on a static setup is to either reconfigure to parallel (halves your combined Voc) or swap the controller for a 150V input unit. Victron's 150/35 isn't dramatically more expensive and gives you proper headroom for Scottish winters.

Also check the Victron app's history graphs — if it's logging "input high voltage" protection events, that confirms exactly what's happening rather than leaving you guessing.

KeithWebb | 892 posts

@ExJoiner6 Scottish Borders winters are no joke for this — I'd actually work out your worst-case Voc before assuming anything's definitely broken. Take your panels' open-circuit voltage, then apply the temperature coefficient down to say -15°C (not unheard of up your way). If you're running two 200W panels in series, that combined Voc could comfortably breach 100V on a sharp frost. Victron's own sizing tool is brilliant for this calculation. You might find you simply need to rewire to parallel rather than series to bring the voltage down safely.

FenlandVanLifer | 218 posts

Had almost identical symptoms in my Transit last January — turned out the Victron was hitting the absorption voltage earlier than expected because the panels were putting out higher-than-rated voltage in the cold (as @LiamWalker touched on). Worth connecting via the VictronConnect app and actually watching the voltage/current graphs on a cold clear morning. The data logging will show you exactly when and why it's dropping out. Mine needed the absorption voltage threshold tweaking slightly once I understood what was happening. The SmartSolar's Bluetooth logging is genuinely useful for diagnosing this rather than just guessing.

ColPalmer | 1,847 posts

@ExJoiner6 One thing nobody's mentioned yet — check your battery temperature sensor if you're using one. I had a SmartSolar doing something very similar and it turned out the temp sensor had worked loose from the battery terminal. The controller was getting a false reading and throttling charge accordingly. Also worth logging a few mornings via the VictronConnect app; the history graphs will show you exactly when and why it's cutting back. Takes the guesswork out completely. The Bluetooth logging on those units is genuinely brilliant for diagnosing cold-weather oddities. 🏴󠁧󠁢󠁳󠁣󠁴󠁿

KenGraham | 634 posts

@ExJoiner6 Worth checking your open-circuit voltage on those panels during a cold, bright morning — Voc climbs noticeably when temps drop, and two 200W panels in series could be pushing close to or beyond the 100V input limit on your SmartSolar. Victron's MPPT calculator is handy for this. If you're regularly hitting -5°C or colder in the Borders, that Voc spike might be triggering the over-voltage protection rather than a temperature issue per se. Might be worth logging the input voltage through the VictronConnect app to catch what's actually happening at the point of cutout.

FellKev | 412 posts

@ExJoiner6 My static in the Lakes had this exact drama two winters back. Frost-bright mornings are brutal — panels pumping out near peak Voc before the battery's had a chance to warm up and accept any meaningful charge. The Victron gets confused and throttles back hard.

What sorted mine was setting a proper absorption voltage offset for winter in the SmartSolar app and — this is the bit most miss — enabling the low temperature charge current limiting under the battery settings. Saved my Fogstar lithiums from grief too.

@KenGraham makes a solid point about Voc. Check it before assuming the controller's faulty.