Anyone else noticed their MPPT controller readings go a bit daft in winter compared to summer?

Running a Victron SmartSolar 100/30 on a 400W panel setup (two 200W panels in series) on my van, and I've been scratching my head at the numbers lately. During summer I was regularly seeing 18-22A charge current on a decent day, but now we're into December I'm barely cracking 8-10A even when the sky looks reasonably clear. Panel voltage is sitting higher than usual too — hitting around 43-45V open circuit where it was more like 39-40V in July.

I know panels run more efficiently in the cold (lower resistance or something like that), so the voltage thing makes sense I think, but the drop in actual charging current has caught me off guard. I've still got reasonable sun hours, maybe 2-3 usable hours on a good day here in the East Midlands, but I'm struggling to keep my 200Ah lithium above 70% without running the engine alternator to top it up. Not ideal when I'm trying to stay as quiet and fuel-free as possible on longer stays.

Is this just normal winter behaviour and I need to accept reality, or is there something worth checking — shading, wiring connections, the controller settings? Would adding a third panel help meaningfully or is it just throwing money at a sun problem?

@Nobby30 — saw exactly this on my shepherd's hut setup last winter and it took me an embarrassingly long time to work out what was happening.

Short version: cold panels are more efficient, so your Voc climbs noticeably in January compared to July. Your actual harvest drops because the sun's barely bothering to show up, but the voltage figures can look almost alarmingly healthy on a crisp clear morning.

The flip side is your battery accepts charge differently in the cold too — if you're running lithium, check your Fogstar or whatever cells are getting proper low-temp cutoff protection, because that'll skew your charge current figures as well.

The Victron app history graphs are brilliant for spotting the pattern across seasons — worth scrolling back if you haven't already.

The dirty secret nobody tells you when you're drooling over solar specs is that panel wattage ratings are calculated at 25°C cell temperature, so your "400W" array is basically cosplaying as a 400W array while actually delivering considerably less once you factor in low sun angle, atmospheric losses, and the fact that Britain apparently forgot what blue sky looks like between October and March. 😅 Check your Victron app's yield history — if your daily totals are plummeting harder than the temperature, it's almost certainly irradiance rather than a fault. My two Fogstar-charged lithiums barely broke a sweat last January because the panels were doing roughly the work of a slightly enthusiastic candle.

On my boat the panels basically become decorative art in December — lovely to look at, absolutely useless for charging my Fogstar lithiums before teatime.

Yeah this is dead normal. Worth checking your Victron app's yield history — you'll probably see the drop tracks almost perfectly with day length rather than anything being faulty.

One thing I noticed at my cabin setup: even on bright winter days the angle kills you more than the cold does. My fixed-mount panels are optimised for summer and basically graze the low sun at 15° instead of hitting it properly. Added a cheap tilt bracket last February and picked up a noticeable chunk back.

Also worth knowing — if temps are genuinely cold your Voc actually creeps up, so double-check your series string voltage doesn't push too close to your controller's input ceiling on a frosty morning. The 100/30 has 100V max input and two panels in series can surprise you when it's -5°C.

@Nobby30 the bit that trips most people up is the voltage side of things too — cold panels actually push higher Voc, so if you're running two 200W panels in series check your array doesn't spike above the 100V input limit on cold clear mornings. That's a real risk in winter, not just summer inefficiency.

Also worth looking at your bulk/absorption times in the VictronConnect history. Short absorption phases in winter usually mean the batteries aren't getting properly topped off, which compounds over weeks and tanks your usable capacity. Not a controller fault — just physics working against you.

My garden office setup sees maybe 30-40% of peak summer yield right now. It's annoying but expected. Battery maintenance charging from the mains every couple of weeks keeps things sane.

@VoltWill raises something worth expanding on — that elevated Voc in cold conditions isn't just academic. If your panels are rated at, say, 45V Voc each and you're running them in series, you're looking at 90V nominal. Apply the standard -0.3%/°C temperature coefficient across a proper British winter morning at -5°C (25°C below STC), and you're adding roughly 6-7V on top. Suddenly you're nudging 97V on a 100V-rated controller.

On my narrowboat I specifically sized my SmartSolar 150/35 with that headroom in mind precisely because of this. The 100/30's absolute maximum is 100V — worth double-checking your actual cold-morning peaks against that figure in VictronConnect's history, because over-voltage events can permanently damage the input stage without necessarily throwing an obvious fault immediately.

@DefenderAdventure makes a good point about the Voc — worth actually doing the maths on that, @Nobby30. Two 200W panels in series on a cold morning could push Voc surprisingly high, so double-check your combined cold-weather Voc against your SmartSolar's 100V input limit. On the amperage drop you're seeing though, don't overlook panel angle. In winter the sun sits much lower in the sky, so a flat van roof mount takes a real hit. Even propping panels up at a steeper angle temporarily can make a noticeable difference to your harvest.