Anyone else noticed their MPPT dropping efficiency over winter compared to summer figures?

Running a Victron SmartSolar 100/30 with 400W of panels (2x200W Risen in series) feeding a 100Ah lithium (Battle Born). Summer was brilliant — regularly seeing 25-28A going in on a decent day, bulk charge done by mid-morning most days.

Since about November though I'm lucky to see 15A peak even on the clearer days, and the controller seems to be sitting in float embarrassingly early — like 11am sometimes — despite the battery clearly not being full (resting voltage drops back to 13.1V within an hour of loads kicking in). Not sure if it's a dodgy state-of-charge calculation, the panels just not generating enough, or something else entirely going on.

I've checked connections, all look fine. Panel voltage in the app is reading sensible numbers (around 38-42V open circuit which seems right for the cold). Just wondering whether others are seeing similar with their setups over winter, or whether something more specific might be going on with mine. Anyone gone through this and figured out what the actual culprit was?

Yeah @Bazza49 this is completely normal and actually worth understanding properly. Two main factors at play in winter: obviously less irradiance and shorter days, but also your panels are actually running more efficiently in cold temps (lower cell temperature = higher Voc). The real killer is sun angle — your panels are seeing the irradiance at a much shallower angle, so effective watts hitting the cells drops significantly.

Worth checking your actual Voc on a clear cold morning — you might be surprised how high it climbs. Just make sure you're not nudging your 100/30's input limits.

The bigger win for winter is simply tilting your panels steeper if you can manage it — even going from flat to 60° can make a noticeable difference this time of year in the UK. What's your current panel angle?

Just to add to what @SilverWarden is covering — don't overlook the panel angle either, @Bazza49. Low winter sun means you're often catching it at a really poor angle if your panels are flat or on a shallow pitch. Even tilting them up to 60-70° can make a noticeable difference to those winter figures. I went from desperately scraping 8-10A on dull January days to consistently hitting 15A+ just by propping mine up steeper. Dead easy to do if you're not on a fixed roof mount. Your Victron kit itself will be fine — those SmartSolars are rock solid. Check your history graphs in the VictorConnect app and you'll likely see the MPPT itself is performing perfectly well given what it's actually got to work with.

Good points from @SilverWarden and @Thommo53 already. One thing worth adding specifically for your setup: two 200W Risen in series means your Voc will be roughly 80-90V in cold weather — panels run significantly higher voltage when cold. Your 100/30 handles that fine (100V max input), but worth confirming with Victron's MPPT calculator that you're not nudging the ceiling on a frosty morning, as over-voltage will cause the controller to shed power or worse.

Also worth checking your Battle Born's charge parameters in VictronConnect — lithium at lower temperatures may be throttling charge acceptance on the battery side rather than the MPPT itself, which can look identical in the stats. Battle Born cells don't want to accept full charge below about 0°C.

Have you got the Bluetooth history graphs? They'll show whether it's input-limited or output-limited.

Great thread this. @Bazza49 one thing nobody's mentioned yet — have you checked your battery temperature? Battle Born LiFePO4 cells have built-in low-temp protection and will actually throttle incoming charge current when it's cold, even if the BMS doesn't completely cut off. If your battery's sitting in an uninsulated space in a van or outbuilding, it could be restricting what the Victron will push in regardless of what the panels are producing.

Worth having a look in VictronConnect at the charge current graph over a typical winter morning — if you see it climbing slowly rather than jumping straight to bulk, cold batteries are likely your culprit as much as anything else. A bit of insulation around the battery makes a surprising difference up here in Yorkshire through December and January!

Good thread. One thing I'd add that hasn't come up yet — have a look at your Voc on cold mornings through the VictorConnect app. In winter, panel voltage actually rises with the cold (panels are more efficient at lower temps), so you might find your array pushing closer to or even briefly over your MPPT's input ceiling on a crisp clear morning. Worth logging a few days' data to see what's happening at the top end. If your Voc is nudging the 100V limit of the 100/30, the controller could be throttling back to protect itself. Not saying that's your issue, but it's a quick thing to check before assuming the drop is purely down to sun angle or battery temp as @YorkshireExplorer mentioned.

Really good shout from @PanelRuss on the Voc point — worth stressing that on a frosty morning your open-circuit voltage can spike quite significantly above the rated spec. With 2x200W Risen in series you want to double-check your combined Voc doesn't creep uncomfortably close to that 100V input ceiling on the SmartSolar 100/30. Most panels are rated at 25°C STC, and Voc rises as temperature drops — roughly 0.3-0.4% per °C depending on the panel's temperature coefficient.

I had a similar winter head-scratch on my narrowboat setup before I properly understood the maths. Worth pulling up the Risen datasheet and doing the cold-temperature Voc calculation properly rather than assuming summer figures are representative. If you're marginal, that could also be causing occasional protection cutouts which would obviously tank your daily harvest figures without it being obvious why.