Anyone else noticed MPPT efficiency dropping in winter low-light conditions?

Running a Victron SmartSolar 100/30 with 400W of panels (two 200W Renogy monos in series) and I've been logging the data through VRM all this winter. On overcast days where the panels are sitting at maybe 80–120W input, I'm seeing the MPPT conversion efficiency drop noticeably compared to summer — sometimes only 88–91% rather than the 96–97% I'd expect from the spec sheet. Ambient temps are around 4–8°C here in the array, so thermal losses on the controller side shouldn't be the issue.

My working theory is it's related to operating point instability — at very low irradiance the IV curve flattens out and the MPPT algorithm struggles to find a stable peak, so it's hunting and wasting a chunk of what little energy there is. I've also wondered whether the 12V nominal battery voltage (Fogstar Drift 100Ah LiFePO4, currently sitting at 13.2–13.4V resting) creates a wider voltage conversion ratio that hits efficiency harder at low input wattages.

Has anyone done proper back-to-back comparisons between different MPPT controllers in these conditions? Curious whether something like the Victron 75/15 with its smaller conversion ratio would actually outperform the 100/30 on a small system during British winters, or whether I'm chasing ghosts in the data.

@Ozzy yes, noticed exactly this with my 150/35 setup. Worth checking your panel voltage-to-battery voltage ratio on those grey days — if the input voltage is barely clearing the minimum operating threshold, the MPPT algorithm doesn't have much headroom to do its job properly.

One thing that genuinely helped me was re-angling my panels steeper in November — from 30° up to around 55°. Even diffuse light has a directional bias in winter and you pick up a surprising amount more.

Also check your absorption/float voltage settings aren't cutting the charge short prematurely when the panels are already struggling. VRM will show you if it's dropping out of bulk charge too early.

What's your battery bank — lithium or lead? Makes a difference to how aggressively you can configure the charge profile.

Same MPPT controller here (100/30) on my shepherd's hut build. One thing I'd add — on really flat grey days I've found the controller barely gets out of bulk phase before light drops off, so the efficiency looks terrible but it's partly just the tiny harvest window.

Worth checking your Voc drops off more than rated in cold damp conditions too. My Renogy panels actually perform slightly better in cold bright spells (higher Voc) but on thick overcast the string voltage can dip close to where the MPPT algorithm struggles to find the curve properly.

@LizStewart makes a good point on that voltage ratio — if your battery is near full and panels are weak, the controller's just not got much to work with.

Great thread @Ozzy. One thing worth checking that I don't think's been mentioned yet — have a look at your absorption and float voltage settings relative to ambient temperature. Most MPPT controllers have temperature compensation built in, but if you're not using a battery temperature sensor the controller's just guessing based on its own internal temp, which in a cold shed or outside enclosure can be miles off.

On my setup I noticed the controller was actually over-charging slightly on cold mornings because of this mismatch, which paradoxically made the overall harvest figures look worse over the day. Adding a proper battery temp sensor sorted it. Victron's own sensor is a few quid and clips straight on — genuinely worth it for winter running. Might not be the whole answer but worth ruling out before digging deeper.