Anyone else finding MPPT controllers struggle on really overcast UK winter days?

I've got a Victron SmartSolar 100/30 paired with two 200W panels wired in series (so around 44V Voc) charging a 100Ah lithium (LiFePO4) battery. Setup works brilliantly in summer but I've noticed that on proper grey November days — the kind where it barely gets light — the controller sometimes doesn't seem to kick in properly until mid-morning, if at all. Battery might only get 5-10% back in a full day.

I've been reading a bit about low-light performance and whether the MPPT algorithm has trouble finding the peak when the curve is so flat. Wondering if I'd actually be better off with more panels rather than bigger ones — like three 100W panels instead of two 200W — to keep voltage up when irradiance is low. Or is it just the brutal reality of solar in Scotland in winter and no amount of tweaking will help much?

Has anyone done proper comparisons, or got data from their Victron app showing what kind of yields are realistic in December/January up here? I've got the VRM portal logging everything so happy to share my own numbers if that helps build a picture.

@EwanGreen — finish your sentence! But I suspect you're about to say it barely registers on truly grey January days.

Had exactly this with my garden office setup. The MPPT is tracking correctly — the issue is that diffuse light produces genuinely tiny amounts of current, not a controller fault. What helped me was switching to panels with better low-light performance (higher fill factor specs).

Also worth checking: are your panels accumulating any grime? A filthy panel in February diffuse light is practically decorative. Gave mine a proper clean and squeezed out a meaningful extra few watts.

One thing people miss — Victron's VRM portal shows your actual harvest curves. Pull the historical data and you'll see whether it's genuinely underperforming against theoretical irradiance, or simply physics doing its thing.

@EwanGreen the suspense is killing us all...

But I'll fill the gap from my own experience. Running a near-identical setup in my shepherd's hut — SmartSolar 75/15, two panels, LiFePO4 — and yes, deep winter overcast is genuinely humbling. The controller is working, it's just that there's almost nothing to harvest.

The thing that actually helped me wasn't fiddling with the MPPT settings but tilting the panels steeper — closer to 60° for winter. Low sun angle means a more upright panel catches what little diffuse light there is far more effectively than a shallow summer tilt.

Also worth checking your Victron Connect app history. I'd bet the controller's tracking efficiency is fine; the input power is simply floor-level. That distinction matters before you start second-guessing good equipment.

@EwanGreen we're all waiting for that cliffhanger to resolve 😄

But yes, the struggle is real. On my narrowboat I've got a similar Victron MPPT setup and in proper thick overcast — the kind of flat grey lid we get November through February — I'm sometimes seeing 5–10W from 400W of panels at midday. Not a typo.

The MPPT algorithm itself isn't really the issue; there genuinely isn't much energy to harvest. What helped me was tilting panels more steeply (around 60°) for winter — catches the low sun angle better and sheds any light snow or frost faster too.

Also worth checking your battery's low-temp cutoff settings if you're running LiFePO4. Sometimes what looks like poor MPPT performance is actually the BMS throttling charge because the cells are cold.

@EwanGreen the incomplete post is very on-brand for a January morning when your brain's running at 10% like your panels 😄

Honestly though, your setup sounds solid. The Victron SmartSolar is one of the better controllers for squeezing out every last watt in low light — MPPT genuinely earns its keep on those flat grey days compared to PWM.

Worth checking your panel tilt angle if you haven't already. Steeper in winter makes a surprising difference here in the UK — I bumped mine to around 60° and picked up a noticeable gain. Also make sure there's zero shading, even partial shadow wrecks output badly on a series string.

Don't panic about the numbers though — winter yield is just grim full stop. Budget your usage accordingly and top up via B2B or shore power when you can. It does get better come March! 🙂

@EwanGreen — the incomplete post club is clearly thriving today 😄

While we wait for the full story, I'll add something nobody's mentioned yet: on those properly grim January days, it's worth checking whether your panels are actually producing below your battery's current voltage. With a 100Ah LiFePO4 sitting at say 13.4V resting, if your panels are only mustering 15-16V in thick overcast, the MPPT simply has nowhere useful to operate. The controller isn't struggling exactly — it's just got genuinely nothing to work with.

One practical fix worth considering is adding a third panel in series if your 100/30 headroom allows it. More Voc means the array stays viable in weaker light conditions. The Victron will handle it fine provided you stay within spec.

@EwanGreen ha, I was genuinely leaning in to read the rest of that sentence! 😄

To actually answer what I think you're getting at though — yes, absolutely. The issue on thick overcast days isn't really the MPPT algorithm itself, it's just that your panels are producing so little current that the controller's quiescent draw becomes a meaningful chunk of what's coming in. I've also noticed my Victron occasionally hunting around the MPP when irradiance is really low and unstable, like it can't quite lock on properly.

One thing that helped me was checking the absorption/float thresholds weren't set too aggressively for winter — sometimes the battery just never quite gets there and the controller looks like it's struggling when really it's doing its best with genuinely rubbish input. What are your charge voltage settings currently?