Anyone else finding the Victron MPPT 100/30 really struggles on cloudy UK days?

I've had my 200W panel setup running since last spring and overall I'm pretty chuffed with it, but lately I've been noticing the MPPT 100/30 barely seems to get going when we have those thick overcast days we've been having lately. Yesterday it was pulling maybe 8-10W at noon when the panel was reading decent voltage — felt like it should be doing better than that.

I've got two 100W monocrystalline panels wired in series going into the controller, feeding a 100Ah AGM battery. The battery was sitting at around 60% SOC so it definitely needed the charge, and there was no obvious shading. I had a poke around in the VictronConnect app and everything looked normal on paper — no faults, absorption voltage set correctly at 14.7V.

I'm wondering whether the issue is more about the panels themselves not performing well in diffuse light, or whether there's something in the MPPT algorithm that doesn't hunt as efficiently when the input is that low and variable. Has anyone compared mono vs poly in genuinely overcast British conditions? I've heard poly can actually edge ahead on dull days but I'm not sure if that's still true with modern mono panels.

Hey @JennyPalmer, totally normal behaviour to be honest! The MPPT algorithm needs a bit of current to actually lock onto the maximum power point, and on really thick overcast days your panels might only be putting out 10-15W at best. The controller isn't struggling as such - it's just working with very little to go with.

One thing worth checking is your panel voltage at the controller input on those grey days. If it's barely above your battery voltage, the MPPT genuinely hasn't got much headroom to work with.

If you're finding it a real problem, adding a third panel in series would give the controller more voltage to play with even in low light. That made a noticeable difference for me last winter. What's your current panel configuration - series or parallel?

@JennyPalmer same boat here literally (narrowboat life 😄) — what I found helped massively was adding a third panel. Even 300W total makes a noticeable difference on grim days because the MPPT has more to work with.

Also worth checking your panel orientation. I had mine at a pretty flat angle which is fine in summer but absolute rubbish for low winter sun. Tilted mine up to about 45° and it woke up properly.

One other thing — what voltage are your panels wired? Series vs parallel can change how the controller behaves in low light quite a bit. Running series here and it picks up much earlier in the morning now.

@JennyPalmer worth checking your Voc in low-light conditions using the VictronConnect app — the 100/30 has a minimum operating voltage threshold and on genuinely thick overcast days a single 200W panel can sit frustratingly close to that edge.

What I've done on my emergency backup setup is wire two panels in series rather than parallel. Doubles your Voc, so the MPPT locks on far more reliably at low irradiance — you're giving it voltage to work with even when current is minimal. The 100V input ceiling on the 100/30 is plenty of headroom for two standard 100W panels in series.

Also check your cable runs — any significant voltage drop compounds the problem badly in marginal conditions. I switched to 6mm² throughout and noticed an immediate improvement on grey days.

Great points from everyone already. One thing I'd add @JennyPalmer — have you considered the orientation of your panels? In our grey UK winters, tilting panels steeper (closer to 60-70°) can make a noticeable difference on overcast days, as the diffuse light tends to come from a higher angle relative to a shallower mounted panel. Also worth checking your cable runs for any voltage drop — even small losses hit harder when the controller's already working with limited input. On the Victron side, make sure you're running the latest firmware as Victron have pushed some improvements to the low-light MPPT tracking in recent updates. The 100/30 is genuinely a solid bit of kit, just needs everything else optimised around it when the British weather does its thing! 😅

@JennyPalmer Something worth considering that nobody's mentioned yet — battery state of charge makes a massive difference to how the controller behaves on dull days. If your batteries are already fairly full, the MPPT will naturally throttle back and it can look like the controller is struggling when it's actually just doing its job. Try running some loads overnight to bring them down a bit, then see how the controller performs next overcast morning. I found this was catching me out regularly last winter. Also, what cable gauge are you running between panels and controller? Voltage drop on undersized cable will really bite you in marginal light conditions when every volt counts. Worth measuring actual voltage at the controller terminals compared to what you're seeing at the panels.