Anyone else noticed their MPPT readings go haywire in this weird patchy British summer weather?

Been having a frustrating few days with my Victron SmartSolar 100/30 giving me really inconsistent numbers. Yesterday we had that classic mix of bright sunshine followed by heavy cloud every ten minutes or so, and the controller was bouncing between 180W and basically nothing constantly. Batteries never really got above 80% SOC all day despite it technically being a "sunny" day according to the forecast.

I've got two 200W panels wired in parallel on the roof of my converted Sprinter, feeding into a 200Ah lithium setup. On a solid clear day I'll regularly see 340-360W coming through, so the kit itself seems fine. It's just these patchy days where I feel like I'm losing more than I should — almost like the MPPT can't track fast enough when the clouds are rolling through every few minutes.

Has anyone done any proper comparisons between MPPT controllers for how quickly they re-acquire the power point after a cloud passes? I'm wondering whether my Victron is actually slower to respond than some of the cheaper units, or whether I'm just imagining it because I'm staring at the VictronConnect app too much. Would love to hear if anyone has swapped controllers and noticed a real difference.

Trevor1975 | 847 posts | ⚡ Solar Enthusiast

@AngliaSolar totally feel your pain mate! That stop-start cloud cover is genuinely the worst scenario for MPPTs - the controller is constantly hunting for the new peak power point every time irradiance changes dramatically.

One thing worth checking is whether your Victron is set to "Fast" sweep speed rather than the default. Makes a noticeable difference when conditions are shifting every few minutes. Also have a look at your absorption voltage logs - you'll probably find it's struggling to ever reach proper absorption because a cloud rolls in just as it's getting there.

The SmartSolar app's history graphs are brilliant for diagnosing this properly rather than just watching live readings dance about. Worth exporting a week's data to see the pattern.

Honestly though, sometimes British summer weather just wins! 😅

WonkyDrifter | 1,203 posts | ☀️ Off-Grid Convert

Something worth checking @AngliaSolar - in patchy conditions your MPPT is constantly re-scanning for the true maximum power point as irradiance shifts, so the numbers will look all over the place. That's actually it working correctly rather than a fault.

What I'd suggest is having a look at your VictronConnect history graphs rather than watching the live readings - gives you a much clearer picture of actual harvest across the day. The live display during cloud-edge events is genuinely misleading sometimes.

Also worth noting that cloud-edge effect can briefly push your panel output above rated STC figures, so don't be surprised if you occasionally see higher wattage than you'd expect. The 100/30 handles it well in my experience. What's your battery state of charge doing throughout all this?

AshSeeker | 214 posts | 🔋 Still Learning

Interesting thread — I've noticed something similar on my narrowboat setup with a Victron SmartSolar 75/15. The rapid cloud transitions seem to cause the MPPT algorithm to hunt around for the new maximum power point, which takes a few seconds each time. On a day with clouds rolling through every few minutes, that's a lot of lost efficiency.

Curious whether anyone has experimented with the "Equalization" settings or adjusted the absorption voltage thresholds to compensate? I haven't touched mine yet — slightly nervous about getting it wrong.

Also worth asking @AngliaSolar — are you monitoring via the VictronConnect app? Being able to log the yield data over a patchy day has helped me at least understand what's happening, even if I can't fully fix it.

WattCharlie | 412 posts | 🏕️ Shepherd's Hut Solar

Worth mentioning the panel temperature swings too — when cloud rolls in, panels cool rapidly, then when sun bursts back through you can briefly see Voc spike noticeably higher than your standard STC figures. On my hut setup I've caught the SmartSolar logging short bursts well above what I'd expect from my rated panels.

Also check your absorption voltage isn't triggering prematurely during a bright patch, only for the controller to drop back to bulk when cloud returns. Creates that "haywire" appearance in the VRM graphs but it's actually the MPPT doing its job — just working overtime.

@AshSeeker narrowboat shading from bridges and bankside trees adds another variable on top of the patchy cloud. Might be worth enabling the Equalisation log in VictronConnect to see what's actually happening minute-to-minute.