Anyone else finding the Renogy 40A MPPT a bit erratic on cloudy days?

Had the same drama with mine last spring — turns out it was having more of an existential crisis than I was during lockdown.

Swapped it out for a Victron SmartSolar 100/30 and honestly the difference on a grey British sky is night and day (ironic, given the lack of actual day). The Victron just hunts for whatever scraps of photons are knocking about, whereas the Renogy seemed to give up the moment a cloud looked at it funny.

A few things worth checking before you bin it though:

• PV array voltage — if it's dropping too low on overcast days, the MPPT might be struggling to even wake up properly
• Firmware — Renogy did push some updates that apparently helped with low-light tracking, though I'll be honest I never noticed much improvement myself
• Cable losses — undersized DC cable between panels and controller will murder your efficiency when input is already marginal

The van I run now has a pair of Fogstar 100Ah lithium batteries and the Victron combo, and even a proper miserable November in Wales pulled enough to keep the fridge ticking over, which feels like witchcraft.

Anyone else made the switch away from Renogy for cloudy-climate use, or found a fix that actually worked? Genuinely curious whether I jumped ship too early or whether it's a known weak spot with that 40A unit specifically.

@RiverFinn mine did the same on my static — turned out the Renogy was basically napping every time a cloud drifted past, whereas my Victron SmartSolar just gets on with it like a proper northerner in a drizzle. 🌧️

@RiverFinn @MariaJones yeah the Renogy "napping" thing is real — had one on the boat before I switched and it drove me absolutely mental on those grey November days where you're getting some juice but the controller just... gives up?

Victron SmartSolar sorted it for me too. The MPPT algorithm actually hunts properly in low light rather than throwing a strop. Night and day difference honestly.

Only gripe is the Victron costs about double but when you're relying on it in winter with 4hrs of weak sun you can't afford a controller having an identity crisis every 20 minutes 😤

Worth checking your panel wiring too before swapping — had a dodgy MC4 connector making mine worse than it needed to be.

@RiverFinn had a Renogy 40A on my shed setup for about eight months — the cloudy day behaviour was infuriating. It wasn't just slow to recover, it'd sometimes lock onto a completely wrong voltage point and just sit there looking pleased with itself.

The Victron's MPPT algorithm is genuinely in a different league for diffuse light. UK winters basically demand something that can squeeze every watt out of grey skies.

One thing worth checking before you swap though — make sure your panel Voc isn't borderline for the controller's input rating. Had a mate convinced his Renogy was faulty when it was actually just poorly matched to his array.

@RiverFinn same pattern on my boat build — Renogy would lose track completely then take ages to re-sweep and find the MPP again after cloud cover. Victron's algorithm is noticeably quicker at re-acquiring.

One thing worth checking: panel voltage at the controller under overcast conditions. If it's dropping below the Renogy's minimum tracking threshold that'll make it worse. I had a marginal string voltage situation that amplified the problem.

Switched to a Victron SmartSolar 75/15 on the smaller array and the Bluetooth logging made it dead easy to see exactly what was happening during those patchy days. Data doesn't lie.

Really interesting thread — and completely matches what I've seen on the narrowboat over the years.

One thing worth adding: the Victron SmartSolar's low-light MPPT algorithm is genuinely in a different league. Even on a proper grey British winter day it'll still be nibbling away at whatever's available. The re-sweep issue @BatteryRay mentions is the killer — you're losing productive minutes every time it has to rediscover the curve.

Also worth checking your panel wiring for any loose connections before assuming it's purely the controller — intermittent resistance can mimic exactly this behaviour and I chased that rabbit for weeks once! 😅

@RiverFinn — good call switching when you did. The SmartSolar paired with the Victron Connect app gives you proper visibility into what's happening, which alone is worth the price difference over budget units.

Had exactly this on the nb. Perseverance last winter — grey February sky, Renogy sat there looking at my 200W panel like it had forgotten what electricity was. I swear it needed a motivational poster.

What nobody mentions is that the erratic behaviour gets worse when you've got partial shading from a bridge or overhanging tree —