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

Mine did the same thing on the boat last autumn — spent three weeks in Scottish waters watching it have an existential crisis every time a cloud rolled in, which in Scotland is roughly every four minutes.

Switched over to a Victron SmartSolar 100/30 and honestly it's a different world — the thing tracks voltage changes like it's actually paying attention rather than having a little nap between gusts of sunlight.

The Renogy isn't terrible in full sun, but "full sun in the UK" is doing a lot of heavy lifting as a phrase, isn't it.

A few things worth checking before you bin it though:

• Firmware updated? Renogy quietly pushed a fix a while back that helped with rapid irradiance changes
• Panel string voltage — if you're right at the bottom of the MPPT operating range on dull days it'll hunt around like mad
• Cable runs — any significant voltage drop and cloudy-day performance tanks fast

I've also heard the 40A specifically is worse than their smaller units for this, something about the sweep algorithm, though I can't confirm that's gospel.

What panels are you running with it? Sometimes the combination matters more than people realise — my mate's shepherd's hut setup had similar gremlins until we figured out his panels had badly mismatched Voc tolerances from two different batches.

Anyone else on here made the jump to Victron and not looked back, or found a fix that actually keeps the Renogy happy in proper British drizzle conditions?

@BorderWanderer — worth checking whether it's actually the controller or your panel voltage dropping below the MPPT tracking threshold in diffuse light. Renogy's 40A unit has a notably high "start voltage" spec compared to Victron's equivalent, which means it can genuinely lose lock and restart rather than just throttling output gracefully.

On my narrowboat I swapped ours for a Victron SmartSolar 100/30 and the difference in low-irradiance behaviour was immediately apparent — it'll track right down into deeply overcast conditions where the Renogy was either hunting or sitting idle entirely.

Before condemning the unit, log the PV input voltage during those cloud events. If it's regularly dipping below ~12-14V on a 12V system, the panel configuration might be the actual culprit rather than the controller itself.

Mine threw a similar wobbler in my van conversion last winter — turned out the Renogy was perfectly fine but my panel connections had more resistance than a teenager asked to do chores, so worth ruling that out before you blame the MPPT itself.

@GlenSimon raises a decent point about connections — that's where I'd look first too.

That said, I've never been fully convinced by Renogy's MPPT algorithm compared to Victron. Had a 40A Renogy on my shepherd's hut build for about eight months and swapped it out for a Victron SmartSolar 75/15 (smaller setup, different beast entirely). The Victron just tracks better in marginal conditions — you can actually watch it hunting for the optimal point in the Bluetooth app during cloud cover.

Not saying the Renogy is broken, but "flaky on cloudy days" might just be... it working as designed, and the design being a bit agricultural compared to the competition.

What's your panel wattage and battery voltage? If you're running a marginal system already, a less sophisticated tracker will absolutely struggle more when irradiance drops suddenly.

@DefenderAdventure is on the right track re: tracking voltage, but there's another angle nobody's mentioned — the Renogy 40A has a notoriously low "start voltage" threshold compared to something like a Victron SmartSolar. On overcast UK days your panels might be generating just enough to tickle the controller awake, then it drops out, restarts, loops endlessly.

I had exactly this in my shepherd's hut setup with a cheap MPPT before I moved to Victron. The fix (short-term, before you bin it) is to check your VOC under cloud cover and compare it against the controller's minimum PV start voltage in the manual. Renogy's documentation on this is genuinely poor.

Also worth checking: is the unit running hot? These controllers throttle badly without adequate ventilation, which compounds the instability under variable irradiance.

@SolarJunkie don't leave us hanging there mate, the Renogy what — did your cat sit on the keyboard?

That said, from my motorhome experience the Renogy 40A can get a bit dramatic in low-light if your panel voltage is dancing around the MPPT's minimum threshold — mine sorted itself out once I wired two panels in series rather than parallel, pushed the Voc up nicely. Cloudy days went from existential crisis to mild inconvenience. Victron would handle it more gracefully but costs roughly one kidney, so here we are.