Victron MPPT keeps dropping out on cloudy days - normal or faulty?

Had this exact issue last winter with my 100/20. Drove me mad for weeks thinking it was faulty.

Turns out it's pretty normal behaviour — when irradiance drops really low the MPPT can't maintain a stable operating point and just resets rather than hunting around inefficiently. Victron's algorithm is actually being sensible rather than broken, annoyingly.

Few things worth checking though:

• Panel voltage — if it's dropping below the controller's minimum start voltage on overcast days, that's your culprit
• Wiring losses — thin cable run will make dropout worse when current's already marginal
• VE.Direct logs — worth pulling the history if you've got a Cerbo or just the app connected, you can see exactly what the unit was doing before each drop

What's your panel setup? If you're running a smallish array the dropout threshold becomes more noticeable. I added a second panel in series last spring and the cloudy-day behaviour improved noticeably — higher voltage means the MPPT locks on more reliably even when watts are low.

Also check your firmware is current. There were some MPPT algorithm tweaks in updates from a couple of years back that helped stability in low-light conditions.

Genuine faults do happen but dropout on grey UK days is usually just physics being miserable. What does VictronConnect show for your array voltage when it cuts out?

Mine did the same on the garden office install last February — three weeks of assuming the Victron was dying, turned out it was just Yorkshire doing Yorkshire things with the sky.

Worth checking your absorption voltage threshold in VictronConnect though — if it's set too high the controller basically throws its hands up and quits when there's not enough grunt to hit it on a grey day.

Dropping the float/absorption gap helped mine stop having what I can only describe as a solar-powered sulk every time a cloud passed over.

@LB_Camper glad you cracked it — would've saved me three weeks of forum rabbit holes and one very unnecessary call to my installer.

Brilliant thread this — glad it's getting some traction because it catches out so many people new to solar 🙌

Worth adding: if you're monitoring via VRM, the "No input power" events in the history log will stack up on those grim grey days and look absolutely terrifying at first glance. Totally normal though.

I saw the same on my boat install last November — three weeks of solid overcast and the logs looked like the MPPT was having a breakdown. Wasn't. Just British winter doing its thing 😅

@LB_Camper welcome to the forum by the way — great first post, this exact question must come up every October like clockwork! Stick around, loads of good knowledge here.

One practical tip: check your absorption voltage is being reached at least occasionally. If it genuinely never hits target voltage even on decent days, then start worrying.

Good point from @LB_Camper. Worth adding that the dropout threshold on Victron MPPTs is configurable via VictronConnect — the "Start voltage" and "Tail current" settings are often overlooked.

On my off-grid setup I adjusted the rebulk voltage offset down slightly, which reduced the frequency of unnecessary restarts during patchy winter cloud cover. Made a noticeable difference.

Also worth checking your panel VOC at low irradiance against the controller's minimum operating input — if you're running marginal panel sizing for a 12V system the MPPT simply won't see enough voltage headroom to initialise properly in diffuse light.

Log the data in VRM if you have connectivity — the pattern of dropouts tells you everything. Random restarts suggest a genuine fault; regular dropouts around dawn/dusk or heavy overcast are almost certainly just physics doing its thing.

Yeah had this on my narrowboat setup last winter — absolute nightmare trying to diagnose it when you're living aboard and the batteries are dropping.

@Bazza60 is spot on about VictronConnect, but also worth checking your panel voltage at low light. If it's barely scraping above your battery voltage the MPPT just gives up — it's not broken, it's doing the right thing mathematically.

My fix was adding a second 100W panel in series. Bumped the Voc enough that even on a grim grey January day it had something proper to work with. Marginal cost, massive difference in behaviour.

Also — absorption/float thresholds matter here too. If your batteries are near full already the controller will drop in and out by design anyway.

Good shout from @Bazza60 on the VictronConnect settings — worth also checking your panel voltage vs battery voltage ratio while you're in there. I had a period last autumn where my 175W panel was barely pushing above the absorption voltage on grey days, so the controller just kept hunting around rather than dropping cleanly.

Also worth logging a few days of data via the VRM portal if you're connected. Took me a while to bother setting it up on my static but it made diagnosing this sort of thing so much easier — you can actually see the irradiance curve against output and spot exactly where it's losing the plot.

Battery state of charge matters too — a nearly full bank will cause the MPPT to throttle right back, which can look like a dropout if you're just glancing at the panel indicator.