Victron MPPT keeps dropping out on cloudy days — is this normal?

Seen this exact behaviour with my SmartSolar 100/30 last winter up in Scotland — overcast days are basically a stress test for MPPT algorithms.

The short answer is: yes and no. Some dropout is completely normal when irradiance is dancing all over the place, but persistent dropout suggests something worth investigating.

A few things worth checking:

• Panel voltage at the controller input — on heavy overcast, are you actually getting above the minimum threshold to wake the MPPT? Victron units need a meaningful margin above battery voltage to engage properly
• Absorption voltage settings — if your battery is already sitting near full from yesterday, the controller will throttle right back and look like it's dropping out when it's actually just satisfied
• Wiring resistance — a marginal connection that's fine at peak output will show its ugly face under low-irradiance conditions when every watt counts
• Panel configuration — series vs parallel matters enormously on cloudy days. Series raises voltage (good for MPPT engagement), parallel raises current

The Victron Connect app is your best friend here. Pull up the live graphs on a dodgy day and watch what the input voltage and current are actually doing. If you're seeing the voltage collapsing, that's a panel/wiring issue. If the controller just isn't "waking up", check your absorption/float settings — Victron's defaults are sometimes oddly conservative depending on firmware version.

What panels and battery chemistry are you running? That'll change the diagnosis considerably. Lithium and lead-acid behave very differently when the sky turns British.

Mine did the same last winter parked in a layby near Keswick — Victron's algorithm basically has an existential crisis when the panels can't decide if they're generating 40W or 4W every thirty seconds.

Worth checking your absorption voltage threshold in the VictronConnect app — if it's set too high for your battery state, the controller gives up and restarts the search cycle rather than holding a sensible tracking point.

@DailySolar has the right idea calling it a stress test; I'd also add that shading from a single cloud edge hitting one cell can tank the whole string more than people expect — serious argument for a parallel wiring setup if your panel count allows it.

@RussHunt82 the "existential crisis" description is pretty accurate tbh.

Worth checking your absorption voltage threshold — if it's set too high for your battery chemistry, the MPPT will hunt constantly in low irradiance because it can't sustain the voltage long enough to satisfy the algorithm.

I've got a SmartSolar 150/35 paired with a Fogstar Drift 100Ah LiFePO4. On proper grey UK days I dropped my absorption voltage by about 0.2V and the dropouts almost entirely stopped. The controller stopped chasing a target it couldn't realistically hit.

Also check your panel voltage under load vs open circuit — if your array is marginal for the controller's input threshold in overcast conditions, you're right on the edge of the operating window.

VictronConnect's detailed history graph will show you exactly when and why it's dropping. Worth pulling that data before assuming hardware fault.

Had this exact faff with my garden office setup last January. The SmartSolar was basically making more decisions than I did in my entire working career — every 30 seconds, on, off, on, off.

Worth checking your panel wiring too. Loose MC4 connectors act up something rotten in low-irradiance conditions — voltage marginal enough that

@RetiredElectrician that "more decisions than I" cliffhanger is killing me 😄

Had similar bother with my 100/50 during that grey spell we had February. What sorted it for me was tweaking the tail current setting down in VictronConnect — factory default is often too aggressive for low-irradiance conditions and it keeps bouncing in/out of absorption.

Also worth checking your panel voltage at the controller input on a dull day. If it's barely scraping above your battery voltage, the MPPT's got nothing to work with regardless of settings.

Fogstar batteries behave differently to lead-acid here too — if you're running lithium the thresholds need proper adjusting or you'll see exactly this.