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

Had this exact behaviour with my 100/20 on the shepherd's hut build last autumn. Turns out it's almost always one of three culprits:

1. Panel voltage dropping below the controller's reconnect threshold
On heavily overcast days, a single 12V nominal panel can sag well below what the MPPT needs to "see" to reconnect. Check your VPanel readings in VictronConnect during the dropout — if it's hovering around your battery voltage plus a few volts, that's your answer.

2. Absorption/float cycling
If your batteries are near full and the sun's patchy, the MPPT will legitimately switch off rather than backfeed. Fogstar Drift LiFePO4 cells are particularly aggressive about rejecting charge near 100% SOC.

3. Wiring resistance
Undersized DC cable between panels and controller causes voltage drop under load, which can trigger the low-panel-voltage cutoff even when irradiance seems adequate.

Worth checking in VictronConnect → History → whether you're seeing actual faults logged or just normal on/off cycling. There's a meaningful difference. A genuine fault will show in the alarm log; normal threshold behaviour won't.

What's your panel configuration and battery chemistry? That'll narrow it down considerably. Also — which specific MPPT model, and have you checked whether the firmware is current? Victron pushed a fix a while back that addressed some spurious dropout behaviour on the 75-series controllers.

Anyone else seen this with partial shading setups? I've wondered whether running two smaller strings into separate MPPTs (rather than one combined string) might reduce this on grey UK days generally.

Hey @WattEd, good summary of the usual suspects. One thing I'd add that catches people out — check your cable run lengths and wire gauge. I've seen undersized cable cause enough voltage drop that the controller sees marginal voltage even when the panels are actually producing decently. Especially on longer runs to shepherd's huts and cabins where people often underestimate the distance. Chuck your figures into a voltage drop calculator before assuming it's a controller fault. Victron kit is generally solid, so if the hardware checks out fine I'd look at the wiring first before sending anything back.

Great points from both @WattEd and @SparkySailor. One thing I'd throw in that often gets overlooked — have a look at your battery state of charge when the dropouts happen. If your bank is already sitting fairly full, the MPPT will deliberately throttle back or disconnect during low-irradiance periods because there's simply nowhere useful to push the current. Victron's absorption and float thresholds can make it look like a fault when it's actually working exactly as intended. Chuck the VictronConnect app on your phone and watch the live data through a cloudy patch — you'll see pretty quickly whether it's genuinely dropping out or just doing sensible power management. The history graph in the app is dead useful for this too.

Worth adding — partial shading on even one cell can kneecap your whole string voltage faster than you'd think, so if you're on a narrowboat or van with stuff mounted near panels (vents, aerials, that rogue seagull), check for shadow paths throughout the day before blaming the Victron. 🐦

Also, the VictronConnect app will show you the absorption/float history and any error codes, which takes all the guesswork out — if you're still poking around in the dark without it, that's your first job. Mine flagged a low voltage warning on my shepherd's hut setup immediately once I actually looked.

Really useful thread, cheers @WattEd. One thing nobody's mentioned yet — have a look at your battery temperature compensation settings if you're running AGM or gel. On colder autumn days the Victron can behave oddly if the compensation coefficient is set wrong for your battery type, pushing the absorption voltage outside the expected range and causing the controller to cycle unnecessarily. Worth connecting a BMV or the SmartShunt via Bluetooth and watching the live data for a few minutes on a patchy day — you'll quickly see whether it's a voltage issue, a current issue, or something in the charge algorithm itself. The VictronConnect app makes this dead straightforward and saves loads of guesswork.