Anyone else's MPPT completely losing the plot on cloudy UK days?

My Victron SmartSolar 100/30 seems to think a grey February sky in the Midlands is basically a solar eclipse — drops to practically nothing even when there's clearly some diffuse light getting through.

Running two 200W Renogy panels in parallel and on a decent day I'd expect at least 40–60W from overcast conditions, but I'm lucky to see 8W on the display. Battery's a Fogstar 100Ah LiFePO4 sitting at 70% so it's not like it's refusing a charge.

Is this an MPPT tracking issue, a wiring gremlin, or is the British weather just genuinely that tragic? Bonus points if you've actually fixed it rather than just moved to Spain.

@MultiPlusFan this is almost certainly a combination of your absorption/float thresholds being set too high for winter conditions and the MPPT's algorithm being overly conservative when array voltage is unstable.

On overcast days here in the islands, diffuse irradiance can sit around 50–80W/m² and the SmartSolar genuinely struggles to "lock on" cleanly — particularly if your panels are marginal voltage spec for that controller.

Worth checking:

• Panel Voc in cold diffuse conditions — counterintuitively panels run cooler on overcast days, pushing Voc higher, which can confuse sweep algorithms
• Battery state — if you're near full, absorption termination kicks in aggressively
• VictronConnect app → check the yield graph hourly resolution, the controller may actually be harvesting more than the live readout suggests

My setup (3kW east/west split feeding a cabin and EV charger) showed identical symptoms until I dropped absorption voltage 0.2V. Made a noticeable difference.

Yeah, had exactly this with mine last winter. Worth checking your panel VOC against what the controller's actually seeing in those conditions — diffuse light can drop your array voltage significantly, and if it's hovering around the MPPT's minimum startup threshold it'll keep cutting in and out rather than settling into a proper tracking cycle.

Also, have you got the BatteryLife algorithm enabled? On a partially discharged battery during a dull day it can actually throttle input quite aggressively trying to protect the bank. Sometimes disabling it temporarily just to diagnose whether that's the culprit is worth doing.

What's your battery bank — lithium or lead? Makes a difference to how the controller behaves in these borderline conditions. @IslandVanLifer makes a fair point about thresholds too, both things could be happening simultaneously.

Been there with my boat setup — February diffuse light is genuinely brutal for MPPT efficiency.

One thing neither @IslandVanLifer nor @ILK_Marine mentioned: check your battery temperature compensation. If your Victron isn't reading battery temp correctly (or the sensor's disconnected), it'll apply default compensation that can actually raise your thresholds in cold conditions, making the controller think it needs stronger input before committing to bulk charge.

Also worth enabling the "street lighting" or low-light optimisation in VictronConnect if you haven't — some firmware versions have it buried under the load settings.

My SmartSolar 100/20 on the shepherd's hut struggled similarly until I dropped the absorption voltage by about 0.3V for winter and suddenly it started harvesting those grey-sky mornings properly. Small tweak, noticeable difference.

Worth adding — the panel orientation makes a huge difference in diffuse conditions that nobody's mentioned yet. My array is tilted at 52° (roughly matches Welsh latitude) and I get noticeably better harvest on overcast days than when I had it at the "summer optimal" 35°. Steeper angle catches that low, scattered light better.

Also check your MPPT's minimum input voltage threshold relative to your actual VOC under cloud — diffuse light can drop panel voltage surprisingly close to the controller's wake-up point, causing it to constantly drop in and out rather than tracking properly.

The Victron app's history graph is gold for diagnosing this — look for that characteristic "sawtooth" pattern on cloudy days which usually points to the controller hunting rather than a genuine lack of available power.

Has anyone actually checked what their panel tilt angle is doing over winter? I've got fixed mounts on my static caravan and I wonder if the combination of low sun angle plus diffuse light is basically the worst of both worlds — the MPPT is fighting poor irradiance AND poor angle simultaneously.

Is there a meaningful difference between, say, 30° and 45° tilt for February in the UK? I've been meaning to adjust mine but never got round to it.

Also — does the Victron app actually log enough granular data to tell you whether it's a threshold issue (as @IslandVanLifer suggests) versus just genuinely terrible input? I can never quite tell from the history graphs whether the controller is behaving oddly or just reporting accurate misery.