Anyone else noticed their MPPT acting weird during mixed cloud/sun days in the UK?

My Victron SmartSolar 100/30 has been throwing a proper tantrum lately — hunting between absorption and bulk every few minutes when we get that classic British "sun, cloud, sun, cloud, oh wait now it's raining" weather. Garden office setup, 400W of Renogy panels on the roof, 200Ah Fogstar Drift lithium.

Voltage is bouncing between 26V and 28.8V like it can't make its mind up, and the VictronConnect app just looks like a seismograph reading. Panels are clean, wiring's solid, battery's healthy — fairly sure it's just the algorithm struggling with the rapid irradiance changes rather than a fault as such.

Anyone tweaked their absorption voltage threshold or played with the tail current settings to smooth this out, or is this just par for the course on a rainy Tuesday in November? Wondering if there's a sweet spot in the settings that stops it flip-flopping constantly.

StuThompson | 847 posts | ⚡ Solar Enthusiast

@BitsAndBobs totally normal behaviour mate, your controller is just doing its job honestly. When irradiance drops suddenly, battery voltage sags back below the absorption threshold so it kicks back into bulk — then the sun returns and it shoots back up again. It's not really "hunting" so much as accurately responding to genuine voltage changes.

Worth checking your cable sizing and connections though — any resistance in the circuit will exaggerate the voltage swings the controller sees. Also have a look at your absorption voltage setpoint; sometimes nudging it down a touch reduces the frequency of these transitions on patchy days without meaningfully affecting charge quality.

The Victron app logs are your friend here — pull up a graph of a cloudy day and you'll see it's probably perfectly rational behaviour once you see the voltage trace alongside it.

WonkyWarden | 1,203 posts | 🔋 Off-Grid Convert

@BitsAndBobs worth checking your absorption voltage threshold and re-bulk voltage settings in the VictronConnect app. If your re-bulk offset is set too tight (default is often 0.4V), the controller will keep dropping back to bulk every time a cloud knocks the voltage down slightly. Try bumping it to 0.8V or even 1.0V — made a massive difference on my setup during our famously "glorious" British summers! Also, if you haven't already, enable the "BatteryLife" algorithm, which handles these fluctuating conditions a fair bit more gracefully. What battery chemistry are you running? AGM and lithium behave quite differently in these scenarios.

CopperSparky | 134 posts | 🔌 EV Charging Nerd

@BitsAndBobs something that helped me was enabling the "BatteryLife" algorithm in VictronConnect — it adapts the charge curve based on actual conditions rather than rigidly chasing fixed voltage targets. Made a noticeable difference on those classic grey-bright-grey October days.

Also worth checking: what's your panel array doing in terms of partial shading during cloud transitions? If some cells are dropping out intermittently, your MPPT is getting contradictory input signals on top of the voltage hunting. I've got a south-facing setup and even a chimney shadow in the afternoon was causing bizarre MPPT behaviour I initially blamed on the controller itself.

What firmware version are you running? There were some stability improvements in recent Victron releases worth checking via the app.

SilverSpanner | 412 posts | 🔧 Hands-On Tinkerer

@BitsAndBobs the hunting you're describing is actually your controller doing exactly what it should — it's just more visible on days like that. One thing worth adding to what's already been said: have a look at your tail current setting in VictronConnect. If it's set too low, even a brief cloud shadow will convince the controller it's finished absorption and it'll drop back to bulk the moment the sun returns. I bumped mine up to around 2-3% of battery capacity and it settled down considerably. Also worth checking your panel wiring connections whilst you're at it — a slightly loose MC4 can cause oddly similar symptoms and it's easily overlooked. That classic British "twenty minutes of everything" weather does make diagnosis tricky, I'll grant you that!

LakelandNomad | 2,847 posts | ☀️ Solar Obsessive

@BitsAndBobs this is bread-and-butter UK solar behaviour, unfortunately. What's often overlooked is the tail current setting — in VictronConnect, under the absorption parameters, you can set a tail current threshold (typically 2-4% of battery capacity in amps) so it transitions to float based on actual charge acceptance rather than just time or voltage alone.

On my boat setup with Fogstar Drift 100Ah LiFePO4s, dialling this in completely eliminated the bulk/absorption hunting on those classic Lakeland "ten clouds a minute" days. The controller stops second-guessing itself when a cloud rolls in.

Worth cross-referencing with your battery manufacturer's recommended charge profile too — Victron's defaults are sensible but not always optimised for every chemistry.

Curly66 | 89 posts | 🏕️ Van Dweller & Tinkerer

@BitsAndBobs I had almost identical behaviour with my 75/15 last autumn — classic Pembrokeshire weather doing its worst! One thing nobody's mentioned yet: check your battery temperature compensation settings. If your controller thinks the battery is colder than it actually is, the absorption voltage target shifts and you'll get that hunting behaviour compounded on top of the irradiance fluctuations. Also worth having a look at your absorption time settings — dropping it slightly can stop the controller getting frustrated trying to hold a voltage it can't maintain when a cloud rolls in. The VictronConnect app lets you tweak this fairly easily. Not a complete fix but it smoothed things out noticeably for me.