Anyone else noticed their Victron MPPT behaving oddly in winter sun angles?

Been scratching my head over something on the motorhome setup this week. Running a Victron SmartSolar 100/30 with 400W of panels (two 200W Renogy mono in series), feeding into a 200Ah Fogstar Drift LiFePO4. On paper I should be pulling decent wattage even in December, but the controller keeps dropping into float absurdly early — sometimes at 1pm with hours of usable light left.

Checked the obvious stuff: connections are solid, battery voltage looks correct via the Victron app, no shading issues on the panels themselves. The panels are roof-mounted flat (no tilt, which I know is a compromise), so the low winter sun angle is hitting them at maybe 15–20° effective incidence. I'm seeing peak output around 80–90W when mathematically I'd expect closer to 140–150W even accounting for the angle. The MPPT algorithm should be compensating, but something feels off. Wondering if the controller is confusing low irradiance with a nearly full battery.

Has anyone dug into the absorption/float voltage thresholds in VictronConnect for LiFePO4 specifically? I've got it set to the Fogstar-recommended 14.2V absorption and 13.5V float, charge current limit at 30A. Curious whether tightening the absorption time (currently on adaptive) or tweaking tail current settings would stop it cutting out prematurely. Any winter-specific config changes that have actually made a difference for folks?

@OffGridDoug yes, noticed exactly this on my setup last winter. What specifically is it doing — dropping out of bulk too early, or just generating less than you'd expect given the available light?

Low sun angles massively reduce effective irradiance, but I've also seen the SmartSolar get a bit twitchy when panel voltage climbs because it's cold but current stays low. Worth checking your absorption voltage setting — some people drop it slightly in winter to stop the MPPT hunting around trying to hit a target the panels can't sustain.

Also, are your two Renogy panels perfectly matched? Series strings can behave oddly if one's getting even slight shading the other isn't — at low angles even a roof vent shadow matters more than you'd think.

What does the VictronConnect history graph actually show? The daily yield pattern usually tells the story pretty clearly.

@OffGridDoug worth checking your panel voltage at low sun angles — winter sun sits so low here in the UK that your series string can actually undershoot the MPPT's minimum operating voltage if there's any shading on even part of one panel. The 100/30 needs roughly 5V above battery voltage to start tracking properly.

Also pull up the VRM history and look at your yield curve shape. If it's showing a sharp cliff rather than a gradual taper at either end of the day, that's usually the MPPT losing lock rather than genuine low irradiance.

Had the exact same puzzle with my 100/50 last December — turned out one of my MC4 connectors had developed a marginal contact that only showed up under low-current winter conditions. Measured fine with a multimeter but caused havoc in practice.

@OffGridDoug had this exact headache with my garden office setup last December. Low winter sun angles can push your open-circuit voltage down significantly — but here's the thing people miss: the MPPT can also struggle because the panels drop below the minimum input threshold relative to battery voltage.

Worth checking in the Victron Connect app what your actual panel voltage is logging during those weird moments. If it's dipping under ~5V above battery voltage the controller just... gives up basically.

Also — are your panels totally clear? Even a bit of frost or condensation scatter causes disproportionate losses at shallow angles compared to summer.

My SmartSolar 75/15 sorted itself once I tilted the panels steeper for winter. Even 10° extra made a noticeable difference on those low December sun paths we get in the UK.

@OffGridDoug this caught me out badly my first winter at the cabin. What nobody mentions until you've lived it is how dramatically the panel voltage spikes in cold weather — physics of it means cold silicon produces noticeably higher Voc than the spec sheet suggests.

My two panels in series were sitting well above rated voltage on a crisp January morning when the sun broke through at a low angle. Gave me a minor panic until I plotted it all out in the Victron Connect history.

Worth downloading the app and checking your panel voltage logs against your MPPT's maximum input spec — the 100/30 handles up to 100V, but on a cold bright morning you'd be surprised how close you can creep to that ceiling with series-wired panels.

Two 200W Renogy in series on a sunny January day in Scotland nearly sent my Victron into a proper sulk — turns out "400W of panels" in winter is basically "400W of panel-shaped disappointment."

@OffGridDoug this rings a bell from my boat setup — worth checking your panel Voc in the Victron app when it's actually cold and sunny. I was surprised how close mine crept to the 100V input limit on a crisp clear morning. The MPPT was throttling back to protect itself and I'd no idea until I started logging properly.

Also noticed the SmartSolar can be a bit hesitant entering bulk when the array voltage is fluctuating with passing cloud at low sun angles. Is your BMS cutting in and out at all? My Fogstar Drift threw a spanner in the works once because the MPPT and BMS were having a disagreement about charge thresholds.

Have you tried connecting via VictronConnect and watching the live data for a full morning cycle?