Anyone else finding their MPPT controller wildly overestimating battery state of charge in winter?

Running a Victron SmartSolar 100/30 with a 200Ah lithium (a Lion Energy UT 1300) on my narrowboat. During summer it was spot on — hit 100% around midday, held it, all made sense. Now we're into December and I'm getting readings of 95–100% SoC by early afternoon even on genuinely grim days where I've had maybe 2–3 hours of weak sun and the panels (2x 175W rigid, south-facing on the roof) can't possibly have put much in.

I've been checking voltage directly at the battery terminals with a multimeter and it's sitting around 13.1–13.2V at "rest" after the controller switches to float — which to my understanding is nowhere near full on lithium. The SmartSolar app is cheerfully telling me I'm at 98% though. I've got the battery profile set to the Lion Energy presets (absorption 14.2V, float 13.5V) so I don't think it's a settings issue, but something clearly isn't adding up.

Has anyone else seen this in winter specifically, or is this a calibration drift thing that needs a full charge cycle to reset? Wondering whether I should try forcing a manual equalisation or just do a proper discharge down to 20% and charge back up to let it recalibrate. Open to any suggestions — don't want to be caught short on a cold mooring thinking I've got plenty of juice when I haven't.

Familiar story. My van setup does exactly the same thing every autumn — the Victron thinks it's full because the absorption voltage was hit quickly, but the actual capacity charged is laughable compared to what the panel actually pulled in summer.

The culprit is usually the tail current threshold. In weak winter sun the charge current is already so low that the controller interprets it as "battery satisfied" rather than "panel's just rubbish today."

Worth digging into VictronConnect and bumping up your absorption time or lowering the tail current percentage. On my SmartSolar I dropped mine from 4% to 2% of battery capacity and it made a noticeable difference.

Also worth checking — is your temperature sensor actually connected? A cold lithium accepting less charge is normal, but the controller needs to know it's cold to compensate properly.

Great thread. One thing worth checking that nobody's mentioned yet — have you looked at your tail current settings? In winter your panels are delivering so little current that the controller can hit the absorption end-current threshold almost immediately, even when the battery's nowhere near genuinely full. The Victron effectively "sees" the tail current it expects from a full battery simply because total available current is tiny to begin with. Try dropping your tail current percentage down in the VictronConnect app, or better yet switch to a fixed time-based absorption period for winter months. It won't fully solve the underlying issue but it should stop it declaring victory quite so prematurely. @AndyButler's point about this being a seasonal pattern is spot on — it's really a function of low irradiance rather than anything wrong with the kit itself.

@ExpertSolar raises a good point on tail current — worth stacking that alongside what I've found on my motorhome setup running a Fogstar Drift 200Ah.

The core issue is that absorption voltage is reached far faster in cold weather because charge acceptance drops dramatically. The MPPT hits target voltage, interprets that as "battery nearly full," and transitions to float prematurely. Your actual state of charge could be 70-75% at that point.

Two things that helped me:

• Increase absorption time — either fixed duration or ensure tail current threshold is set low enough (I run 2% of capacity, so ~4A on a 200Ah bank)
• Check your temperature compensation — if you're not running a SmartSense or battery temperature sensor, the controller is working blind

The Victron app's history graphs will show you absorption duration each day — if it's consistently under 20 minutes in December, that's your smoking gun.

Good thread this. @CoastalWanderer one thing I'd add — have you checked whether your absorption voltage is actually being reached in the first place during shorter winter days? On my boat I found the panels were barely hitting absorption before the sun dropped, so the controller was essentially calling it done prematurely based on time rather than genuine charge acceptance.

Worth pulling up the Victron Connect history and checking how long it's actually spending in absorption versus bulk. If it's cutting absorption short repeatedly, your battery never gets a proper top-up regardless of what the SoC display claims.

Also worth noting that lithium resting voltage curves are quite flat, which makes coulomb counting drift worse over time if you're not hitting proper full charges to reset the calibration. A proper 100% cycle sorted mine right out.

Good thread. Something I haven't seen mentioned yet — have you checked your battery temperature compensation settings? If you're running the SmartSolar without a temperature sensor attached, it's working blind during these cold spells. Lithium batteries actually have fairly flat voltage curves across temperature compared to lead-acid, so Victron's default compensation profile (designed with lead-acid in mind) can genuinely confuse the SOC algorithm when ambient temps drop sharply.

Worth grabbing a SmartSense or even just the battery temp sensor dongle — they're not expensive. Once mine was properly connected last January the SOC readings settled down considerably on my setup. The Victron Connect app will also tell you whether temperature compensation is currently active, so that's a quick first check before buying anything.