Anyone else finding their MPPT controller wildly optimistic about state of charge in winter?

Running a Victron SmartSolar 100/30 with a 200Ah AGM bank and a single 200W panel on the roof of my Transit conversion. All summer it was spot on — battery hitting 14.4V absorption, finishing the day sitting happy at 13.2V float, SOC showing 95–100% by early afternoon most days.

Now we're into January it's a different story. Panel's only pulling maybe 3–4A on a bright day, and the controller is still claiming 80–85% SOC when I know the batteries are struggling — lights dimming, the 12V compressor fridge cycling slower, and the resting voltage sitting around 12.3V after a few hours off load, which to me says more like 50–60%.

I've got the battery temperature sensor fitted and the charge profile set correctly for AGM, so I don't think it's a settings issue. My suspicion is the controller is just not getting enough charge cycles to properly recalibrate the SOC estimate over winter. Has anyone else seen this, and is there a fix short of getting a proper battery monitor like a Victron BMV-712? I keep going back and forth on whether it's worth the £90-odd for the shunt kit.

WattRoger | Posts: 847

@Hamish1980 Classic winter headache this one! A few things worth checking — AGMs are genuinely temperature-sensitive, so if your bank is sitting in an uninsulated space, the actual capacity could be 20-30% down on what it was in summer. Cold batteries just don't deliver the same.

Also worth noting that a single 200W panel in winter, especially if it's flat-mounted, is going to struggle massively with those low sun angles. You might be finishing absorption at noon but with far less actual charge put in than the Victron's reporting.

Have you got the VictronConnect app pulling history logs? The "charged energy" figures over the day will tell you more than the SOC estimate. The SmartSolar's SOC calculation can drift badly if it's not hitting full absorption regularly. A proper battery monitor like a BMV-712 would sort this out properly.

SmartSolarGuy | Posts: 1,203

@Hamish1980 Something nobody's mentioned yet — low temperatures actually reduce AGM capacity, sometimes 20-30% below rated. So your 200Ah bank is effectively a 150Ah bank on a cold morning, but your Victron is still doing its maths against the full 200Ah figure. The voltage readings look "correct" but the usable capacity just isn't there.

Had exactly this in my motorhome last January parked up in the Peaks. Battery appeared fine until I ran the heating overnight and woke up to low voltage alarms.

Worth enabling temperature compensation if you've got a Victron Smart Battery Sense — genuinely transformed my winter accuracy. Cheap bit of kit, pairs over Bluetooth directly with the SmartSolar.

One 200W panel in winter is also doing heavy lifting on short grey days. That's a separate conversation though. 😄

DownsCamper | Posts: 2,341

@Hamish1980 Worth adding to what's already been said — your MPPT is calculating SoC primarily from voltage, and the problem in winter is that a cold AGM sitting at 12.6V genuinely looks fully charged by voltage alone, but capacity has dropped significantly. On my static caravan setup I solved this by enabling the battery temperature sensor (Victron sell a dedicated one for the SmartSolar range) and adjusting the temperature compensation coefficient — typically -4mV/°C/cell for AGM. Without this, your controller is making absorption/float decisions based on assumptions that simply don't hold below around 10°C. Also check your tail current setting in VictronConnect; raising it slightly helps the controller declare absorption "done" more realistically when panels are producing weakly in December light.

RockyTinker | Posts: 312

Had the exact same thing with my tiny house setup last January. One thing nobody's flagged yet — panel angle makes a massive difference in winter. My 200W Renogy was nearly flat-mounted and I was losing maybe 40% of potential harvest just from the sun being so low. Tilted it up to about 60° and the numbers came back to life almost overnight.

Also worth checking your cable runs for voltage drop — cold resistance increases slightly and every tenth of a volt matters when the sun's only giving you a 4-hour window anyway. My Victron was reading phantom absorption because it was seeing inflated voltage at the controller end, not at the battery terminals.

Temp-compensated charging is great but only works if the sensor's actually on the battery, not just floating nearby.

PartnerNomad | Posts: 847

@Hamish1980 This caught me out badly with my cabin setup last February. One thing worth trying — have you enabled the temperature compensation feature in VictronConnect? It adjusts absorption/float voltages based on a temp sensor reading. Without it, your controller is just assuming a fixed 25°C and charging accordingly.

The Victron SmartSolar supports this natively if you wire in a BMV battery monitor with the temp sensor — makes a noticeable difference to accuracy in cold spells.

Also, what are your actual absorption and float voltage settings? For AGM in winter, some people nudge absorption up slightly to compensate. Might be worth posting your current config so others can spot anything obvious.

Compo55 | Posts: 1,156

@Hamish1980 One thing worth adding that I haven't seen mentioned — cold temperatures actually increase your AGM's internal resistance, which means the battery can reach absorption voltage faster whilst still being genuinely undercharged. The controller sees 14.4V and thinks "job done," but the resting voltage an hour later tells a different story. Try checking voltage 2-3 hours after the controller goes to float and compare that to your summer readings. Bet you'll see a noticeable difference. Temperature compensation on the charge voltage helps enormously if your controller supports it — worth enabling if you haven't already.

MistyTrekker | Posts: 1,089

@Hamish1980 One thing I'd add that nobody seems to have mentioned yet — check your battery temperature. AGMs lose significant capacity in the cold, sometimes 20-30% at near-freezing temps, but your MPPT has no idea unless you've got a temperature sensor fitted. The Victron SmartSolar supports a battery temperature sensor accessory, and honestly it's worth every penny for winter van life. Without it, the controller is making assumptions based on ambient conditions at best. I fitted one to my own setup last November and it made a noticeable difference to how accurately the charging behaved. Also worth double-checking your absorption voltage is set correctly for AGM specifically — some units default to flooded lead-acid settings out of the box.

CotswoldNomad | Posts: 2,341

@Hamish1980 Classic winter MPPT optimism — the controller equivalent of "it's not that cold, put a jumper on."

Worth checking your temperature compensation is actually wired up and not just spiritually connected. The Victron SmartSolar has a dedicated

LutonDream | Posts: 847

@Hamish1980 Something worth considering that nobody's touched on yet — a single 200W panel on a van roof is going to be seriously compromised by low winter sun angles. You might only be getting effective output for a couple of hours either side of midday, and even then at a fraction of rated power. The SmartSolar will dutifully report a completed absorption cycle, but it's genuinely not had enough time to properly top the bank off. Consider adding a second panel if roof space allows — it made a massive difference on my Luton over winter.