Anyone else had grief with MPPT controllers in cold weather?

Pulled the van out of the garage yesterday morning after it sat overnight in -4°C and the Victron SmartSolar 100/30 was throwing a weird absorption voltage — looked like it was pushing nearly 15.2V into my 100Ah AGM, which felt high. Checked it again an hour later once the garage warmed up a bit and it had settled back down to the usual 14.4V. Two 175W panels on the roof, wired in series giving me roughly 44Voc.

From what I can gather, the lower the temperature the higher the voltage a lead-acid battery will actually accept, so the controller doing this might technically be correct behaviour if temp compensation is working properly. I've got the temperature sensor dongle connected to the battery, so in theory it should be doing the right thing. Still made me a bit nervous watching it though — haven't seen it that high before.

Has anyone else noticed this on cold mornings, particularly with AGMs? Wondering if 15.2V is within safe limits for a Varta LFD90 or whether I should be tightening up the compensation curve in the VictronConnect app. Also curious whether those of you running lithium setups even bother with temp compensation or just lock the charge voltages solid.

CotswoldVanLifer | Posts: 847 | Joined: Mar 2019

@Lee1963 Yep, seen this with mine in the Cotswolds last winter during that cold snap we had in January. Thing is, your Victron is actually doing exactly what it should — lithium or AGM? If it's AGM, the controller compensates upward at lower battery temperatures, so 15.2V at -4°C is textbook temperature compensation working correctly. It's roughly -24mV per °C below 25°C for most lead-acid chemistries.

Worth checking your temperature compensation coefficient in the VictronConnect app and making sure the temp sensor (if you've got one fitted) is actually reading correctly rather than defaulting to an assumed ambient. A faulty sensor can cause it to overcorrect wildly.

What battery chemistry are you running? That'll change the advice considerably.

DustyHiker | Posts: 312 | Joined: Sep 2020

@Lee1963 Worth checking whether your battery temperature sensor is actually connected properly — if the Victron is running without temp compensation data it'll default to assuming 25°C and apply standard absorption voltage regardless. At -4°C with proper temp sensing it should be pushing higher voltage, that's actually correct behaviour, lithium aside obviously. If you're running AGM or wet lead-acid, a bit above 15V in proper cold is perfectly normal with compensation active. Check your VictronConnect app under the battery settings and see what temperature it's actually reading. If it shows "--" or 25°C when it's clearly freezing outside, your sensor wire has likely come loose. Happened to me parked up in the Peaks last February — sorted it in two minutes once I spotted it.

PanelEwan | Posts: 1,204 | Joined: Jun 2018

@Lee1963 This is actually correct behaviour — Victron's temperature compensation is working as designed. At -4°C you'd expect roughly +0.3–0.4V above the nominal absorption voltage on a standard lead-acid/AGM setup. The algorithm applies around 4–5mV per cell per °C below 25°C.

Worth checking your compensation coefficient in VictronConnect though — default is -16mV/°C for most profiles but some battery manufacturers (notably Fogstar's LiFePO4 guidance) recommend disabling temp compensation entirely on lithium.

On my narrowboat setup I actually saw similar numbers last February moored in Scotland. Turned out my temperature sensor had partially detached from the battery terminal, so the controller was using ambient air temp instead — which exaggerated the compensation significantly.

Is your SmartSolar reading battery temp directly via the VE.Smart network or just assuming ambient?