Anyone else had grief with MPPT controllers underreporting battery voltage in cold weather?

Been scratching my head over this one for a few weeks now. Running a 400W panel setup on the van (two 200W monos in series) going into a Victron SmartSolar 100/30, charging a 100Ah lithium (a Fogstar Drift if it matters). Everything looked fine over summer but now we're into the colder months I'm noticing the controller is reading battery voltage about 0.3–0.4V lower than my BMV-712 shunt monitor. Not a massive difference on paper but it's causing the SmartSolar to hit absorption earlier than it should and I don't think the battery is actually getting a full charge.

I've already checked cable connections and they're all solid. The SmartSolar is mounted inside the van on a wooden panel, nowhere near the battery — about 1.8 metres of 6mm² cable run between them. My suspicion is there's enough resistance in that run that the voltage drop is throwing things off, and maybe temperature is making it slightly worse? The Victron app does let you set a voltage compensation offset but I'm not sure if that's really the right fix or if I'm barking up the wrong tree.

Has anyone else seen this with the SmartSolar range specifically, or is it a general MPPT thing? Wondering if enabling the voltage sense via the VE.Direct cable or adding a proper remote battery sense wire would sort it properly rather than just bodging the offset setting.

@ExpertBuild your Victron's probably reading voltage at the controller terminals rather than at the battery — run a proper temperature-compensated voltage sense wire direct to the battery and watch the discrepancy vanish like my motivation to do any more cable runs in January.

Worth adding to what @Chalky's getting at — temperature compensation can actually work against you with lithium anyway. That feature's really designed for lead-acid, where charge acceptance genuinely changes with temperature. Most people running lithium disable it entirely in the Victron app. Also worth checking whether your Fogstar BMS is doing any voltage reporting of its own via Bluetooth — if the readings differ significantly from what the SmartSolar's showing, that'll help you pinpoint whether it's a wiring/resistance issue or something in the controller settings. I had similar head-scratching on my own setup last winter and it turned out to be a slightly loose terminal causing a small voltage drop that only really showed up when current draw was higher in the cold. Worth doing a proper torque check on all your connections while you're at it.

Had almost identical grief last winter with my static caravan setup. The penny dropped when I noticed my SmartSolar was consistently reading 0.3–0.4V lower than the Victron BMV-712 sitting right at the battery terminals.

The culprit in my case was a long cable run — about 4 metres between controller and battery. Even with decent cable, resistance adds up, especially when you're pulling decent amps.

@Russ1992 makes a solid point about temperature compensation fighting against lithium chemistry. Worth disabling that entirely in VictronConnect if you haven't already — it's buried under the battery settings and defaults to a figure designed for lead-acid.

Once I sorted both issues the discrepancy vanished almost completely. The SmartSolar is a brilliant bit of kit but the defaults really do assume you're running AGM.

Been through exactly this on the narrowboat last winter — bitter mornings when the cut was iced over and my SmartSolar was doing all sorts of odd things. What nobody's mentioned yet is absorption timeout. When the controller sees a falsely low voltage, it can keep pushing current longer than it should trying to hit target, which actually stresses the cells more than the underreporting itself does. Worth checking your Victron Connect app logs — look at the absorption duration column over several days. If you're seeing unusually long absorption phases coinciding with cold mornings, that's your smoking gun. I ended up tightening my absorption time limit as a precaution until I sorted the underlying voltage sense issue properly.

Really important point that hasn't been raised yet — check your battery cable runs and connections first before assuming it's purely a controller issue. I spent two weeks convinced my Victron was lying to me, only to discover a slightly loose Anderson connector on the battery positive was introducing enough resistance to skew the sense voltage under load, which obviously looks worse in cold when internal resistance rises anyway.

If your SmartSolar has the dedicated battery sense terminals (the small screw terminals, not the main lugs), make sure you're actually using them. The controller will then read true terminal voltage rather than inferring it through the charge cables.

Also worth checking in the VictronConnect app — under the history tab you can see if the absorption phase is cutting short, which would confirm whether it's genuinely underreading or something else entirely.

Just to add something slightly different to what's been covered — have you checked whether your Fogstar has its own BMS temperature compensation? Some lithium BMS units will actually throttle the charge acceptance in cold conditions independently of what the MPPT is doing, which can create a confusing feedback loop where the controller looks like it's misreporting when it's actually just responding to reduced current acceptance from the battery end.

Worth connecting via the VictronConnect app and watching the live data during a cold morning charge cycle. If you see the absorption voltage being hit but amperage dropping off sharply, that's likely your BMS doing its job rather than a controller measurement issue. @Finn1991's point about connections is still worth ruling out first though — a dodgy crimp in the cold can mimic all sorts of strange behaviour.

My narrowboat Victron spent all of last January convinced the batteries were at 11.8V when they were absolutely fine — turns out the temperature sensor lead had worked loose, so it was essentially guessing like a drunk playing darts 🎯