Anyone else finding their Victron SmartShunt wildly optimistic about battery health in cold weather?

Mine's sat in the motorhome reading 87% SOC but the lights are already dimming — classic sign the Fogstar 100Ah LiFePO4 is actually closer to 40% in this 2°C overnight nonsense we've been having.

Wondering if it's a BMS thing cutting capacity before the SmartShunt catches on, or whether I need to tweak the Peukert settings. Running a fairly standard setup: 200W Renogy panel, 30A MPPT, single 12V 100Ah battery.

Has anyone dialled in their SmartShunt config specifically for LiFePO4 in UK winter temps, or am I just going to have to accept that January is the enemy of accurate SOC readings?

@MarineGeoff lived this exact scenario on the narrowboat last January — moored up near Braunston, temperature dropped to -1°C overnight and my SmartShunt was cheerfully telling me 79% whilst the inverter was cutting out under load.

The issue isn't really the shunt lying — it's that LiFePO4 voltage curves flatten dramatically in cold, so the shunt's voltage-based cross-referencing goes completely haywire.

What fixed it for me was going into the Victron app and setting a proper Peukert exponent alongside updating the charge efficiency factor down to around 97%. Also worth checking your battery temperature compensation settings — if your Fogstar has a temp sensor output, get that wired into the SmartShunt directly. Night and day difference once I did that. The readings became genuinely trustworthy even through a cold snap on the cut.

Simon1988 | 47 posts

@MarineGeoff the SmartShunt itself isn't really the culprit here — it's faithfully reporting based on charge/discharge counting, but LiFePO4 cells genuinely deliver less usable capacity in the cold. Your battery is at 87% by charge counting, but the available capacity at 2°C has shrunk considerably, so you hit voltage sag much earlier than the SOC figure suggests.

Worth diving into the SmartShunt's temperature compensation settings if you've got a temperature sensor connected. Also check your Peukert exponent — slight adjustment can help reflect real-world cold performance better.

Longer term, some insulation around the battery compartment makes a surprising difference. Even a simple foam wrap keeps temps a few degrees higher overnight and you'll notice the difference immediately. @Jess1989 probably found similar on the narrowboat I'd imagine!

Rachel1995 | 134 posts

Worth adding to what @Simon1988 is getting at — LiFePO4 cells genuinely lose usable capacity in the cold, so that 87% might be accurate for what's stored, but the cells simply can't deliver it properly at 2°C. Your BMS will also start throttling discharge current to protect the cells, which explains the dimming lights before the SOC looks critical.

One thing that helped me massively was insulating the battery compartment rather than fighting the readings. Even a layer of closed-cell foam made a noticeable difference to how my Fogstar performed overnight last winter. The SmartShunt will look after itself — it's the battery temperature you want to manage. Do you have any heating in that compartment at all, @MarineGeoff?

OffGridDoug | 312 posts

@MarineGeoff worth checking your SmartShunt's Peukert exponent and charge efficiency factor in VictronConnect — both need adjusting for LiFePO4 (Peukert closer to 1.05, charge efficiency ~99%). Default settings assume lead-acid behaviour and will skew SOC calculations significantly.

More practically though: if your Fogstar BMS has a temperature sensor, cross-reference what it's actually reporting to the cells versus ambient. I've seen a 4°C differential inside a well-insulated battery box versus outside air temp, which meaningfully changes where on the discharge curve you actually are.

My own setup runs a Dallas temperature probe wired directly into the SmartShunt's aux input — gives me a compensated voltage reference that's far more honest in winter than SOC percentage alone. Voltage under load at 2°C is your ground truth here, not the shunt's accumulated calculations.