Anyone else finding their lithium battery hits a wall around 20% SOC in cold weather?

Been having a frustrating few weeks with my 200Ah LiFePO4 (it's a Fogstar Drift, about 18 months old). As soon as the temperature drops below about 5°C overnight, the BMS seems to cut off discharge way earlier than it should — I'm losing what feels like 30-40Ah of usable capacity before it trips out. During the day when the van warms up a bit it's absolutely fine again.

I've got a Victron SmartShunt keeping an eye on things and the SOC reads around 18-22% when it shuts down, so it's definitely not a genuine low-voltage cutoff. My suspicion is the BMS is protecting the cells from low-temp discharge, which I understand is a thing with lithium, but I didn't expect it to kick in this aggressively at temperatures that aren't even that extreme.

Has anyone found a practical workaround that doesn't involve keeping the heating on all night just to babysit the battery? I've seen people mention wrapping batteries in insulation or using a small self-regulating heat mat on a timer — curious whether that's actually worth doing or just a faff. Would love to know what others are running and whether this is just the reality of LiFePO4 in a UK winter.

EwanMorris76 | 847 posts

@Wendy1967 Classic cold weather behaviour, that - completely normal for LiFePO4 unfortunately. The chemistry genuinely struggles to deliver current at low temperatures, so your BMS is doing exactly what it should by cutting off early rather than letting the cells get damaged.

What's your battery's physical location? If it's in an uninsulated space like a garage or exterior cabinet, even just wrapping it in some rigid foam insulation makes a remarkable difference. Mine's in a wooden enclosure with 50mm PIR board and it rarely sees below 8°C even when it's -2° outside.

Worth checking whether Fogstar publish their low-temperature discharge specs - most LiFePO4 starts derate noticeably below 10°C and protection typically kicks in around 0°C. Your 5°C threshold suggests the cells themselves are probably sitting colder than the ambient air temperature.

BoxerLife | 1,203 posts

@Wendy1967 To add to what @EwanMorris76 is saying - worth checking whether your Fogstar has a low-temp cutoff threshold you can actually monitor via the app. The Drift series usually reports cell temperatures separately to ambient, and you might find your cells are sitting a degree or two colder than you'd expect, especially if the battery's in an uninsulated space.

A simple fix that's worked wonders for me is wrapping the battery in a bit of closed-cell foam camping mat material - costs next to nothing and keeps residual heat in overnight. Obviously nothing that blocks ventilation, mind you.

Also worth knowing - LiFePO4 recovers its capacity pretty quickly once temps rise again, so if you're seeing permanent capacity loss rather than temporary, that's a different conversation entirely.

CopperMaker | 412 posts

My static van's Fogstar spent last January doing its best impression of a stroppy teenager — wrap the battery in a bit of closed-cell foam insulation and suddenly it remembers it's supposed to be a power source.

Sam1975 | 634 posts

@CopperMaker that stroppy teenager description is painfully accurate 😄

My static up in Derbyshire went through exactly this two winters ago with a non-heated LiFePO4. The penny-drop moment for me was realising the battery wasn't actually failing — it was protecting itself, which is the BMS doing its job correctly.

What actually solved it: I fitted a small 12V reptile heat mat (the kind tortoise keepers use) underneath the battery, wired through a cheap thermostat probe set to kick in at 3°C. Total cost was about £18 from Amazon. Battery compartment now sits at a reliable 8-10°C even when it's freezing outside.

@Wendy1967 the Fogstar Drift is a solid unit — I'd be surprised if there's anything actually wrong with it. Thermal management is almost certainly your culprit here.