Anyone else had their Fogstar Drift cells sag badly below 20% SOC in cold weather?

Picked up a 200Ah 12V LiFePO4 from Fogstar last autumn and she's been brilliant right up until the temperatures dropped — now anything below about 5°C and the voltage nosedives from 13.1V to 12.6V the moment I put the kettle on.

Running a Victron SmartShunt so the data's all there in black and white: discharge curve looks textbook until roughly 20% SOC, then it falls off a cliff faster than my bank balance after a marine parts order.

BMS is cutting out at 11.8V under load which trips my Victron MultiPlus and then I'm sat in the dark wondering why I didn't just buy more cells — is this normal cold-weather LiFePO4 behaviour or is my BMS being overly cautious with its cutoff threshold?

@MarineClare — classic LiFePO4 cold behaviour, not a fault as such. The internal resistance climbs sharply once you're below 5°C and under any real load that resistance shows up as voltage drop. Perfectly healthy cells doing exactly what cold chemistry does.

On my narrowboat last January, same story — Fogstar 280Ah pack, diesel heating off overnight, woke to 3°C in the bilge and the pack was sagging badly under the inverter load. Fitted a small self-regulating heat mat under the battery box, wired through the BMS aux port, and it transformed things completely.

Worth checking whether your actual capacity has dropped too, or just the voltage presentation under load. If it recovers quickly when the load lifts, you're fine — that's pure resistance, not cell degradation. A proper capacity test at warmer temps will tell you the real story.

@MarineClare totally normal unfortunately — LiFePO4 is particularly notorious for that flat discharge curve collapsing under load in the cold. Worth checking what your BMS low voltage cutoff is set to, as some will trip prematurely if it's reading a momentary sag rather than true resting voltage.

One thing that helped me on my van build was adding a bit of insulation around the battery — even a simple foam wrap keeps the cells a few degrees warmer overnight and makes a noticeable difference. Also try to avoid heavy loads first thing in the morning before the cells have had chance to warm up slightly from charging.

If you've got solar, a slow morning absorption charge will warm them gently before you start drawing hard. What inverter/loads are you running when you see the sag?

@MarineClare same issue with my tiny house setup last winter. Below about 3°C I was seeing the same sag, especially under any decent load — kettle, inverter spooling up, that sort of thing.

What helped me was insulating the battery box properly rather than fighting the chemistry. Foam board around the cells kept them above 8°C overnight even when ambient was -2°C outside.

Also worth checking your BMS low-voltage cutoff isn't set too aggressively — mine was tripping at 12.4V which felt like a dead battery but wasn't really. Bumped it down slightly and got more usable capacity back.

Fogstar's cells themselves are decent, it's just LiFePO4 being LiFePO4 in winter. Nothing wrong with yours by the sounds of it.

@MarineClad worth checking whether your BMS has a low-temperature cutoff that might be triggering earlier than expected — some units are set quite conservatively around 5°C and will throttle output before the cells themselves actually become a problem. Also, if you're on a boat and have any engine charging, try to keep a bit more buffer at the top end during cold snaps, maybe targeting 95% rather than letting it cycle fully. Insulating the battery box makes a surprising difference too — even a basic foam enclosure can keep temps a few degrees above ambient overnight. The cells themselves generate a little heat under load which helps, but they need somewhere to retain it.

@MarineClare had exactly this on my boat last February with a 100Ah Drift. The flat discharge curve basically vanishes in the cold — what's happening is internal resistance shoots up, so under any decent load the voltage drop looks catastrophic even though actual capacity is still there.

What helped me was insulating the battery box properly and adding a small self-regulating heat mat underneath (wired through a thermostat, kicks in below 5°C). Made a noticeable difference to the sag under load.

Also worth adjusting your low-voltage disconnect upward slightly in winter — I run mine at 12.4V rather than 12.0V during cold months, otherwise the BMS can cut out suddenly rather than giving you a graceful warning.

Really common with LiFePO4 chemistry generally — the internal resistance climbs sharply in the cold and that's what's causing the voltage sag under load rather than actual capacity loss as such.

What helped me with my shepherd's hut setup was wrapping the battery in a bit of closed-cell foam insulation and letting the system's own heat retention do some work overnight. Not glamorous but it made a noticeable difference.

Also worth looking at your charge settings — I dropped my absorption voltage slightly over winter on the Victron MPPT and that seemed to reduce the stress on cells that were already borderline cold.

@AshHermit makes a good point about the BMS cutoffs too — worth logging with something like a Victron BMV-712 if you haven't already, so you can actually see whether it's a genuine voltage sag or the BMS tripping early.