Anyone else noticed their Fogstar Drift cells losing capacity faster in winter?

I've got a 200Ah 12V LiFePO4 setup using Fogstar Drift cells, been running it about 18 months now. Over the last few weeks I've noticed I'm only getting what feels like 160-170Ah of usable capacity compared to the full 200Ah I was seeing in summer. The BMS (a Daly 100A unit) isn't throwing any faults.

I know LiFePO4 doesn't like the cold and capacity drops are expected, but I'm sitting in the Scottish Borders where we've had a fair few nights below 5°C lately. The cells are in an insulated box under the van but not actively heated. Is this kind of 15-20% drop normal at those temps, or should I be worried something else is going on?

Has anyone fitted a battery heating pad to their LiFePO4 setup? Wondering whether a low-wattage heat mat on a thermostat is worth doing, or whether I'm just overthinking a normal seasonal pattern. Would love to know what kind of temperature range people are actually maintaining their cells at through winter.

@JoeTurner this is almost certainly temperature-related rather than genuine capacity loss — worth distinguishing between the two before you worry.

My shepherd's hut setup went through exactly this last January. The cells themselves were sitting around 4-6°C overnight and the BMS was cutting discharge headroom significantly as a result. LiFePO4 genuinely delivers less usable capacity below about 10°C — it's electrochemistry, not degradation.

What sorted it for me was insulating the battery compartment properly and adding a small self-regulating heat mat wired through a thermostat. Capacity came right back once the cells stayed above 10°C.

A few things worth checking:

• Actual cell temperature — not just ambient
• Whether your BMS has low-temp discharge protection kicking in early
• Resting voltage after a full charge

18 months is nowhere near old enough for meaningful capacity fade on Fogstar cells in normal use.

@DaleVicky is spot on about that distinction. Worth adding that LiFePO4 chemistry specifically starts showing noticeable capacity reduction once you're consistently below about 10°C, and it gets more pronounced as you approach 0°C. The cells aren't actually degrading — they're just performing differently in the cold.

@JoeTurner a quick way to check: bring your battery into a warmer space (or just wait for a mild day) and run the same load test you normally would. If your capacity comes back close to what you'd expect, it's definitely thermal rather than genuine cell ageing.

Have you got any insulation around your battery bank? Even wrapping it in some old duvets or foam board can make a surprising difference over winter. Some folk also add a small low-wattage heat mat on a thermostat, though obviously that has its own parasitic draw to factor in.

@JoeTurner to put some numbers on what @DaleVicky and @CamperRachel are touching on — LiFePO4 typically loses around 20-30% usable capacity at 0°C compared to 25°C rated figures. If your cells are sitting in an uninsulated space hitting 2-5°C overnight, that 160-170Ah figure is entirely plausible and not degradation.

On my boat setup I wrap the battery bank in 25mm closed-cell foam and gained back roughly 15Ah through winter just from that alone. Fogstar Drift cells are solid — at 18 months you're nowhere near meaningful capacity fade territory with LiFePO4.

Worth checking a few things:

• Actual cell temperature (a cheap probe thermistor on your BMS helps enormously)
• Whether your Victron/charger is doing a proper absorption phase in cold conditions
• Resting voltage after full charge — should still hit 13.6V+

Run a proper capacity test at a controlled indoor temperature before drawing any conclusions.