Anyone else find their Fogstar Drift batteries run cooler than spec in a shepherd's hut over winter?

Running a pair of 100Ah Fogstar Drifts in my hut and noticed they barely break a sweat even when the Victron MPPT is hammering them at full charge current on a rare sunny January day — sitting at about 18°C ambient inside while the spec sheet starts fretting at 0°C.

Wondering if the insulated tongue-and-groove walls are doing more thermal work than I expected, or if I'm just lucky that my 400W of Renogy panels rarely produce enough in December to stress-test anything properly.

Has anyone actually logged cell temps through a proper cold snap — talking -5°C outside and below? Curious whether a bit of rockwool behind the battery box is worth bothering with or pure overkill.

@BatteryJason interesting one — I've had something similar on the narrowboat over winter. The Drifts seem to throttle themselves a bit once ambient drops below about 5°C, which I reckon is the BMS doing its job protecting the cells from fast charging in the cold.

Worth checking your Victron MPPT charge log — does the absorption phase seem longer than usual? I noticed mine was stretching out considerably on cold mornings before the batteries warmed up enough to accept full current.

Have you got any temperature compensation set on the MPPT? I'm genuinely not sure whether Fogstar recommend it for the Drift chemistry — been meaning to dig into that myself. The manual isn't exactly verbose on the cold weather stuff 😅

What temperatures are you actually seeing inside the hut when this happens?

Yeah, this matches what I've seen too. Worth bearing in mind that LiFePO4 chemistry generally runs more efficiently at lower temperatures in terms of heat generation during charge — less internal resistance losses becoming heat. The flip side of course is that below about 5°C you really want to make sure your BMS is blocking charge altogether, which the Drift's built-in protection should handle.

In a shepherd's hut though, ambient temp probably stays above that threshold most of the time if you've got any heating running. I'd be more curious whether @BatteryJason is seeing any capacity reduction on those cold mornings — that's where you tend to notice the temperature effect more than the heat signature. My 200Ah setup dropped noticeably in usable capacity during that cold snap we had in December. Not a fault, just physics!

Has anyone actually checked whether low ambient temps are affecting charge acceptance as well as heat output? Wondering if the BMS is quietly de-rating the charge current in cold conditions, which would obviously make the cells run cooler — but you'd also be getting less usable capacity than you think.

Asking because I'm looking at adding a Fogstar Drift to my setup specifically for EV top-up charging during winter, and if the BMS is throttling things below, say, 5°C, that changes my calculations quite a bit. Do the Drifts have any kind of low-temp charge cutoff built in, or is that something you'd need to manage via the Victron side with temperature compensation settings?

Would be useful to know what @BatteryJason's MPPT logs actually show for charge current vs ambient temp across a few winter days.