Does anyone actually get 100Ah from a 100Ah LiFePO4 in winter?

I've just swapped out my old AGM bank for a single 100Ah LiFePO4 (a Fogstar Drift, if it matters) in my Transit-based camper, and I'm a bit puzzled by what I'm seeing in cold weather. Sitting overnight at around 3–4°C, I'm only pulling maybe 70–75Ah before the BMS cuts out, which is noticeably less than the rated capacity. I've got a Victron BMV-712 doing the monitoring so I'm fairly confident the numbers are accurate.

I know LiFePO4 cells lose usable capacity in the cold, but I wasn't expecting quite this much of a drop. The battery doesn't have any built-in heating, and I'm not doing any pre-warming before discharge. My loads are pretty modest — a 12V compressor fridge running at around 4–5A average, some LED lighting, and a phone or two on charge.

Is this just normal behaviour and I should budget for roughly 70–80% capacity through winter, or is there something I can actually do about it — heating pad, insulating the battery box, something else? Would love to hear from anyone who's run LiFePO4 through a proper British winter.

@DailyConvert what temperatures are you actually seeing overnight? I've noticed in my shepherd's hut build that once the cells drop below about 5°C the BMS starts getting conservative, and below 0°C some will cut charging entirely to protect against lithium plating.

The usable capacity doesn't disappear as such — it's more that the BMS is protecting the cells rather than the chemistry being fundamentally limited. Worth checking whether it's a discharge issue or a charging issue, because they're quite different problems.

On my van conversion I solved the worst of it by insulating the battery box and letting a small parasitic load keep temps marginal. Not elegant but it works.

What voltage are you seeing under load compared to resting? That'd tell us quite a bit about what's actually happening.

@DailyConvert worth checking whether your BMS is throttling charge acceptance before you even get to the discharge question. On my narrowboat last January I was seeing the Victron SmartShunt reporting noticeably less usable capacity — turned out the battery hadn't fully charged the previous day because the BMS had restricted charging below about 5°C.

So you might not actually be losing capacity on discharge, you could just be starting from a lower state of charge than you think.

What's your charging source — solar, alternator, or shore power? And are you running any battery heating? The Fogstar Drift doesn't have self-heating built in, so in a Transit overnight you could genuinely be sitting at sub-5°C which changes things considerably.

WattRoger | Posts: 847

@DailyConvert Short answer: yes, pretty much, but only on the discharge side. LiFePO4 capacity drop in cold is actually quite modest compared to AGM — you might lose 5-10% at 5°C, a bit more below zero, but nothing dramatic.

The bigger gotcha with the Fogstar Drift specifically is that the BMS will flat-out refuse to charge below 0°C, which @VE_Boats is hinting at. So if your van's been sitting overnight in a cold snap, your solar or alternator won't be doing anything useful until the cells warm up a bit — either from ambient or from discharge load warming them internally.

Practically speaking, wrap it in some thin insulation if it's in an exposed locker. Even a bit of closed-cell foam makes a surprising difference to overnight temperature retention.

What's your overnight low been roughly?

KMV_Marine | Posts: 312

@DailyConvert To add to what @WattRoger is getting at — the Fogstar Drift specifically has low-temperature charge protection built into the BMS, which will cut charging below around 0°C. So you might be hitting a full 100Ah out overnight but then struggling to put it back in the morning if your van's been sat in sub-zero temperatures.

On a boat we insulate battery compartments partly for exactly this reason. Even a cheap closed-cell foam wrap around your battery box makes a surprising difference — the cell retains some warmth from its own discharge and stays above that charge cutoff threshold longer.

Worth logging your BMS data if you have the app connected, as it'll tell you definitively whether it's a capacity issue or purely a charge acceptance issue. Two very different problems with different solutions.

AlanPalmer57 | Posts: 43

Watching this thread closely — I've got a Fogstar Drift 100Ah in a static shed setup and noticed similar things over winter.

One thing nobody's mentioned yet: what voltage is your charger set to? I found my Victron SmartSolar was absorbing at 14.2V which seemed fine on paper, but in genuinely cold conditions (below 5°C in my case) it was cutting the charge cycle short because the battery was hitting the voltage target before it was actually full. Dropped absorption to 14.0V and extended the absorption time instead, and capacity came back noticeably.

Is your Transit getting direct solar or are you relying purely on the alternator? That might be relevant to where the shortfall is actually coming from.

CallumHobbs | Posts: 156

Worth adding the charging side of the story here. I spent a winter on the boat with two Fogstar Drift 100Ah batteries and learned the hard way that the BMS cutting charge below around 5°C is actually protecting you, not robbing you.

What helped me was fitting a small self-regulating heating pad under each battery — cheap off Amazon, runs off 12V, barely draws anything. Once ambient temp climbs a few degrees the BMS opens back up and charging resumes normally.

@AlanPalmer57 your static shed setup might benefit most from this — a shed can get genuinely brutal overnight in a Scottish January. Even wrapping the battery in closed-cell foam insulation buys you a couple of degrees and delays that BMS lockout.

Discharge capacity in cold is largely fine in my experience. It's the charge acceptance that catches people out.