Does battery capacity drop noticeably in winter? Seeing odd figures from my Fogstar 100Ah LiFePO4

Been running a small off-grid setup for my garden office since spring — 200W of Renogy panels, a Victron SmartSolar MPPT, and a Fogstar Drift 100Ah LiFePO4. All summer it performed brilliantly, hitting 95–100% most days and easily powering my monitors, lighting, and a small fan heater on the occasional cool morning.

Now that temperatures have dropped (we're regularly sitting at 3–8°C overnight here in the Midlands), I'm noticing the battery rarely climbs above 80–85% even on decent sunny days, and the Victron app is showing absorption ending much earlier than it used to. I haven't changed any settings.

Is this normal LiFePO4 behaviour in cold weather, or should I be looking at my charge parameters? I've read that lithium cells don't accept charge as well below about 5°C, but I'm not sure how much of a real-world difference that makes versus just reduced panel output from the lower sun angle.

Would insulating the battery box make a meaningful difference, or is that overcomplicating it?

@Liz1979 yes, absolutely normal — LiFePO4 cells lose usable capacity as temperature drops, typically around 2-3% per degree C below about 10°C. Your Fogstar will still accept a charge fine, but the actual deliverable capacity shrinks noticeably.

What you'll likely find is that once the cells warm up slightly through use, the figures creep back towards normal. My own Fogstar Drift 200Ah setup showed similar "phantom" capacity loss last January — looked dire on the Victron BMV-712 at 7am, but recovered as the day progressed.

Worth checking:

• Is your battery in an uninsulated space?
• What's the actual cell temperature reading via the Fogstar app?

A simple insulated enclosure (even camping foam matting) makes a surprising difference. The chemistry hasn't failed — it's just physics doing its thing.

@Liz1979 Fogstar's own spec sheet quietly mentions reduced capacity below 10°C — it's in the small print nobody reads until January when their office goes dark at 3pm. 🕯️

Below about 0°C your Fogstar will also refuse to charge entirely — most BMS units have low-temp cutoff and yours will just sit there looking smug while your panels produce nothing useful.

@Liz1979 worth checking your Victron app history rather than guessing — the SmartSolar logs SOC and voltage over time, so you can actually see when the drop started correlating with ambient temps dropping. That'll tell you whether it's genuinely thermal capacity loss or something else (loose connection, shading, etc.).

My shepherd's hut battery is in an insulated box with a small heat mat on a thermostat — keeps it above 8°C and the difference in winter performance is measurable. Not a complicated fix if you've got 12V to spare.

As @48VQueen says, the charge cutoff is the real gotcha — no point having panels if the BMS won't accept charge on a cold morning.