Anyone else had issues with Fogstar Drift cells dropping capacity in cold weather?

Finally got my 200Ah Fogstar Drift LiFePO4 fitted on the narrowboat last autumn — brilliant bit of kit overall, Victron SmartShunt keeping an eye on everything. But over winter I noticed the usable capacity seemed to drop noticeably once the bilge temps got down to around 4–5°C. Not catastrophic, but definitely losing maybe 15–20% of what I'd expect.

I know LiFePO4 doesn't love the cold, and the Drift cells aren't self-heating. Wondering if it's worth adding a small heating pad underneath — seen a few people do this with a basic thermostat set to kick in around 5°C. Running 400W of solar on the roof so power shouldn't be the issue.

Has anyone actually measured the capacity drop on their Fogstar cells at low temps, or is this just normal chemistry behaviour I need to accept? Also curious whether anyone's used a battery heating solution that doesn't involve spending a fortune.

@HazelSoul LiFePO4 chemistry basically throws a strop below about 5°C — you're not losing capacity permanently, the cells just have higher internal resistance when cold, so your Victron will show lower SoC readings that "magically" recover once the boat warms up a bit.

Key things worth checking:

• Charge cutoff — most BMS units will block charging below 0°C to prevent lithium plating, which looks like capacity loss
• Resting voltage — LiFePO4 has a notoriously flat discharge curve anyway, even worse when cold
• Fogstar's own spec sheet shows roughly 80% capacity at 0°C, which is pretty standard across the chemistry

My cabin setup sees similar nonsense every January — I just accepted that winter = 80% usable and sized accordingly. A battery blanket or locating the bank somewhere with residual heat (engine bay adjacent?) sorts it properly.

@HazelSoul same boat (literally) — had the same thing last winter with mine. What made a noticeable difference was insulating the battery compartment with some cheap closed-cell foam. Doesn't need to be fancy. The cells hold enough residual warmth from charging/discharging that once you stop the heat escaping, temps stay above that 5°C threshold @FogstarFan mentioned.

Also worth checking your SmartShunt low-temp data in VictronConnect — you can see exactly when capacity dropped and correlate it with ambient temps. Helped me understand the pattern rather than just guessing.

The Drift cells themselves seem fine once spring came round, full capacity back no bother. Not a fault, just physics.

@HazelSoul Worth mentioning that your Victron SmartShunt can actually help you track this properly — log your capacity readings against ambient temperature over a few weeks and you'll see a clear correlation.

One thing neither @FogstarFan nor @LDVWanderer have touched on: avoid charging when the cells are below 0°C full stop, not just because you'll lose usable capacity but because you risk lithium plating which is permanent damage. Most decent BMS units will have low-temperature charge cutoff but worth double-checking yours is configured correctly.

On a narrowboat you've also got the advantage that the engine bay retains quite a bit of heat — if your battery location is anywhere near there, even indirect warmth makes a real difference through the night. How are yours mounted, bow or stern?

@HazelSoul This took me a while to get my head around when I first set up my garden office system. What nobody tells you upfront is that the charging side is where cold weather gets really nasty — LiFePO4 cells genuinely don't want to accept charge below freezing, and forcing it can cause lithium plating that does permanently nick your capacity. Most decent BMS units will cut charge input as protection, which looks like a fault but is actually working exactly as intended. Worth double-checking your BMS low-temp charge cutoff threshold in the settings if you haven't already. Mine kicked in at 5°C and initially had me convinced something had failed catastrophically. It hadn't.

Really common with LiFePO4 on boats — the chemistry just doesn't like the cold. One thing worth adding that nobody's touched on yet: your BMS will likely be cutting things off earlier than you'd expect in cold temps because it's protecting the cells from being charged below a certain threshold (typically around 5°C for most units). So some of what you're experiencing might not actually be capacity loss as such — the cells may simply be refusing to accept a full charge. Check whether your BMS has low-temperature charge cutoff settings, and if so what they're set to. Some folk on here have added a small heating mat directly to the battery bank with a thermostat, which kicks in overnight when moored up. Modest power draw but keeps the cells in their happy range. @LDVWanderer's insulation point is spot on too — the two approaches work well together.