Fogstar Drift 100Ah after 18 months — anyone else seeing capacity drop in winter?

Picked up two Fogstar Drift 100Ah LiFePO4 cells back in spring of last year for the narrowboat. Wired them in parallel through a Daly 100A BMS, feeding a Victron SmartShunt and a 400W Renogy panel array up top. Summer was brilliant — hitting 195Ah usable without breaking a sweat.

Come January though, I'm seeing the SmartShunt reporting maybe 165–170Ah actual before the BMS trips. I know LiFePO4 loses some capacity in the cold — canal moorings aren't exactly tropical — but I'm wondering if this is just temperature derating or if the cells are genuinely degrading faster than they should be. Water temp sitting around 4–6°C overnight, battery box is insulated but not heated.

Has anyone else got Fogstar Drift cells past their first winter and noticed similar numbers? I've read the spec sheet says capacity starts dipping noticeably below 10°C, which tracks, but 15–18% feels steeper than I'd expect. Considering adding a self-heating pad (seen the Fogstar Drift 2 has it built in — typical, isn't it) but want to know if it's worth the faff before I start rewiring anything.

Also curious whether running them at a lower float — I'm currently set to 3.45V/cell — makes any odds in cold conditions or if that's a red herring.

Hey @WezFisher, worth checking whether it's actual capacity loss or just the BMS getting confused by the cold. LiFePO4 voltage curves flatten out even more in winter, so your SmartShunt can lose its reference points and start reading conservatively.

Try doing a full charge cycle on a warmer day (above 10°C ideally) and let the SmartShunt recalibrate from a proper 100% starting point. Also worth checking your peukert exponent settings in the Victron — boats tend to draw in longer slower cycles which can throw it off.

Genuine capacity fade at 18 months would be unusual for Drifts unless they've been stored or charged below 0°C repeatedly, which is the real killer for these cells. Has the boat been sitting unheated over winter?

@WezFisher same boat here (literally). My Fogstar Drifts did the same thing last winter moored up in Scotland — looked like proper capacity loss but turned out the cells were just cold-soaking overnight and the internal resistance was spiking.

Worth checking your SmartShunt's SOC figure against an actual resting voltage reading when the cells are properly warmed up mid-afternoon. Mine showed ~20% less capacity on cold mornings vs what I actually had.

One thing nobody mentions — if you're running a calorifier or diesel stove, the engine bay heat helps massively. I moved my battery bank closer to the engine bulkhead and it made a noticeable difference through Feb/March.

Real capacity fade after 18 months on LiFePO4 would be unusual unless you've been regularly pushing below 10% SOC.

@WezFisher LiFePO4 in winter is basically that mate who says they'll help you move house but goes mysteriously quiet when it's -3°C outside — the capacity isn't gone, it's just sulking until things warm up a bit.

Worth noting: your Fogstar Drifts will temporarily derate below about 5°

@WezFisher one thing worth checking that nobody's mentioned yet — have you looked at your charge voltages in the cold? LiFePO4 internal resistance climbs noticeably below around 5°C, which means your Victron might be hitting the absorption voltage earlier than it should and cutting the charge short before the cells are actually full. You end up thinking you've got a full battery when you haven't really.

If your SmartShunt is showing a lower charged Ah figure than expected, that's a decent clue. Worth logging a full charge cycle through VictronConnect and seeing what absorption time looks like compared to summer. Reducing charge current slightly in winter can actually help the cells accept a more complete charge too. Proper capacity loss at 18 months on a lightly cycled narrowboat setup would be unusual — I'd be fairly confident it's temperature behaviour rather than genuine degradation.

Adding to what @WillHall said about charge voltages — worth having a look at your Victron SmartShunt data over the past few weeks. If your resting voltage is sitting lower than expected even after a full charge cycle, it's worth ruling out a cell imbalance before assuming straight capacity loss. Cold temps can mask a drifting cell because the voltage curves compress and the BMS can trip early thinking it's full when one cell is lagging behind.

Have you got the Victron app logging your charge/discharge history? The trend data over a fortnight or so can tell you a lot. Also, what's your absorption voltage set to on the charger side — some people drop it slightly in winter thinking they're being kind to the cells but actually end up chronically undercharging them.

Living this exact nightmare on the water — my Fogstar Drifts threw a strop around December and I nearly blamed the BMS before realising my boat had been sat at 8°C for a fortnight and LiFePO4 basically phones it in below 10°C like it's working from home.

@WezFisher funny you post this now — I was staring at my battery monitor last week thinking the same thing up at the cabin.

What caught me out wasn't the cells themselves but the resting voltage plateau. In proper cold, LiFePO4's flat discharge curve gets even flatter and the SmartShunt's SOC calculation starts drifting badly if your charge efficiency factor isn't dialled in.

Went through two winters convinced my Fogstars were degrading. Turned out my SmartShunt was reading 60% when the cells were genuinely sitting at 78%.

Try forcing a full absorption charge on a mild day and letting the shunt resync — gave me back what felt like 15Ah overnight.

Really useful thread, this. @WezFisher one thing nobody's mentioned yet — have you checked the actual resting voltage after a full charge cycle and letting it sit for a few hours? LiFePO4 has that famously flat discharge curve which can make the SmartShunt's SOC calculations drift over time if it hasn't had a proper recalibration. Worth doing a full charge to absorption, disconnect the loads, and let it settle. The SmartShunt should then reset its reference point. Might find your "lost" capacity is partly a measurement issue rather than genuine cell degradation. Not saying that's definitely it, but worth ruling out before assuming the worst.

Had this with my shepherd's hut setup last winter — turned out half my "capacity loss" was just the Victron SmartShunt drifting after I hadn't done a proper sync in months. Worth ruling that out first before assuming the cells themselves are degrading.

Also, parallel LiFePO4 without individual cell monitoring is a bit of a guess really. One cell could be lagging and you'd never know from the top-level voltage alone.

@Midge66 raises a good point about resting voltage — but also check your tail current setting on the SmartShunt, mine was way off default.

@DalesCruiser makes a fair point about the SmartShunt drift, but I'd also look at your charge voltage before writing off the cells themselves.

Had my garden office Fogstar setup running through its second winter now — noticed what looked like capacity loss until I realised my Renogy MPPT was pulling back absorption voltage slightly in cold conditions. The cells weren't getting a proper full charge, so the SmartShunt never hit 100% SOC to resync properly.

Worth checking your charge profile logs if your MPPT supports it. Small voltage discrepancy compounds over weeks into something that looks like genuine degradation but isn't.