Anyone else had issues with Fogstar Drift cells losing capacity over winter in a static caravan?

Picked up two 100Ah Fogstar Drift LiFePO4 cells last spring and built a 24V bank for the caravan. Worked brilliantly through summer — barely touched 80% DoD even on cloudy days with the 400W Renogy panels keeping things topped up.

Left the caravan unoccupied from November through to February, with the bank sitting at around 50% SoC (as recommended). Came back to find both cells showing noticeably reduced capacity on my Victron BMV-712 — roughly 78Ah each instead of the rated 100Ah. Temperature in the caravan was probably dropping to -5°C or so on the worst nights.

Is this permanent degradation or does LiFePO4 just need a few charge cycles to "wake up" after sitting cold? I've done three full cycles now and it hasn't really improved. Wondering whether I should've used a low-temp cut-off on the BMS or whether the Fogstar cells just don't handle UK winters well when left unattended.

OakTom | 847 posts

@PanelPaula Drift cells are solid, so worth ruling out the basics first. LiFePO4 capacity does drop noticeably in cold temps — not permanent damage, just chemistry slowing down. If your static is unheated over winter, the cells themselves could be sitting at 5°C or below, which can knock 15-20% off usable capacity right there.

Worth checking: is your BMS cutting off charge when temperatures drop near freezing? Charging LiFePO4 below 0°C causes lithium plating which is permanent damage. Some cheaper BMS units don't have low-temp charge protection.

What BMS are you running with them? That'll help narrow it down considerably. Also, are the cells recovering to full capacity when they warm back up, or is the loss sticking around? That distinction matters a lot for diagnosis.

AGM_Pro | 134 posts

@PanelPaula Worth checking what your BMS cut-off voltage was set to over winter. I had something similar on the narrowboat — cells seemed like they'd lost capacity but it turned out the low-temp charge protection was kicking in early and the Victron SmartShunt had drifted on its SoC calibration. Did you do a full synchronisation cycle (charge to 100%, hold at absorption, let it trickle to tail current) after the cold spell? That often sorts the apparent capacity loss on LiFePO4. Also, what temperature were the cells sitting at? Below about 5°C they'll genuinely deliver less usable capacity even if nothing's actually degraded permanently.

Kingy | 1,203 posts

Had almost the exact same head-scratcher on my narrowboat last winter. Turned out the culprit wasn't the Fogstar cells at all — it was the charger profile on my Victron MPPT quietly doing something unhelpful once ambient temps dropped into single digits.

LiFePO4 doesn't like being charged below about 5°C, and most MPPTs won't throttle back unless you've specifically enabled the low-temperature cutoff. Mine hadn't been set up properly, so it was essentially stress-cycling the bank every chilly morning.

Worth checking if your Renogy controller has a temperature compensation or low-temp charge cutoff feature — and whether it's actually connected to a temp sensor or just assuming. Some come with the sensor in the box but nobody fits it. 🙄

How's the bank been stored — any periods with no load or charge at all? Deep self-discharge over months can skew your capacity readings temporarily.

NeilBarker | 412 posts

@PanelPaula One thing nobody's mentioned yet — passive loads over winter. Even with everything "off," a lot of caravans have phantom draws from alarm systems, gas detectors, or even the BMS itself ticking away. Over several cold months that adds up considerably. I'd stick a clamp meter or basic current monitor on the negative bus and see what's actually drawing overnight. Also worth noting that LiFePO4 self-discharge increases slightly in cold, but it's genuinely minimal — if you're seeing significant capacity loss rather than just a lower state-of-charge reading, I'd lean toward an undetected parasitic drain as the first suspect before blaming the cells themselves. Fogstar's Drift cells have a decent reputation and I'd be surprised if they were the problem here.

Midge55 | 287 posts

@PanelPaula Something worth adding to what's been said — LiFePO4 really doesn't like sitting at very low state of charge in cold temps for extended periods. If the BMS disconnected the bank in autumn and it just sat there all winter at whatever voltage it landed on, you could be seeing some recoverable capacity loss rather than permanent degradation. Try a few slow charge/discharge cycles and measure actual capacity properly — not just resting voltage. Also, what temperature did it get inside that caravan? Below about 0°C and you really shouldn't be charging at full current anyway. Some BMS units don't have low-temp charge cutoff configured by default. Fogstar Drifts are decent cells so I'd be surprised if they've genuinely degraded badly — more likely something in the management side of things. What BMS are you running?

KellyRobinson | 47 posts

Had this exact worry when planning my tiny house battery setup — ended up doing a lot of reading on storage SoC before committing to Fogstar cells.

One thing nobody's flagged yet: have you checked whether your Renogy MPPT is actually reaching your float voltage correctly in winter? Low irradiance can mean the controller never completes a proper absorption cycle, so the cells creep down over weeks without you noticing.

What are your charge settings currently? Some Renogy units ship with AGM defaults still active — seen it catch people out before.

Also worth logging voltage at the battery terminals rather than the controller display. I use a basic Victron BMV-712 and the difference between what the MPPT thinks it's delivering versus terminal voltage surprised me on colder days.

RockyMender | 203 posts

My garden office Fogstar bank sulked all January like a teenager told to tidy their room — turned out the Victron BMV was logging it sat at 100% SOC for weeks, which LiFePO4 absolutely despises long-term. Threw it on a gentle 50% storage charge before the next cold snap and she's been right as rain since. @KellyRobinson the research is sound but nothing beats accidentally killing something once to really learn it.

EcoFlowMaster | 412 posts

@PanelPaula Had something similar with my motorhome bank last winter — except I thought I'd set the storage charge at 50% on the Victron, turned out I'd fat-fingered it and left them sat at 98% for three months like an overfed lab

BrambleElla | 891 posts

@KellyRobinson welcome to the forum — great to have you here, hope you find the thread useful for your tiny house planning!

Worth adding something nobody's mentioned yet: storage SoC matters hugely with LiFePO4 over winter. If your cells sat at high charge for weeks unused, that accelerates capacity fade regardless of temperature. I store my van bank at around 50% when parked up for extended periods — Victron MPPT has a absorption voltage limit you can drop deliberately to achieve this. Fogstar Drift cells are solid but they're not immune to the usual LiFePO4 rules.