Anyone else noticed Fogstar Drift cells running warmer than spec under high discharge?

Running a 200Ah 12V LiFePO4 bank in my garden office setup — four Fogstar Drift 200Ah cells in parallel with a Daly 200A BMS. Victron SmartSolar 100/30 on the charge side. Generally solid, but I've been logging cell temps with a cheap Inkbird sensor and noticing they creep up to around 35–38°C when I'm pulling 80A+ for any sustained period (kettle, fan heater on boost, etc.). The spec sheet suggests they should handle that comfortably without much heat, so I'm a bit puzzled.

I know 38°C isn't catastrophic — far from it — but it's noticeably warmer than my old CALB cells ever ran in the same enclosure. The bank is in an insulated but ventilated wooden cabinet inside the office, ambient usually 18–22°C. So the delta is maybe 15–18°C under load, which feels high for LiFePO4 at that discharge rate.

Wondering if this is a Drift-specific thing, a cell matching issue, or whether the Daly BMS itself is generating heat and skewing my sensor readings. Has anyone done a proper IR scan across a Fogstar bank under load? Would be curious whether it's uniform across cells or if one is carrying more of the current.

HeatherOllie | 847 posts

@NeilJackson interesting one — worth double-checking your busbars and cell terminal connections first before assuming it's the cells themselves. Loose or undersized connections create resistance that shows up as heat under load, and it's easy to overlook when everything looks tidy.

Also, what are you actually pulling from the bank? Four cells in parallel should spread the current pretty evenly, but if your BMS is seeing close to its 200A limit regularly, that's 50A per cell which is only 0.25C — shouldn't be causing warmth at all. So I'd be suspicious the heat source isn't where you think it is.

Have you got a cheap IR thermometer? Run it across terminals, busbars, and the BMS FETs under load. Often tells you the story immediately. 🙂

DailyDream | 1,203 posts

@NeilJackson what discharge rates are you actually hitting during peak load? With four cells in parallel you're effectively sharing the current across all of them, but if your busbar layout isn't properly balanced you could have one or two cells doing the heavy lifting while the others coast — that'll show up as localised warmth rather than uniform heating across the bank.

Also worth checking whether it's consistent warmth or spikes under specific loads. Garden office kit like monitors, kettles, or space heaters can cause some nasty transient spikes that a 200A Daly might not be throttling quickly enough.

What does your BMS data actually show for individual cell temperatures? If you're not already logging it, the Daly Bluetooth app gives you decent enough readouts to spot patterns over a few charge/discharge cycles.

Jonno71 | 412 posts

@NeilJackson I had something similar crop up with a different LiFePO4 build — turned out my cell-level fusing was adding meaningful resistance and generating heat I hadn't accounted for. Worth ruling that out if you haven't already.

Also, what's the ambient temp in your garden office? These cells aren't bad in cooler conditions but a poorly ventilated cabinet in summer can skew your readings considerably versus published spec. The Drift cells themselves have a decent reputation so I'd be looking at the installation variables before assuming the cells are the issue.

What's your actual measured voltage sag under load? If you're seeing significant sag that might point toward internal resistance being higher than expected — could be worth grabbing a proper cell IR meter if you don't have one.

HollyWatson | 412 posts

@NeilJackson I had something similar with my Drift cells last summer — worth checking whether your Daly BMS is actually load-balancing evenly across all four cells in parallel. Mine was showing one cell consistently shouldering more current than the others due to slightly unequal cable lengths between the parallel connections. Even a small difference in resistance can skew things noticeably under sustained load.

I'd suggest measuring the voltage drop across each individual cell's terminals while under load rather than just the overall bank voltage — it can be quite revealing. Also, what temperature are you actually seeing? "Warmer than spec" covers a lot of ground — LiFePO4 cells are generally fine up to around 45°C, but anything beyond that warrants proper investigation before assuming it's normal.

RobThompson | 312 posts

Had similar with my shepherd's hut build — turned out the Daly BMS itself was generating noticeable heat under sustained discharge, bleeding into my temp readings. Worth isolating where exactly the warmth is coming from before assuming it's the cells themselves.

Also worth checking your parallel bus setup. Four cells in parallel with uneven cable lengths can cause one cell to shoulder more current than the others — that cell will run hotter. Matched cable lengths, same gauge throughout, makes a real difference.

What's your peak load actually drawing? If you're regularly pushing past 0.5C on those Drifts I'd expect some warmth, but nothing alarming. Anything spiking beyond that consistently might point to the Daly struggling to balance things cleanly at high discharge.

NeilBarker | 847 posts

@NeilJackson Worth checking your cell-level internal resistance with a proper analyser if you haven't already — even brand new cells can have slight variation between them, and in a parallel configuration one cell doing more of the heavy lifting will run noticeably warmer than the others. I'd also have a look at your busbars and terminal connections. Loose or undersized connections add resistance exactly where you don't want it, and that heat has to go somewhere. What gauge cable are you running between cells and to the BMS? For a 200A continuous setup I wouldn't go below 70mm² personally, and even then keep the runs as short as possible. Also worth logging your actual discharge current over a typical day — you might be hitting peaks above what you think.

GrumpyBuilder | 1,203 posts

Four cells in parallel with a single Daly is asking for trouble honestly. That BMS isn't balancing anything useful at the cell level when you've wired it that way — it's just overcurrent protection at that point.

What's your actual discharge rate? If you're hammering anywhere near that 200A rating continuously, LiFePO4 will run warm, that's just physics. Fogstar Drift cells aren't magic.

Check your busbar connections too. Loose joints create resistance, resistance creates heat, and everyone blames the cells when it's actually a dodgy M8 bolt they undertorqued six months ago.

HazelHermit | 94 posts

@NeilJackson One thing nobody's mentioned yet — have you checked your busbar and cable termination torque? Loose connections are notorious for generating heat that gets misattributed to the cells themselves. With four cells in parallel you've got eight terminals plus your BMS connections, and any one of them being slightly under-torqued will cause localised resistance and warmth. Fogstar spec M6 terminals at 4Nm if I recall correctly. Grab an infrared thermometer and scan all your connections under load — you'll spot any hot spots immediately. Sorted a very similar "mystery" warmth issue on my own setup doing exactly that.