Anyone else had grief with JK BMS dropping cells at low temps?

Been running a 280Ah LiFePO4 bank in my shed-based off-grid setup since last autumn — four EVE 280Ah cells in a 12V configuration with a JK BMS (the 200A active balancer version). All summer it was absolutely rock solid, barely had to think about it.

Come November, I started getting random low-voltage disconnects on individual cells, usually cell 3. Pulled the data logs off the JK app and noticed it's nearly always happening when the shed temperature drops below about 5°C overnight. The cell voltage would sag to around 2.8V under even modest load (running a 300W inverter for maybe an hour), then the BMS trips out. Warm it back up to 10°C+ and everything reads fine again — cells balance back out within 20 minutes or so.

I've got a Victron SmartShunt on the bank as well, and the SOC never looks particularly low when this happens — usually sitting around 40-50%. I'm wondering whether cell 3 has developed some internal resistance issues, or whether this is just a known quirk of the JK's temperature compensation settings. Has anyone adjusted the low-temp cutoff parameters in the JK app to account for this, or is that just masking a deeper problem with the cell itself?

ForestSolar | 847 posts | ⚡ Solar Obsessive

@CopperGaffer Yeah, classic winter headache with LiFePO4. The JK's low-temp cutoff is your friend really — it's protecting the cells from lithium plating during charging, which causes permanent damage. Worth checking your BMS app settings though; the default charge cutoff temp is often set around 5°C which catches people out.

Practical fix that worked for me: a cheap seedling heat mat under the battery box with a simple thermostat controller, keeps the bank above 8°C overnight for pennies. Also make sure your charge disconnect threshold is set higher than your discharge cutoff — some units ship with oddly matched defaults.

What ambient temps are you actually seeing in the shed? Below zero regularly, or just hovering around freezing? That'll change what I'd suggest next.

GeorgeSmith97 | 203 posts | 🔋 Still Learning

@CopperGaffer Had almost identical grief last January with my 304Ah bank in an uninsulated outbuilding. What sorted it for me was setting a low temperature cutoff in the JK app — mine's configured to stop charging below 5°C now. Discharging at low temps is generally fine with LiFePO4, it's the charging that'll damage the cells.

Worth wrapping the battery box in some rigid PIR insulation board too — made a surprising difference overnight. I picked up offcuts from a local builders merchant for next to nothing.

What are your current BMS temp protection settings? The JK app gives you quite granular control over the thresholds, so if you haven't dug into that menu yet it's probably your quickest win before spending anything.

ExFarmer79 | 1,204 posts | 🐑 Powered by Spite and Solar

JK's low-temp cutoff is doing exactly what it should — charging LiFePO4 below ~5°C destroys cells, ask me how I know (RIP my first Fogstar pack, 2022-2023, never forget). Wrap your bank in 25mm Kingspan, chuck a cheap reptile heat mat on a thermostat underneath, job done for under a tenner. Mine in the shepherd's hut hasn't complained since. If you want to raise the cutoff threshold, JK's app lets you adjust the low-temp charge protection value directly — but I'd leave it above 5°C minimum, not below. The BMS isn't the problem here, your insulation is.

DaleSeeker | 412 posts | 🔧 Tinkering Since Forever

Worth checking whether your JK is set to cut on discharge or charge at low temps — they're separate parameters and it catches people out. Charging LiFePO4 below about 5°C is genuinely damaging, but you can often still discharge safely down to -10°C or so, depending on your cells. If you're losing power to your loads entirely in cold snaps, dig into the BMS app and make sure your low-temp discharge cutoff isn't set unnecessarily high. I had mine accidentally mirrored from the charge settings when I first configured it. Also, a cheap wireless thermometer inside the battery enclosure is well worth it — knowing actual cell temps versus ambient shed temps was a bit of an eye-opener for me last February. 🌡️

BayPete | 847 posts | 🚐 Perpetually Parked Somewhere Damp

Had this exact drama in my motorhome last February — woke up to a dead 12V system at about 3°C ambient. The JK had done its job perfectly, which was cold comfort at 7am in a Cornish layby.

What sorted it for me was adding a cheap self-regulating heating mat directly against the cell faces, wired through a Victron temperature sensor feeding a relay. Kicks on before the BMS even gets twitchy.

@DaleSeeker raises the right question — discharge and charge cutoffs are separate parameters in the JK app and plenty of people only check one. Worth scrolling through the full protection settings rather than just the headline numbers.

Also worth knowing: EVE cells specifically get grumpy below 0°C for charging but can discharge somewhat lower — make sure your thresholds reflect that distinction.

KangooNomad | 2,891 posts | 🏠 Tiny House, Massive Spreadsheets

Something worth adding that nobody's mentioned — the JK's temperature sensor placement matters enormously here. Mine was reading 2–3°C warmer than actual cell surface temp because I'd zip-tied it near the busbars rather than directly against a cell face. Was triggering cutoff later than it should have, paradoxically. Moved it to the coldest cell in the pack (bottom of the stack, furthest from any heat source), and suddenly the behaviour made much more sense.

Also check firmware version — JK pushed updates last year that changed how the low-temp protection hysteresis behaves on the 200A active balancer units specifically.

MultiPlusNerd | 2,891 posts | ⚡ Grumpy But Correctly Wired

Slap a self-regulating heating pad on your cells and a cheap DS18B20 temp probe feeding back to the BMS — sorted it in my van last winter when the Fogstar cells started sulking below 5°C, though obviously your shed isn't rattling down the A303 at midnight like mine. The JK's just doing its job protecting you from yourself, really.

Smithy51 | 673 posts | ⚡ Bodging it properly since 2019

One thing nobody's mentioned yet — have you checked where you've actually mounted the temperature sensor? Mine was cable-tied to the busbar rather than against the cell body, so it was reading ambient rather than cell temp. Took me an embarrassingly long time to spot that one. Once I stuck it properly against the side of the middle cells with some kapton tape and a jubilee clip, the readings became far more accurate and the phantom cutoffs stopped.

Also worth insulating the battery bank itself — even just wrapping in some old sleeping bag material inside the shed. Keeps the cells above that critical threshold overnight without any need to fiddle with BMS settings. Addresses the root cause rather than just moving the goalposts on the cutoff values.