Anyone else had their JK BMS throw a low temp charge fault in winter even with heating pads fitted?

Running a 280Ah LiFePO4 bank in my van (four 70Ah EVE cells in series) with a JK BMS — the 2A active balancer version. Fitted a 100W silicone heating pad under the battery box back in October, controlled by an Inkbird IBS-TH2 thermostat set to kick in at 5°C. Thought I had it sorted.

Problem is, last week when temps dropped to around -3°C overnight near Inverness, the BMS threw a low temperature charge protection fault and cut off charging at about 7am when the solar started coming up. The Inkbird was reading 8°C at the cell surface, so the pad had clearly done its job — but the JK's own NTC sensor was sat on the side of one cell and was reading 3.8°C. BMS limit is set to 5°C, so it tripped.

I'm wondering if the issue is just sensor placement — maybe I need to move the NTC to somewhere the heating pad actually warms up first, or raise the BMS low temp threshold slightly. Has anyone else wrestled with this? I'm a bit reluctant to just bump the threshold up without understanding why there's a 4°C gap between the two sensors in the first place.

Had the exact same thing last January in my motorhome. The pad was working fine but the sensor placement was the issue — JK reads cell temp not ambient, so if the probe isn't actually touching the cells properly you'll get false readings even when the box is warm.

Worth checking where your temp probes are seated. Mine were just loosely resting against the side rather than properly taped down with thermal paste/tape. Fixed that and the faults stopped.

Also double-check your low temp charge cutoff threshold in the JK app — mine shipped with it set at 5°C which is quite aggressive. Bumped mine down to 0°C and that helped too during marginal temps.

HarbourSoul | 847 posts

@FinnCampbell70 Classic JK quirk this one — the temperature sensor placement is everything. The BMS is reading from its own NTC probe, not from where your cells actually are. If that probe is sitting close to the box wall or near a vent gap, it'll see ambient before your heating pad has had chance to do anything useful.

Try relocating the NTC probe so it's sandwiched directly against one of the middle cells with a bit of self-amalgamating tape holding it flush. Middle cells tend to be the coldest in a series bank, so you're getting a truer reading AND satisfying the BMS quicker.

Also worth checking your low-temp charge cutoff threshold in the JK app — some units ship with it set surprisingly high (around 5°C). Dropping it to 0°C is perfectly safe with LiFePO4 if your pad is maintaining that.

Yep, @Clive1999 and @HarbourSoul have the sensor placement nailed — but also worth checking your low temp charge cutoff threshold in the JK app, as it ships set quite conservatively (often 5°C) and your pad might be keeping the box warm whilst the cells themselves lag behind on a cold morning start.

Two quick fixes:

• Move the BMS temp sensor directly against a cell, not the bus bar or case
• Drop the charge cutoff to 0°C if you're confident your pad maintains that minimum

Also worth sticking a cheap kapton tape probe thermometer on cell 1 and cell 4 simultaneously — you'll likely find a surprising delta across the bank in a van environment.

Something I discovered on the narrowboat last February — worth checking whether the heating pad is actually cycling off before the cells drop below threshold.

Mine was on a basic thermostat that cut out at 5°C, which sounds fine until you realise the air gap between pad and cell means thermal lag of several degrees. By the time the pad kicked back in, the BMS had already seen 2°C at the sensor and latched the fault.

Moved to a Victron Battery Protect arrangement with the pad running continuously off a dedicated small lead-acid buffer during cold nights. Solved it entirely.

The JK's low temp cutoff being a latching fault rather than auto-recovery catches a lot of people out too — you'll need to physically reset it once temps recover, which isn't obvious from the manual.

Russ1992 | 234 posts

Had exactly this on my build last January. One thing nobody's mentioned yet — check the actual wiring to your heating pad. Mine was drawing enough current that there was a voltage drop causing the pad to run significantly cooler than rated. Measured the pad surface at only 18°C when the thermostat said it should've cut off at 22°C. Switched to a heavier gauge cable and proper ring terminals and the problem largely went away. Also worth verifying your thermostat probe is sandwiched directly against a cell rather than sitting in free air inside the box — ambient air in there can be 5-6°C warmer than the cells themselves on a cold morning, so your pad cuts out thinking everything's fine when the cells are still cold.

Thommo53 | 891 posts

Good thread this. One thing I'd add to what @Russ1992 is hinting at — double-check the wiring to your heating pad. I had a similar nightmare last winter and eventually traced it to a poor crimp on the positive feed to the pad. Pad was drawing just enough current to fool me into thinking it was working, but wasn't generating meaningful heat. Stuck a clamp meter on it and the draw was way below what a healthy 100W pad should pull. Swapped the connector, sorted immediately.

Also worth mentioning — silicone pads heat from the bottom up, so if your BMS temperature sensor is mounted on the side of a cell rather than underneath, you might be seeing cold readings even when the cells themselves are adequately warm. Just a thought before you start tweaking BMS parameters.

KN_Builds | 1,247 posts

Worth adding — have you checked the cell temperature sensor placement on the JK itself? The BMS reads from its own NTC sensors, not from any external thermostat controlling your heating pad. If those sensors are tucked against the BMS board rather than properly contact with the cells, it'll be reading ambient air temp inside the box, not actual cell temperature. Cold air pocket forms overnight, BMS panics and pulls the charge cutoff even if your pad has warmed the cells themselves. A bit of thermal paste and some kapton tape to secure the sensors flush against the cell casing sorted mine completely.

DaiWalker61 | 412 posts

Good shout from @KN_Builds on sensor placement — I'd extend that and say also check which temperature sensor the JK is actually using to trigger the low temp cutoff. On mine I found the BMS has its own onboard sensor as well as the external ones, and depending on firmware it can prioritise differently. Worth connecting via Bluetooth and watching the temp readings live while the pad is running — you might find one sensor is sat in a cold dead zone while the pad is happily warming elsewhere. Repositioning the external sensor closer to the actual cells sorted it for me.

PeakVanLifer | 634 posts

Had almost identical faff last January in my shepherd's hut build. Turned out my heating pad thermostat was cutting out way too early — set to kick off at 5°C but the BMS sensor was reading 2°C because it wasn't in the same thermal zone as the pad.

Fixed it by wrapping the whole battery box in 50mm celotex and setting the pad to trigger at 8°C instead. Zero issues since.

Also worth checking — JK app lets you adjust the low temp charge cutoff threshold directly. Mine was default 5°C which is tighter than needed. Dropped it to 0°C for winter.

VickyMurray | 891 posts

@FinnCampbell70 one thing nobody's mentioned yet — check your JK BMS app settings for the actual low temp charge cutoff threshold. Mine was factory-set to 5°C, which sounds reasonable until you realise the BMS acts on cell temp, not ambient. Even with a pad running, my cells were sitting at 3–4°C on really cold mornings before the pad had chance to do its job. I bumped the threshold down to 0°C and added a 30-minute timer on the pad via a cheap thermostat controller so it pre-warms before any charging starts. Sorted it completely.