Anyone else had issues with JK BMS dropping cells randomly in cold weather?

Had this exact problem last winter with my motorhome build. Running a 200Ah Fogstar Drift LiFePO4 pack with a JK 200A BMS and when ambient temps dropped below about 4°C, it would randomly disconnect cell 3 from the balancing circuit — not a full BMS trip, just the cell reading would flatline momentarily then recover.

Spent weeks chasing what I thought was a dodgy cell before realising the JK's NTC temperature sensor placement was the culprit. Mine was zip-tied to the busbar rather than seated against the cell body properly, so the BMS was reading artificially low temps and triggering the low-temperature protection threshold inconsistently.

A few things worth checking:

• NTC sensor placement — it needs direct contact with a cell surface, not floating near metalwork
• Low temp charge protection setting — JK defaults vary wildly depending on firmware version; mine was set to 5°C which is arguably too aggressive for LiFePO4
• Firmware version — there were some known balancing bugs in earlier JK Active Balancer firmware that caused phantom cell drops

I reflowed the sensor with some thermal paste and repositioned it, then nudged the low-temp threshold down to 2°C (not below zero, obviously — not willing to risk lithium plating), and it's been solid through this winter so far.

What firmware are you running? And is it actually dropping the cell from balancing specifically, or are you seeing a full protection trip? The behaviour is quite different and points to different root causes.

Would also be curious whether anyone's tried the JK with a secondary external temperature sensor for better cold-weather accuracy — seems like an obvious gap in the product.

@DefenderDream classic JK cold weather behaviour - it's the low temperature cutoff kicking in, but the issue is often the NTC thermistor placement rather than the BMS itself being faulty.

On my narrowboat I had identical symptoms last winter. The thermistor was reading air temp near the BMS rather than actual cell temp, so it was seeing ambient drop and triggering the LTO cutoff prematurely.

Fix that worked for me:

• Repositioned the NTC probe directly onto the centre cell group with thermal paste and kapton tape
• In the JK app, check your Cell Low Temp Protection setting - mine was factory set at 5°C which is far too conservative for LiFePO4
• Set it to 0°C for discharge, -10°C for charge (Fogstar Drift cells are rated down to -20°C discharge)

Also worth checking firmware version - JK pushed an update mid-2023 that improved thermal sensor handling significantly.

Experienced almost the identical thing in my motorhome last January parked up in Scotland. The frustrating part nobody mentions is that the JK app often shows the cell temps as fine right up until the disconnect happens — turns out my temperature sensor wasn't making proper contact with the cells themselves, just floating near them inside the enclosure.

Once I repositioned the sensor properly against the cell body and used a tiny dab of thermal paste, the readings became accurate and the BMS behaviour made much more sense.

Worth checking before you start tweaking any cutoff thresholds in the app — changing those settings to compensate for a dodgy sensor reading is masking the problem rather than fixing it. The Fogstar Drift cells themselves handle cold reasonably well in my experience; usually the sensor placement that's the culprit.

Worth adding that I've seen this in my static caravan setup too — JK BMS was cutting out intermittently and I spent ages chasing a supposed cell imbalance before realising temperature was the actual culprit.

One thing that helped me was fitting a small self-regulating heating pad directly to the battery pack, wired to a Victron Smart Battery Protect so it only activates below a set voltage threshold. Keeps the cells above that critical 5°C mark without draining the bank unnecessarily overnight.

Also worth checking your BMS parameters via the JK app — the low temp charge cutoff and discharge cutoff are sometimes set very conservatively from the factory. Mine were both higher than they needed to be for LiFePO4 chemistry. You can tweak them carefully, just don't go too aggressive or you risk actual cell damage in genuine cold conditions.

@DefenderDream one thing worth checking that nobody's mentioned yet — have you looked at your cell internal resistance readings in the JK app during the cold spells?

In my experience the BMS can trip on high resistance thresholds before it even hits the low temp cutoff. Cells that read fine at 20°C can spike massively when cold, and if your protection settings are left at defaults they're often way too tight for real-world winter conditions.

I had similar grief with my static setup last February. Went into the app and bumped the cell resistance protection threshold up slightly — problem largely went away. Worth logging the data rather than just looking at voltage when it drops out.

Also worth insulating the battery compartment itself. A bit of Kingspan makes more difference than people realise before fiddling with BMS settings.