Anyone else had issues with JK BMS dropping cells randomly on cold mornings?

Had this exact problem last winter on the boat. Three cells would just vanish from the JK display on mornings below about 4°C, then magically reappear once the cabin warmed up a bit. Drove me absolutely mental trying to diagnose it.

Turned out to be the balance leads rather than the BMS itself — the connectors were getting moisture ingress overnight and the resistance was enough to confuse the JK's cell voltage sensing. Cleaned everything with contact cleaner, added a bit of self-amalgamating tape over the connector housing, and it's been solid since.

That said, I'm still not 100% convinced the JK handles cold-related cell resistance changes as gracefully as it should. When I compare notes with a mate running a Daly on a similar Fogstar lithium bank, his BMS seems far less twitchy in the cold. Could be coincidence, could be firmware, hard to say.

Worth checking:

• Connector integrity first — moisture is the usual culprit on boats especially
• Firmware version — JK pushed some updates last year that supposedly improved low-temp sensing stability
• Cell temperature sensors — are they actually making proper contact with the cells?

What firmware are you running? And is this on a fixed install or somewhere with temperature swings and potential dampness? Makes a big difference to how you'd approach it.

Curious whether others are seeing the same thing or whether mine was just a dodgy connector situation all along.

@ExBrickie mate, your BMS is basically doing what we all do on cold mornings — refusing to function until it's had a warm-up. 🥶

Jokes aside, classic contact resistance issue — thermal expansion/contraction loosening the cell sense wire crimps on the balance connector. Pull the JST plug, reseat every sense wire crimp individually, and dab a tiny bit of dielectric grease on the connector before you push it back in.

Mine did identical on my 280Ah Fogstar cells last February — turned out one crimp was barely making contact and the cold was just enough to break it. Twenty minutes with a crimping tool and it's been rock solid since, even through some properly Baltic Scottish mornings.

@ExBrickie saw almost identical behaviour on my shepherd's hut setup last February. Turned out the balance wire connectors had developed just enough resistance at low temps to confuse the JK into thinking those cells were gone. Reseating everything with a tiny bit of dielectric grease sorted it completely.

Worth checking your connector crimps with a multimeter before assuming it's the BMS itself — I chased a firmware red herring for two weeks before finding a dodgy JST connector that was perfectly fine at room temp but borderline at 3-4°C. Classic cold solder/connection issue rather than a genuine BMS fault.

The JK units are generally solid in my experience, so I'd rule out the physical connections first.

@ExBrickie this is a known weakness with the JK's cell voltage sensing at low temps — the balance wire connectors @WingAndPrayer mentioned are only part of the story. The bigger culprit is usually the JK's internal voltage reference circuit drifting below about 5°C, causing it to misread cells as undervoltage and effectively "drop" them from the display.

Seen this on my narrowboat setup with a 200Ah Fogstar Drift pack. Fixed it by:

1. Insulating the BMS itself, not just the cells — neoprene wrap made a noticeable difference
2. Checking all JST connector pins for oxidation and applying Servisol
3. Enabling the JK's low-temperature charge cutoff properly so it fails safely rather than randomly

If you're running below 0°C regularly, honestly consider a Victron SmartShunt alongside the JK — gives you independent monitoring so you're not flying blind when the JK gets confused.

@WingAndPrayer and @LH_Marine are both pointing in the right direction on the connector front.

Had this exact scenario in my shepherds hut build last January — ambient dropped to -3°C overnight and the JK was reporting three "missing" cells by 6am. Swapped out the stock JST-style balance leads for proper crimped Dupont connectors with a thin smear of dielectric grease on each pin. Haven't seen a phantom dropout since, across two full winters now.

Worth also checking your balance lead routing — if they're running anywhere near a cold metal chassis or floor, the connector bodies themselves can contract just enough to lose contact intermittently. I rerouted mine up and away from the hut's steel subframe and cable-tied them loosely to give a bit of thermal buffer.

The JK firmware doesn't handle marginal connections gracefully at all in low temps. Fix the physical connection first before chasing software settings.