Anyone else had issues with JK BMS dropping Bluetooth connection on cold mornings?

Got a JK BMS (the 200A 8S model) wired up to a 280Ah LiFePO4 bank in my van build and I keep losing the Bluetooth connection on cold mornings — talking anything below about 5°C. The BMS itself seems to be functioning fine, balancing looks correct and the cells are all sitting happy around 3.28–3.30V at rest. It's just the app connection that goes completely dead until the van warms up a bit.

I'm running the JK BMS app on Android (Samsung A54), and I've tried forgetting the device and re-pairing, toggling Bluetooth off and on, all the usual faff. Nothing works until the ambient temp creeps back up. Wondering if it's a known issue with the Bluetooth module on these units or whether it's more likely a phone issue. Has anyone tried a different app like overkill or xiaoxiang with these JK units to see if that makes a difference?

Also curious whether anyone has sorted this with a firmware update — I'm on v11.26 I think, not 100% sure if there's something newer worth flashing. Don't want to brick the thing obviously, so a bit cautious about just diving in without knowing others have done it successfully first.

IanWhite replied:

@BenKing80 yes, seen this exact issue on mine and a few others have mentioned it over the years. The JK's Bluetooth module seems particularly sensitive to cold — the onboard coin cell that powers the BT chip when the main board is in low-power mode loses voltage sharpish below 5°C.

Worth checking whether it's actually losing connection or just taking longer to advertise — try leaving the app open and waiting a minute or two before giving up on it.

If it's genuinely dropping, I'd try keeping the BMS itself slightly insulated from the coldest air in the van. Mine lives in a small enclosure with a bit of closed-cell foam around it and the problem largely went away.

Also make sure you're on the latest firmware — JK did quietly address some BT stability stuff in more recent versions.

StormyNomad replied:

@BenKing80 worth checking whether the BMS is actually losing power momentarily rather than just dropping Bluetooth. On mine I found the Bluetooth module itself draws from a separate tiny capacitor circuit, and when temps drop the internal resistance spikes enough to cause a brief brownout on that side of things.

Try leaving your phone's Bluetooth on and just walking away — if it reconnects automatically after a minute or two without you doing anything, that's likely what's happening rather than a genuine fault.

Also double-check your app version. JK pushed an update a while back that supposedly improved reconnection behaviour in low temps. Doesn't fully fix the root cause but it's less annoying in practice. What firmware version is showing on the BMS itself?

MoorRoger replied:

@BenKing80 I had this with my JK on a similar setup last winter. Worth trying a dedicated short cable directly from the battery to the BMS power input rather than running it through your main bus — I was getting tiny voltage sags on cold starts that were enough to reset the Bluetooth module without triggering any proper fault.

Also, the JK app has a tendency to simply time out if the phone's own Bluetooth stack is struggling in the cold. Try keeping your phone inside your sleeping bag for a few minutes before connecting — sounds daft but it genuinely helped me narrow down whether the issue was the BMS or the phone end.

What firmware version are you on? There were some stability improvements in later releases that helped mine considerably.

Gazza55 replied:

@BenKing80 I've noticed the JK app itself can be part of the problem on cold mornings — the Bluetooth stack on some Android phones behaves oddly when the phone's been sat in a cold van overnight. Worth trying with the phone warmed up first just to rule that out before you go poking around at the BMS wiring.

Also, have you checked your BMS firmware version? JK pushed out a few updates a while back and one of them specifically mentioned stability improvements. You can check and update through the app when you do have a connection. Might be worth grabbing the latest version if you're running something older.

Beyond that, @StormyNomad raises a fair point about power drops — I'd add a small capacitor across the BMS power supply has helped some folks with exactly this kind of intermittent behaviour.

TinaHenderson replied:

@BenKing80 worth knowing the JK BMS Bluetooth module runs off the balance port power, and at low temps the cell voltages can sag just enough to cause the module to brown out — even if the BMS itself stays on. Saw something similar in my shepherd's hut build before I moved to a Victron setup.

One thing nobody's mentioned: try enabling the low temperature charge protection in the JK app properly, and while you're in there check the Bluetooth signal interval setting — some firmware versions let you adjust it. Occasionally a simple firmware update via the app sorted the dropout issue for people on the JK forums.

Also worth insulating the BMS itself if it's mounted somewhere exposed. Mine was clipped to an aluminium frame and acting as a heat sink basically.

PeteDixon89 replied:

@BenKing80 Something I haven't seen mentioned yet — try disabling battery optimisation for the JK BMS app in your Android settings. On cold mornings the phone itself tends to throttle background processes more aggressively, which can kill the Bluetooth handshake before it's properly established. Settings > Apps > JK BMS > Battery > Unrestricted. Made a noticeable difference for me when I was getting similar dropouts on my setup over winter. Also worth making sure you're on the latest firmware for the BMS itself — JK pushed an update a while back that apparently improved the Bluetooth stack stability. You can check via the app when you do manage to connect. Combining that with what @TinaHenderson mentioned about the balance port power should cover most bases.