Anyone else had issues with JK BMS dropping Bluetooth mid-charge on a 280Ah LiFePO4 pack?

Been running a 4x280Ah Winston cells setup in my converted Sprinter for about eight months now, paired with a JK BMS (the 200A active balancer version). Generally it's been rock solid but lately I've noticed the Bluetooth connection on the JK app drops out almost every time the pack hits around 90% SOC during a solar charge. It'll reconnect after a minute or two, but it's making me a bit twitchy given I can't monitor what's happening at the top of the charge curve.

I'm running a Victron SmartSolar 100/30 as the charger, absorption set to 3.55V/cell (14.2V), float at 13.5V. The BMS firmware is on v11.25 — I did check for updates last week and that seems to be current. Phone is an Android 12 handset, tried two different phones with the same result.

Has anyone seen this before? I'm wondering if it's interference from the Victron kicking into absorption and causing some kind of voltage fluctuation that upsets the BMS comms, or whether it's more likely a firmware bug. I did spot a thread on a German forum suggesting a dodgy solder joint on the BMS Bluetooth module can cause exactly this, but I'd rather not start poking around inside with a soldering iron just yet.

Curious whether anyone's solved it cleanly, or if there's a setting I'm missing in the JK app that might help stabilise the connection.

Ben1960 | 847 posts

@ShaunWard had the exact same thing on my 4S 280Ah EVE build last summer. Turned out to be a firmware issue — JK released an update that sorted it for a lot of people. Worth checking which firmware version you're on; mine was stuck on an older build and the BT stack was clearly struggling under higher charge currents.

Also worth checking your phone's Bluetooth settings — some Android versions aggressively kill background connections. Try keeping the app in the foreground during charge cycles as a temporary workaround.

If the firmware update doesn't crack it, there's a known fix involving adjusting the "BT disconnect timeout" parameter in the advanced settings. Happy to walk you through it if needed. What's your current charge current set to?

ShedGenius | 1,203 posts

Worth checking the BMS firmware version first — JK pushed an update a while back specifically addressing BT stability during high-current events. The app's a bit rubbish about flagging when updates are available.

Also seen this blamed on the Bluetooth antenna getting too warm if the BMS is mounted in a poorly ventilated spot. On my static van setup I had to reposition mine away from the Victron MPPT — made a noticeable difference.

One other thing: are you running any other Bluetooth devices nearby? Some inverter/charger combos create enough RF interference to knock JK's BT off temporarily. Worth isolating the variable if you can.

What firmware are you currently on? Should be visible in the app under device info.

Rob1963 | 2,341 posts

Classic JK Bluetooth dropout — nine times out of ten it's the app losing the connection rather than the BMS actually having a wobble. Try the JK BMS app rather than whatever third-party tool you're using, and keep your phone within two metres during high-current charging. The BLE radio on these units shares processing overhead with the balancing logic, so when your 200A active balancer kicks in hard across unmatched cells, Bluetooth gets starved. Had identical grief on my static caravan build before I realised my Winston cells were slightly mismatched from day one. Check your cell voltage spread during the dropout — if it's wider than ~20mV, the BMS is working overtime and dropping BT is almost expected. @Ben1960 and @ShedGenius have covered firmware so won't repeat that, but definitely rule out the mundane before assuming hardware fault.

PanelNige | 412 posts

Had this on my motorhome build — worth checking if it's dropping BT specifically during bulk charge when the active balancer is working hardest. The 200A balancer generates a fair bit of RF noise internally and I reckon it can interfere with the BT module on certain units.

Try moving your phone closer and killing any other BT devices nearby when testing. Also — and this sounds daft — make sure the BMS isn't tucked right against metal enclosure walls. Mine was bolted flush inside a steel battery box and signal was terrible.

If it's genuinely the module dying rather than interference, JK's support is actually decent, got a replacement BT board sent out no bother.

@Rob1963 makes a fair point though — rule out the app first before assuming hardware.

SparkyGrafter | 87 posts

Curious whether this is temperature-related? My JK on the boat drops BT occasionally and I eventually noticed it tends to happen when the BMS enclosure gets warm — not hot, just warmer than ambient. Wonder if there's some thermal throttling going on with the comms chip.

@PanelNige mentions bulk charge specifically — is that when your cells are working hardest and generating the most heat?

Also, are you using the Android or iOS app? I've seen people mention the two behave quite differently for connection stability. Worth trying the other platform if you haven't already — I borrowed a mate's Android just to test and it held the connection far better than my iPhone was managing.

Does the BMS itself continue functioning normally when BT drops, or are you seeing any actual protection events logged?

CamperSam | 634 posts

Worth checking your phone's Bluetooth stack rather than blaming the JK outright — Android in particular loves to silently throttle BT during heavy app activity. Try the JK BMS app on a dedicated old phone or tablet you leave permanently paired. That solved it for me on my cabin setup.

Also, 200A active balancer on Winston cells mid-charge is doing a lot of work. Check the BMS firmware version — JK pushed an update a while back specifically addressing connection drops under high balancing load. Update via their PC tool, not the app.

@SparkyGrafter raises a fair point on temperature but I'd rule out the firmware thing first — it's the lowest effort fix and genuinely sorted several people's issues on here before.

If it's still dropping after all that, honestly just run a serial Bluetooth adapter to a Raspberry Pi and stop fighting the app altogether.