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

Just finishing up a van build with four 70Ah Eve cells wired in series for a 12V 280Ah bank. Running a JK BMS (the 200A active balancer version) and overall I'm dead chuffed with it — balance function is brilliant and the app gives you far more data than I expected for the money. Picked it up off AliExpress for about £45 delivered.

Problem is the Bluetooth drops out almost every time the bank hits around 90–95% SOC during a bulk charge from my Victron SmartSolar 100/30. I'll be watching the cell voltages tick up nicely on the JK app and then it just disconnects. Phone is an Android 13, sitting literally 30cm from the BMS. The charge carries on fine — protection trips haven't fired and the cells look balanced when I reconnect — but it's annoying not being able to watch that top-end balancing happen in real time.

I've tried toggling Bluetooth off and on, force-stopping the app, even tried a different phone with the same result. Read somewhere it might be interference from the Victron MPPT or possibly a firmware thing on the BMS itself. Current firmware is listed as V11.25 in the app.

Has anyone seen this before or found a fix? Worth flashing newer firmware, or is this just a known quirk I need to live with?

WayneClark | 847 posts

@PennineCruiser I had the exact same issue on my narrowboat build last spring. Turned out the Bluetooth was dropping because the BMS was throttling power to the comms chip during high charge current — essentially a firmware quirk rather than a fault as such.

Worth trying two things: first, make sure your phone's battery optimisation isn't killing the app in the background. Second, check you're on the latest JK firmware — there was an update that specifically addressed Bluetooth stability under load.

Also worth noting, the active balancer on that unit does generate a bit of heat during bulk charge, which can affect the onboard electronics if ventilation's poor. Where have you mounted yours?

Cleggy51 | 312 posts

@PennineCruiser Seen this crop up a few times now with the JK units. Worth checking whether it's actually dropping connection or just the app timing out — the JK app has a notoriously short idle timeout and won't always reconnect cleanly without a full restart.

Also, if you're running the BMS near any MPPT or inverter charger, RF interference can absolutely murder the Bluetooth signal mid-charge when switching loads kick in. Try moving your phone closer during a charge cycle and see if it holds — narrows down whether it's interference or a firmware quirk.

What firmware version are you on? There were some stability improvements in later releases worth flashing if you haven't already. The JK PC software via the RS485 adaptor is far more reliable than Bluetooth anyway if you've got a laptop handy for monitoring.

Trevor1967 | 1,204 posts

@PennineCruiser Had this on my shed build about eight months back — same JK active balancer unit. What sorted it for me was making sure the BMS firmware was fully up to date using the JK app on Android rather than iOS (the Apple version seemed flakier for me personally). Also worth checking your phone's Bluetooth isn't set to auto-disconnect low-activity devices — Android has a habit of dropping connections it considers "idle" even when the BMS is mid-cycle. Beyond that, keeping your phone within about two metres during the initial pairing seems to help establish a more stable connection going forward. Bit fiddly but once it's sorted it's solid. Four Eve cells is a cracking little bank by the way — should serve you well. 👍

Nige1977 | 203 posts

Had almost identical grief on my garden office build last year — JK BMS, 280Ah Eve cells, same Bluetooth dropout headache. What finally cracked it for me was repositioning the BMS away from the Victron SmartSolar MPPT. Turns out the two were sat about 15cm apart and the MPPT was throwing out enough interference to knock the Bluetooth off when bulk charging kicked in hard.

Moved the JK to the opposite side of the enclosure, added a short section of foam padding underneath to dampen any vibration, and it's been rock solid since — we're talking eight months without a single dropout.

Worth ruling out before you go down the firmware rabbit hole that @WayneClark and @Trevor1967 have likely already mentioned.