JK BMS Alarms List: Every Alarm, Its Cause and the Fix
A JK BMS reports its alarms as a set of flags in the live data it sends over Bluetooth. The app shows them as red text, and the Detail Log records the matching events. Some alarms are protections that switch a MOSFET off until a recovery condition is met. Others are faults in the hardware or the wiring that no setting will clear. This list covers every alarm the JK02 protocol reports, what it means and where to look.
How to read an alarm
Ask three questions for each alarm.
- Is it a protection or a fault? A protection (over-voltage, over-current, temperature) clears itself when the value returns past its recovery threshold. A fault (MOSFET, coprocessor, sensor) needs a check of the hardware.
- Which setting governs it? Every protection has a trip value, a recovery value or delay, and sometimes a related setting that makes it trip too early.
- How often does it happen? One trip on a cold morning is normal. The same trip every day is a setting or a cell problem. The Detail Log answers this, see reading the Detail Log.
Voltage alarms
| Alarm | What it means | Settings that govern it |
|---|---|---|
| Cell overvoltage | A cell went above Cell OVP, charging stops | Cell OVP, OVPR, RCV, SOC-100% voltage, balance start |
| Pack overvoltage | The whole pack is above its limit | Cell OVP and cell count |
| Cell undervoltage | A cell went below Cell UVP, discharging stops | Cell UVP, UVPR, SOC-0% voltage, power off voltage |
| Pack undervoltage | The pack voltage is below its limit | Cell UVP and cell count |
| Battery fully charged | Information only: the charge reached the full condition | RCV and RCV time |
Cell over-voltage that trips at the top of every charge usually means the charger voltage is above cells × OVP, or one cell runs ahead of the others. The overcharge protection guide goes through both cases. Repeated undervoltage on one cell points to a weak cell, see the weak cell guide.
Current alarms
| Alarm | What it means | Settings that govern it |
|---|---|---|
| Charge overcurrent | Charge current above the limit for longer than the delay | Max charge current, charge OCP delay and recovery |
| Discharge overcurrent | Discharge current above the limit for longer than the delay | Max discharge current, discharge OCP delay and recovery |
| Discharge overcurrent level 2 and 3 | Faster, higher current stages of discharge protection | Hard-coded on most boards |
| Charge or discharge short circuit | A current spike far above the rating | SCP delay and SCP recovery |
| Short circuit release failed | The BMS could not recover after a short circuit | Check the load, then power cycle |
Inverters that start a large motor or a capacitor bank can trip short circuit protection at switch-on. The OCP and SCP guide explains which values are safe to change.
Temperature alarms
| Alarm | What it means | Settings that govern it |
|---|---|---|
| Charge overtemperature | Battery too hot to charge | Charge OTP and OTPR |
| Charge undertemperature | Battery too cold to charge | Charge UTP and UTPR, heating |
| Discharge overtemperature | Battery too hot to discharge | Discharge OTP and OTPR |
| Battery overtemperature | Battery temperature warning on newer boards | Discharge OTP |
| Discharge undertemperature | Battery too cold to discharge (newer firmware) | Discharge UTP and UTPR |
| MOSFET overtemperature | The BMS power stage is too hot | MOS OTP and OTPR |
| Temperature sensor fault | A probe is missing, broken or shorted | Disable temperature sensors (troubleshooting only) |
Charge undertemperature on the first cold night is the most common surprise. Many units leave the factory with Charge UTP at -20 °C, which is far too low for LFP. Set it to 2 to 5 °C, see the temperature and heating guide. MOSFET overtemperature means the board is working too hard: too much current, poor airflow or a loose power connection.
Hardware and wiring faults
| Alarm | What it means | What to do |
|---|---|---|
| Balance wire resistance too high | A sense lead or its connector has high resistance | Check crimps and the wire resistance values, see the wire resistance guide |
| Cell count does not match the settings | Fewer or more cells detected than configured | Fix the Cell Count setting or a loose sense lead |
| Charge or discharge MOSFET fault | The BMS could not switch a MOSFET | Contact the seller, the board may be damaged |
| Discharge switch-on failed | The discharge MOSFET did not turn on, often into a large capacitive load | Use precharge, check the load |
| Coprocessor communication error | The main chip lost contact with the measuring chip | Power cycle; if it returns, contact support |
| Parallel module fault | The parallel current limiter (PCL) reported a fault | Check parallel wiring and the PCL setting |
A cell count mismatch also hides other alarms, because the BMS cannot measure correctly. Fix it first.
Remote and information flags
The list also has flags that are not faults: GPS disconnected and Locked by GPS (for units with a GPS tracking module, used by rental and fleet owners), and Password change due, which some boards show while the default settings password is still in use. See the password guide.
Finding the pattern
A single alarm tells you what happened. A pattern tells you why. Export the Detail Log and drop it into the log analyser. It pairs each trip with its release, shows the cell voltages and temperatures around it and links each finding to the setting that governs it. When you need to change a value, connect your BMS in the browser, read the settings and fix the threshold with a backup in hand. The parameters reference explains every field mentioned here.
FAQ
Can I clear an alarm from the app?
A protection clears when its recovery condition is met. A fault alarm stays until the cause is gone or the BMS restarts.
Why do I see an alarm in the app but nothing in the log?
The log records events, not every live flag. Short warnings such as wire resistance may only show live.
Is an "unknown alarm" dangerous?
It means a flag without a published meaning. Note when it appears and export the log, then ask the seller.
Related guides
- JK BMS Log Messages Explained: Boot, Time calibration, ProtectionsWhat each JK BMS log message means: Boot, Time calibration, APP open charge, protection trips and releases, Discharge On Failed and Unknown Log Code [0].
- JK BMS Parameters Explained: Every Setting by GroupJK BMS parameters explained in one glossary: cell OVP, OVPR, UVP, UVPR, balance, SOC, RCV and RFV, current and temperature limits, with typical LFP ranges.
- JK BMS Temperature and Heating Settings: Safe Values for LiFePO4Set JK BMS charge and discharge temperature limits, MOS OTP and the heating function. Why the -20 °C factory default is wrong for LiFePO4 and what to use instead.