In short
- Most JK alarms are protections doing their job: a cell, the pack or a temperature went past a limit set in the BMS.
- Cell over-voltage at the top of charge and cell under-voltage at the end of the night often point to the same weak cell.
- Wire resistance and cell count alarms are wiring problems: switch the pack off and check the leads and busbars first.
The JK app shows a red line, the inverter goes quiet, and the question is always the same: is it serious? Most of the time it is not. A JK BMS is built to stop charging or discharging the moment a value crosses a limit, and to start again once the value is back. The alarm is the board telling you which limit it hit.
Protections and faults are not the same thing
A protection is the BMS doing its job: a cell too high, a current too strong, a temperature out of range. It stops charging, discharging or both, then recovers on its own when the value returns past the recovery threshold. A fault points to the hardware itself: a power switch, the current sensor, a temperature probe. Protections tell you about your pack and your settings; faults tell you about the board.
Voltage alarms
| Alarm | What it means | First check |
|---|---|---|
| Cell over-voltage | One cell went above Cell OVP; charging stopped | The cell that runs ahead at the top of charge, and the balance |
| Pack over-voltage | The whole pack is above its maximum | The charge voltage set in the inverter or charger |
| Cell under-voltage | One cell dropped below Cell UVP; discharge stopped | The weakest cell, and the low cut-off of the inverter |
| Pack under-voltage | The pack is empty | Recharge soon; check why the inverter did not stop earlier |
A cell over-voltage on a pack that is far from full is the classic sign of imbalance: one cell reaches the steep part of the LiFePO4 curve while the others are still on the plateau. If the same cell also drops first at the end of the night, it is probably the one with the least capacity.
Current alarms
Charge or discharge over-current means the current went above the limit set in the board for longer than its delay. Lower the charge current in the inverter, or look at what was running in the house at that moment. Short-circuit alarms trip in microseconds: inspect the wiring before switching back on. Output switch-on failed often comes from the inrush current of the inverter capacitors; a pre-charge resistor or the pre-charge function of the board helps.
Temperature alarms
- Charge over-temperature and discharge over-temperature: the cells are too warm; the pack pauses until it cools.
- Charge under-temperature: too cold to charge LiFePO4. Never lower this limit below 0 °C.
- MOSFET over-temperature: the board itself is too hot, often a high current in a closed cabinet.
- Temperature sensor fault: a probe is unplugged or damaged.
Wiring alarms
Wire resistance means a balance lead measures more resistance than the board accepts: a loose busbar nut, a poor crimp, a corroded terminal. Cell count mismatch means the board sees a different number of cells from its setting, usually a lead that came loose. In both cases, switch the pack off before you touch anything.
Read an alarm in its context
An alarm at 3 a.m. makes sense once you know the charge, the cell delta and the temperature at that moment. The JK app only shows the present, within Bluetooth range. JK BMS Monitor keeps the history, explains each alarm in plain words on the dashboard and can send you an e-mail the moment the BMS raises one, so you see what led up to it.
Common questions
Why does my JK BMS show cell over-voltage when the pack is not full?
Because one cell reaches its limit before the others. On LiFePO4 the voltage rises steeply near full charge, so a cell a few percent ahead hits Cell OVP while the pack still reads 95 %. Let the active balancer work at the top of charge, or top balance the pack once.
Do I need to reset a JK alarm?
Protections clear by themselves once the value is back past its recovery threshold. Faults on the MOSFETs, the current sensor or a probe may need a restart of the board, and if they come back, a word with the seller of the BMS.