JK BMS Cell Over Charge Protection Keeps Tripping: Causes and Fix

If the JK app shows "Cell 01 over charge protection" followed by "Cell overcharge protection is released" over and over, the JK BMS cell over charge protection is tripping in a loop. Charging stops, restarts, and stops again, sometimes several times an hour. Most of the time the cells are fine. The charger or inverter is asking for a voltage that sits above what the BMS allows per cell.

What the message means

"Cell N over charge protection" is a per-cell over-voltage trip. The BMS watches every cell, and when the highest one reaches Cell OVP it opens the charge MOS. No charge current can flow after that, whatever the charger does.

The BMS then waits and closes the charge MOS again, logging "Cell overcharge protection is released". In the example log this happens about 56 s later with the top cell back near 3.600 V, the OVP value, even though OVPR in the same BMS is 3.40 V. On this firmware the release seems to follow a timer or the OVP value, not OVPR. The number in the message is the cell that crossed the limit, counted from 1.

A single trip at the end of a full charge is normal. A trip that repeats every minute is not.

Why it repeats every minute

The usual reason is a charger or inverter that holds the pack at a fixed voltage above cells × OVP. Charging pushes the top cell over the limit, the BMS cuts off, the cell drops back, the BMS releases after about a minute, and the charger pushes it over again.

The example Detail Log shows it clearly:

• 1090 over-voltage trips in four days, 1083 of them on cell 01.
• Each trip comes at about 3.627 V with 22 to 23 A flowing.
• The pack reads 29.0 V on 8 cells, which is 3.625 V per cell. The limit is 8 × 3.60 V = 28.8 V.
• The release comes after about 56 s, and the next trip follows about 6 s later.

The charger holds roughly 29.0 V, so even a perfectly balanced pack ends up above the limit every time the BMS lets current through. With the cells this close together, the setpoint is the problem. The same config has Cell RCV at 3.45 V, which is 27.6 V for 8 cells, while the charger holds 29.0 V. That charger is not following the BMS request, which is why the fix starts at the charger.

Setpoint problem or weak cell

Look at the spread between the highest and lowest cell at the moment of the trip.

• Spread under 20 mV. The cells are in step and the whole pack is too high. Lower the charger voltage. The log analysis calls it a charger voltage problem when at least 80% of the trips happen with a spread under 20 mV.
• One cell far above the rest. If at least 80% of the trips name the same cell and the spread is 50 mV or more, that cell is the outlier. Check the balance settings, the cell and its sense wire. The weak cell guide shows how to confirm it from the log.

The cell named in the message can mislead on its own. The spread is the better test.

How to stop JK BMS cell over charge protection tripping

Start at the charger or inverter, since that is where the voltage comes from.

1. Set the absorption (bulk) voltage below cells × Cell OVP, with a margin. For 8 cells at 3.60 V OVP that means under 28.8 V. Many users leave 30 to 50 mV per cell of headroom, but check your cell datasheet.
2. Open the editor and lower Cell RCV and SOC-100% voltage so the BMS requests less from a charger that follows its request voltage.
3. Keep the chain Cell OVP recovery < SOC-100% voltage < Cell RCV < Cell OVP. The firmware rejects files that break it, see JK BMS sending failure.
4. Compare the result with the LiFePO4 settings guide, or with the ready values on the presets page.

Balancing does not help here. The cells are already within a few millivolts, so there is nothing left to equalise. Raising Cell OVP only moves the limit closer to the cell's real maximum and is not a fix.

Changing the OVP to OVPR gap does not cure a charger that is set too high. The analysis page calls it cycling when there are 10 or more over-voltage trips and the median time from release to the next trip is under 60 s.

Check it with the log

You can confirm the diagnosis in a few minutes.

1. Export the Detail Log from the JK app. The Detail Log guide shows how.
2. Open the log analysis page and drop the .txt file in. It is read in your browser and never uploaded.
3. Read the finding "Charger voltage above the OVP threshold" and the episodes table. The table gives the cells, the peak cell voltage, how long the protection held and the re-arm time of each episode.
4. Attach the .jkcfg of the same BMS. The charts then draw your Cell OVP, OVPR and RCV as lines, so you can see where the pack voltage sits against them. It also raises "Trips do not match the config" (median trip 3.629 V against OVP 3.600 V). That is expected when trips land above OVP.

After you change the charger and the file, export a new log a few days later. The trips should fall to a handful, usually at the end of a full charge.