JK BMS Overcurrent Protection Settings: OCP and SCP Guide

JK BMS overcurrent protection settings are two current limits, each with a delay and a recovery time, plus a separate short circuit protection (SCP). Set Max charge current and Max discharge current to what your cells, cables and fuse can carry continuously, and always below the BMS rating. Leave the SCP fields at the factory value unless you know your firmware.

How the protections work

The BMS measures pack current all the time. When the current stays above a limit for longer than its delay, the BMS opens the charge or discharge MOSFETs. It keeps them open for the recovery time, then closes them again.

There are three layers:

• Charge OCP. Max charge current, Charge OCP delay, Charge OCP recovery time.
• Discharge OCP. Max discharge current, Discharge OCP delay, Discharge OCP recovery time.
• Short circuit protection. Short circuit protection delay and Short circuit recovery time. This one reacts in microseconds, far faster than OCP.

OCP is for overload. SCP is for a dead short.

JK BMS overcurrent protection settings table

These ranges come from the recommendation table in the editor, which is built from the JK manual, Gobel Power, Clever Solar and forum reports. They are ranges, not a single correct value.

Sizing the current limits

JK Preset Studio derives current limits from the cell C-rate and the pack capacity. For LiFePO4 the three wear levels on the presets page use these C-rates for charge and discharge:

• Gentle: 0.3C charge, 0.5C discharge.
• Medium: 0.5C charge, 0.7C discharge.
• Hard: 1C charge, 1C discharge.

The result is C-rate times capacity in Ah, rounded to whole amps, never below 1 A. Then the editor caps it by the BMS rating. The rating is read from the model name: the digits before a trailing P are multiplied by 10. A JK-PB2A16S20P gives 200 A, a model ending in 15P gives 150 A. If the model has no such suffix, the rating is unknown and no cap is applied.

Example: a 280Ah pack on the medium level gives 140 A charge and 196 A discharge. On a 20P board both stay under 200 A. A 1C hard preset on a 314Ah pack would ask for 314 A, so a 20P board caps it at 200 A.

Sizing OCP for an inverter

Separate continuous current from peak current. The Max discharge current is the continuous limit. Inverters and motors draw a short surge at start-up, often two to three times the rated load.

1. Take the inverter continuous power and divide by the battery voltage. Add 10 to 20 percent for efficiency losses and low-voltage operation.
2. Set Max discharge current above that value, but below the weakest part of the system.
3. Use Discharge OCP delay to ride through surges. A longer delay lets a start-up spike pass. The 300 s default is generous. 10 s gives quicker protection for small systems.

Inrush into inverter capacitors can trip SCP instead of OCP. Users sometimes raise the SCP delay, but precharging the inverter is the better fix.

Cable and fuse coordination

The BMS limit should be the lowest setting in a chain. Cables must carry more than the BMS limit continuously, and the fuse should be rated above the BMS limit but below the cable ampacity. That way the BMS trips first for overloads, and the fuse only acts when the BMS fails or a short outruns it. Cell ratings matter too: Max charge current should respect the cell rating, for example 0.5C for EVE LFP cells.

What happens on a trip

If the overload is still there after the recovery time, the BMS trips again.

On charge OCP or a short circuit, the PCL module (parallel current limiter) can limit the current to about 10 A instead of cutting it. Keep PCL enabled when connecting packs in parallel. With Disable PCL set, the BMS cuts charging and retries after about 60 s.

SCP delay and firmware

Do not copy SCP values between files. Firmware V19 and newer stores a small number here, 5 by default, and rejects anything above 50 on import. Older V15 files hold values like 1500 µs. Importing such a file onto a V19 device fails with "This parameter does not apply to the connected device. (Sending failure)". This was verified on a JK-PB2A16S20P with firmware 19.26. See what to do about Sending failure.

For that reason the editor has no recommended range for SCP delay and recovery. The same device also rejected 60 s for the recovery time. Change SCP in the JK app on the device, or leave the factory value. The editor flags an SCP delay above 50 on V19 as an error.

Overcurrent and short-circuit trips, and the peak currents around them, show up in the Detail Log. Load it on the log analysis page and attach your .jkcfg, and the page flags peaks within 10% of your limits. The JK BMS Detail Log guide explains the analysis, and what each log message means decodes the message text.

FAQ

What is a good JK BMS charge current limit?

Stay at or below the cell rating and the BMS rating. For LiFePO4, 0.3C to 0.5C is gentle and common. A 280Ah pack at 0.5C gives 140 A.

Should the OCP delay be long or short?

Short delays protect better. Long delays tolerate surges. Use 2 to 5 s for charge, and a longer delay on discharge only if your inverter or motor has a start-up surge.

Can I change SCP delay in the file?

Only if you know the firmware stores that value the way you expect. On V19 the value must be 50 or lower. Otherwise change it in the JK app. For the file basics, see export and import of JK BMS settings.

Connect your BMS or open your .jkcfg in the editor to check these currents against your model's rating before anything is written, or start from a ready level on the presets page.