How the JCID battery flex works

• Bypasses Apple’s serial lock: Starting with the iPhone XS, Apple “marries” the battery’s logic board (Battery Management System or BMS) to the phone’s motherboard using a serial number. When a replacement battery is installed, even a genuine one, this lock prevents the phone from displaying battery health and triggers an “Important Battery Message” warning.
• Reprograms the data: The JCID flex cable is a tool that allows a technician to modify the data on the original battery’s BMS. The original BMS must be unsoldered from the old battery and reattached to the new battery.
• Requires a programmer: The flex cable is used in conjunction with a JCID programmer, such as the V1S, V1SE, or V1S Pro. This programmer is the device that actually reads, writes, and modifies the battery data. 

Using the JCID flex and programmer

1. Read old data: Connect the old iPhone battery to the JCID programmer and read its data (including battery health, cycle count, etc.).
2. Unsolder the BMS: Carefully unsolder the small BMS board from the old battery.
3. Weld to new flex: Solder the original BMS board onto the JCID repair flex cable.
4. Modify new data: Connect the flex and programmer to the computer. Use the JCID software to write the old battery data to the new flex cable, making it appear “genuine” to the iPhone.
5. Install the new battery: Install the new replacement battery with the modified JCID flex cable attached.
6. Verify the fix: After booting the phone, the “Important Battery Message” should be gone, and the correct battery health percentage should be displayed. 

Variations of the JCID flex

• Built-in flex: This is a direct replacement for the battery ribbon and requires advanced soldering skills. It results in a clean, “traceless” repair.
• Tag-on flex: This external flex cable is less intrusive and is simply connected to the original battery’s BMS. While easier to use, the added bulk makes for a less elegant, and more temporary, repair