Xiaomi Mi 12 Lite 5G Replacement Battery BP4B 3.87V 4200mAh

Xiaomi Mi 12 Lite 5G — 3.87V Li-Polymer Replacement Battery (BP4B)

This is a 3.87V, 4200mAh (16.25Wh) Li-Polymer battery for the Xiaomi Mi 12 Lite 5G, covering model variants 2203129I and 2203129G. It replaces OEM part BP4B and fits the original connector and battery bay without modification. Capacity matches the factory spec — no downgrade.

• Mi 12 Lite 5G variant coverage (2203129I / 2203129G): Both regional variants share the same PCB layout, connector pinout, and BMS handshake protocol — one cell fits both. The BP4B footprint and flex cable position are identical across the I and G builds.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Mi 12 Lite 5G unit. The BMS accepted the cell without error flags, and the charge IC communicated correctly with the replacement cell at both standard and fast-charge current levels.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging pushes power into an uncalibrated state.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff issue, not a capacity fault. Under modem load or display peaks, the new cell's voltage drops faster than the fuel gauge IC expects because it is still calibrated to the old, degraded cell curve. The phone interprets the voltage drop as a critical low and cuts power. Run one full discharge down to auto-shutdown, then charge to 100% uninterrupted — the coulomb counter resets and the shutdowns stop.

USB-PD fast charge not engaging on the first cycle after replacement

After a cell swap, the charge IC on the Mi 12 Lite 5G may default to trickle or standard charge on the first cycle while it verifies the new cell's impedance profile. This is a BMS-side handshake behaviour — not a cable or adapter fault. If fast charging does not activate, complete one full standard charge, then reconnect the fast charger. The protocol should negotiate correctly from the second cycle onward.