Xiaomi Mi 11X 5G BM4Y Replacement Battery 3.87V 4400mAh

Xiaomi Mi 11X 5G / Poco F3 5G — 3.87V Li-Polymer Replacement Battery (BM4Y)

The BM4Y is the OEM-equivalent Li-Polymer cell for the Xiaomi Mi 11X 5G (M2012K11AI) and Poco F3 5G (M2012K11AG). It sits at 3.87V nominal and delivers 4400mAh (17.03Wh). Both devices share the same battery bay, connector pinout, and BMS handshake, so one part number covers the full compatibility list.

• Mi 11X 5G and Poco F3 5G platform fit: These two models run the same Qualcomm Snapdragon 870 platform, share an identical battery connector, and use the same fuel gauge IC communication protocol. The BM4Y slots into either device without adapter or re-pinning.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Mi 11X 5G platform. The BMS accepted the cell without error flags, the charge IC stepped through trickle, CC, and CV phases cleanly, and the fuel gauge IC registered a full 4400mAh draw before cutoff.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current fast charging pushes load into an uncalibrated register.

Sudden shutdown at 20–30% on the Mi 11X 5G after a cell swap

A fresh BM4Y cell has a slightly different internal resistance profile than the aged cell the fuel gauge IC was trained on. Under high-draw events — 5G modem handoff, screen at full brightness, GPS active simultaneously — the new cell can sag below the shutdown threshold before the percentage readout catches up. The phone cuts out even though the display was showing 25%. One full discharge-charge cycle at standard rate recalibrates the coulomb counter and eliminates most of these events. If shutdowns persist past two full cycles, check that the BMS connector is fully seated at the board-side ZIF socket.

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

On the first charge after a cell swap, the Mi 11X 5G's charge IC may refuse to negotiate fast charge and fall back to 5W standard charging. This happens because the BMS on the new cell has not yet completed its initialisation handshake with the charge controller — it presents as a high-impedance cell, which triggers a conservative charge profile as a safety measure. Unplug the cable, wait 10 seconds, and reconnect. In most cases the charge IC renegotiates and fast charge resumes. If it does not, complete the first full cycle at standard rate and fast charge will activate automatically on the second session.