Xiaomi Mi 5X Replacement Battery BN31 3.85V 3000mAh

Xiaomi Mi 5X — 3.85V Li-Polymer Replacement Battery (BN31)

This is a 3000mAh, 3.85V Li-Polymer cell built to replace the BN31 battery in the Xiaomi Mi 5X and Mi 5X Dual SIM. It fits all Mi 5X variants, including TD-LTE models. Install it when the original cell no longer holds charge or the phone shuts down unexpectedly under load.

• Mi 5X and Dual SIM variant fitment: All Mi 5X variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol. One cell covers every model on the compatibility list — the BN31 part number is consistent across all regional and LTE variants.
• Bench tested on actual hardware: We ran this cell on a Mi 5X unit and confirmed the BMS accepted the new cell without flagging a fault. The charge IC initialised correctly and the fuel gauge began tracking state-of-charge from the first cycle.
• Fuel gauge recalibration after install: After fitting this cell, disable fast charging and run one full discharge-to-charge cycle at standard speed. The Mi 5X fuel gauge IC calibrates its coulomb counter against the cell's discharge curve — skipping this step causes the OS to read percentage from a curve mapped to the old, degraded cell.

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

The Mi 5X uses a coulomb-counter fuel gauge that retains the old cell's discharge curve in memory after a swap. The new cell has a steeper voltage drop at low state-of-charge compared to a worn cell. When the phone hits a load spike — LTE handoff, screen-on burst — the new cell's voltage dips below the PMIC's cutoff threshold while the gauge still reads 25%. The fix is one complete discharge cycle without fast charging, letting the fuel gauge IC remap its curve to the new cell. After that cycle, the percentage reading and actual cutoff voltage will align.

Mi 5X not powering on after the replacement cell sat in storage

Li-Polymer cells self-discharge during storage. If the BN31 cell dropped below 2.5V per cell before installation, the BMS enters lockout mode and blocks current flow to protect the cell from damage. The phone will show no response — no charging indicator, no vibration. Connect the phone to a wall adapter rated at 5V/1A or higher and leave it for 20–30 minutes without pressing the power button. The BMS requires a trickle-charge period to exit lockout; once cell voltage climbs above 2.8V, the BMS releases and normal charging resumes.