Xiaomi Mi 9 Pro 5G Replacement Battery BM4H 3.85V 3900mAh

Xiaomi Mi 9 Pro 5G — 3.85V Li-Polymer Replacement Battery (BM4H)

This is a 3.85V, 3900mAh Li-Polymer replacement battery for the Xiaomi Mi 9 Pro 5G (M1908F1XE). It replaces the original BM4H cell when capacity has degraded to the point where the phone can no longer hold a useful charge. Physical dimensions are 74.90 × 63.78 × 4.84mm — confirm these against your existing cell before installing.

• Mi 9 Pro 5G / M1908F1XE fit: Both model designations run the same BM4H cell, same connector pinout, and the same fuel gauge IC handshake — one battery covers both variants without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Mi 9 Pro 5G unit. The BMS accepted charge from the first connection, protection circuits tripped correctly at low-voltage threshold, and no thermal anomalies appeared during a full charge cycle.
• First-cycle fast charge handling: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes into an uncalibrated state.

Why the Mi 9 Pro 5G reports wrong battery percentage after a cell swap

The Mi 9 Pro 5G uses a coulomb counter that builds its fuel map from the original cell's charge and discharge history. Swap in a new cell and that map no longer matches the actual discharge curve. The OS reads a stale reference and displays percentage values that lag or jump. One full slow discharge down to automatic shutdown — followed by a full charge without interruption — forces the fuel gauge IC to rebuild its map against the new cell. After that cycle, percentage accuracy normalises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC's stored discharge curve diverges from the new cell's actual voltage cliff. The phone sees 25% remaining but the cell voltage drops below the modem and display load threshold before the OS can act. The result is an abrupt power-off with no low-battery warning. Discharge the phone fully until it shuts down on its own, then charge uninterrupted to 100% — this single recalibration cycle realigns the coulomb counter to the new cell's voltage profile and eliminates premature cutoffs.