About this replacement
Xiaomi Mi 11 Pro 5G / Mi 11 Ultra — 3.85V Li-Polymer Replacement Battery (BM55)
This is a 3.85V, 4800mAh (18.48Wh) Li-Polymer replacement battery for the Xiaomi Mi 11 Pro 5G (M2102K1AC) and Mi 11 Ultra. It slots in where the original BM55 cell sits and restores power to the processor, display, and modem. Fit both models because Xiaomi used the same battery bay, connector, and BMS handshake across the Mi 11 Pro and Ultra platforms.
- Mi 11 Pro 5G and Mi 11 Ultra compatibility: Both devices share the same physical footprint (81.00 × 64.58 × 5.50mm), the same four-pin connector, and the same BMS communication protocol. One cell covers both models without modification.
- Bench tested on actual hardware: We ran the BM55 cell through charge and discharge cycles on Mi 11 Ultra hardware. The BMS accepted the cell, the charge IC negotiated current correctly, and the protection circuit tripped cleanly at the expected voltage floor.
- Fuel gauge recalibration on first use: After installation, disable fast charging and run one full discharge down to auto-shutdown, then charge to 100% on standard power. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging begins on an uncalibrated baseline.
Why the Mi 11 Pro Reports Wrong Battery Percentage After a Cell Swap
The Mi 11 Pro uses a coulomb counter paired with a fuel gauge IC that learns the discharge curve of the installed cell over time. When you swap the cell, the IC still holds the old cell's learned curve. It maps voltage readings against that stale curve, so 30% on screen may not correspond to 30% of actual remaining capacity. One complete discharge-charge cycle without fast charging forces the IC to record the new cell's voltage-to-capacity relationship from 100% down to the low-voltage cutoff. After that cycle, percentage readouts track closely to real capacity.
Sudden Shutdown at 20–30% on the Replacement Cell
This shutdown is a voltage cliff, not a capacity failure. Under the combined load of the 5G modem, the 120Hz display, and background tasks, cell voltage can drop sharply below the BMS protection threshold even when the fuel gauge still reads 20–30% remaining. The BMS trips the cutoff to prevent cell damage — the phone goes dark instantly. This is most common in the first few cycles before the fuel gauge IC recalibrates. Run the recalibration cycle described above, then confirm the shutdown stops occurring; if it persists past three full cycles, check that cell voltage at shutdown is not dropping below 3.2V under load.