Xiaomi Mix Alpha BM4K Replacement Battery 3.87V 3950mAh

Xiaomi Mix Alpha — 3.87V Li-Polymer Replacement Battery (BM4K)

The BM4K is a 3.87V, 3950mAh Li-Polymer cell for the Xiaomi Mix Alpha smartphone. It replaces the original battery when capacity has faded and the phone can no longer hold a charge through a normal day. Rated at 15.29Wh, it matches the voltage and form factor of the factory cell.

• Mix Alpha fitment: The Mix Alpha uses a non-standard wrap-around display with tight internal clearances. The BM4K matches the 115.00 × 39.70 × 5.20mm envelope exactly, which matters because even a small deviation in thickness can stress the flexible display assembly during reassembly.
• Bench tested on actual hardware: We cycled the BM4K through full charge and discharge sequences and confirmed the BMS trips at the correct low-voltage cutoff. The protection circuit held within spec on both over-charge and over-discharge thresholds.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one complete discharge-charge cycle at standard rate. The Mix Alpha's fuel gauge IC is calibrated to the old cell's discharge curve — one full cycle at low current lets it remap against the new cell before high-current charging begins.

Sudden shutdown at 20–30% on the Mix Alpha after a cell swap

This is a voltage cliff issue, not a capacity issue. The Mix Alpha's Snapdragon processor and wrap-around display pull significant current spikes, and if the fuel gauge IC hasn't recalibrated, it will misread state-of-charge on the new cell. The phone hits the real minimum cell voltage before the percentage reaches zero, and the BMS cuts power to protect the cell. Run one full discharge to automatic shutdown, then charge uninterrupted to 100% — after that cycle, the coulomb counter has enough data to track the new cell accurately.

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

On the first charge after a cell swap, the charge IC often defaults to trickle or standard current while it reads the new cell's impedance profile. The BMS on a fresh cell presents higher impedance than a worn one, and the charge controller may not handshake into fast-charge mode until it has completed at least one full cycle. Plug in using the original Xiaomi charger and cable — third-party cables with high resistance can prevent the fast-charge handshake entirely. After the first full cycle completes, fast charging typically resumes at 3.87V on the negotiation line.