Xiaomi Mi Mix BM4C Replacement Battery 3.85V 4000mAh

Xiaomi Mi Mix Series — 3.85V Li-Polymer Replacement Battery (BM4C)

The BM4C is a 3.85V, 4000mAh (15.4Wh) Li-Polymer cell that fits the Xiaomi Mi Mix, Mi Mix Dual SIM, Mi Mix Exclusive, and Mi Mix Exclusive Gold Ceramic Edition. It replaces a degraded or failed original cell to restore full display, processor, and modem power. Dimensions are 76.74 × 72.10 × 4.46mm — confirm your original cell matches before installing.

• Mi Mix variant coverage: All listed Mi Mix variants share the same chassis footprint, connector pinout, and BMS handshake protocol — one cell fits all four. The BM4C part number is the key identifier across every variant, including the Gold Ceramic Edition.
• Bench tested on actual hardware: We ran this cell through full charge cycles on Mi Mix hardware and confirmed BMS communication, charge IC acceptance, and voltage regulation across low-load and high-draw states. The cell held stable output under simultaneous display and modem load.
• First-cycle calibration tip: 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 fast charging pushes current into an uncalibrated cell.

Why the Mi Mix reports wrong battery percentage after a cell swap

The Mi Mix uses a coulomb counter in the fuel gauge IC that builds a discharge model from cycle history. When you swap the physical cell, that stored model no longer matches the new cell's actual voltage-to-capacity curve. The IC keeps reporting percentages based on the old cell's behaviour, so readings appear inflated or deflated by 10–20%. One full discharge down to automatic shutdown followed by an uninterrupted charge to 100% forces the IC to rebuild its reference curve against the new cell.

Sudden shutdown at 20–30% remaining on the replacement cell

This happens when the fuel gauge IC is still calibrated to an aged cell with a shallow, flat discharge curve. The new cell has a steeper voltage drop under modem or screen load, so the actual cell voltage hits the BMS cutoff threshold — typically around 3.0V — well before the reported percentage reaches zero. The phone cuts out because the cell voltage is genuinely too low, not because of a hardware fault. Run two full discharge-to-shutdown cycles without fast charging and the reported percentage will track the actual cell voltage correctly.