Xiaomi Max BM49 Replacement Battery 3.85V 4750mAh

Xiaomi Mi Max Series — 3.85V Li-Polymer Replacement Battery (BM49)

The BM49 is a 3.85V, 4750mAh Li-Polymer cell that fits the Xiaomi Mi Max, Mi Max Dual SIM, Mi Max Dual SIM TD-LTE, and 2016002 variants. It replaces a degraded original cell that no longer holds charge through a normal day. Capacity is rated at 18.29Wh.

• Mi Max platform fit: These models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The BM49 cell measures 99.46 x 77.00 x 3.46mm — exact to the original bay. No connector adapters required.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on Mi Max hardware. The BMS accepted charge at full current after the first complete cycle, and the protection circuit tripped correctly at low-voltage cutoff with no false shutdowns.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before fast charging pushes high current into an uncalibrated cell.

Sudden shutdown at 20–30% on the BM49 after installation

This happens because the fuel gauge IC still holds the old cell's voltage curve in memory. The new cell has a steeper voltage drop under modem or screen load, and the phone's hardware under-voltage protection cuts power before the OS percentage counter reaches zero. The fuel gauge reads 25% but the actual cell voltage is already below the cutoff threshold. One full discharge to auto-shutdown followed by a complete charge resets the coulomb counter and eliminates most unexpected shutdowns.

Mi Max not powering on after the BM49 sat in storage

Li-Polymer cells self-discharge in storage, and if the BM49 dropped below approximately 2.5V per cell, the BMS enters lockout mode to prevent damage — the phone shows nothing when you press the power button. Connect the phone to a wall charger rated at least 5V/1A and leave it for 20–30 minutes without pressing any button. The BMS needs a trickle current to recover from lockout before the charge IC hands off to normal fast-charge mode. If the charge LED does not activate within 30 minutes, try a different cable and confirm the charger outputs at least 5V.