Xiaomi BM40 Replacement Battery 3.7V 2100mAh Li-ion

Xiaomi Mi 2A — 3.7V Li-ion Replacement Battery (BM40 / BM41)

This 3.7V, 2100mAh (7.77Wh) Li-ion cell replaces the original BM40 and BM41 batteries in the Xiaomi Mi 2A, M2A, and M3 smartphones. It restores the full operational capacity of the device — calls, data, and display — that a degraded original cell can no longer sustain. Install is a direct board-to-board swap with no firmware changes required.

• Mi 2A and M3 platform fit: Both the 2A and M3 share the same battery bay dimensions and connector pinout, and the BMS handshake runs on the same voltage rail. One cell covers both models without modification.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on a Mi 2A unit. The BMS accepted the charge IC handshake cleanly on the first cycle and held cutoff voltage at 4.2V without deviation.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging for the first complete discharge-charge cycle. The fuel gauge IC is still calibrated to your old cell's discharge curve. One slow cycle lets it remap against the new cell before high-current charging begins.

Sudden shutdown at 20–30% on a fresh Mi 2A cell

A new cell does not automatically fix sudden shutdowns at 20–30%. The fuel gauge IC stores the old cell's discharge curve in memory and calls cutoff too early when voltage sags under modem or display load. The new cell has slightly different internal resistance, so the voltage cliff hits at a different state of charge than the IC expects. Running one full discharge to below 10% followed by a slow charge to 100% forces the coulomb counter to reset against the actual cell behaviour.

Phone not powering on after the replacement cell sat in storage

Li-ion cells in storage self-discharge over time. If the cell voltage drops below 2.5V, the BMS enters a lockout state and will not pass current to the board — the phone shows nothing on screen. Connect the phone to a wall charger, not a PC USB port, and leave it for 20–30 minutes without pressing the power button. The charge IC needs time to trickle current past the BMS lockout threshold before the board will boot. If the screen shows a low-battery icon at any point, the recovery is working — continue charging to at least 3.6V before attempting to power on.