Xiaomi Mi Max 3 Replacement Battery BM51 5400mAh 3.85V

Xiaomi Mi Max 3 — 3.85V Li-Polymer Replacement Battery (BM51)

The BM51 is a 3.85V, 5400mAh Li-Polymer cell that fits the Xiaomi Mi Max 3, Mi Max 3 Dual SIM TD-LTE, M1804E4T, and M1804E4A. It replaces the original factory cell when the phone can no longer hold charge, shuts down unexpectedly, or fails to power on. Capacity is 20.79Wh — matching the original specification.

• Mi Max 3 platform fit: All listed variants — M1804E4T, M1804E4A, single and dual SIM — use the same BM51 footprint, connector, and BMS communication protocol. The cell dimensions are 103.92 × 73.94 × 3.78mm, which match the original battery tray with no modification required.
• Bench tested on actual hardware: We ran the BM51 through charge and discharge cycles on Mi Max 3 hardware. The BMS accepted the cell without fault codes, and the charge IC cycled correctly through CC and CV phases at both standard and fast-charge current levels.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the fuel gauge IC a full reference curve against the new cell before high-current charging begins.

Why the Mi Max 3 reports wrong battery percentage after a cell swap

The Mi Max 3 uses a coulomb counter that builds its charge curve against the old cell's impedance and capacity profile. When a new cell goes in, the counter still references the previous curve, so the percentage on screen drifts from the actual state of charge. This shows up as sudden jumps — the display may read 40% and then drop to 15% within a few minutes under load. One full uninterrupted discharge-to-shutdown followed by a complete charge cycle resets the reference and brings the display back into alignment.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff issue, not a capacity problem. Under combined modem, display, and processor load, the cell voltage sags below the BMS cutoff threshold before the fuel gauge reaches 0%. It happens most often on the Mi Max 3's large screen when brightness is high and a data connection is active. The fix is the same recalibration cycle — one full discharge lets the fuel gauge IC map the actual voltage-to-capacity curve of the new cell. After calibration, the BMS cutoff and the displayed percentage stay in sync, and shutdowns below 3.2V under load stop occurring.