Xiaomi BP40 Redmi K20 Pro Compatible Battery 3.85V 3900mAh

Xiaomi Redmi K20 Pro / Mi 9T Pro — 3.85V Li-Polymer Replacement Battery (BP40)

This is a 3900mAh (15.02Wh) Li-Polymer replacement battery for the Xiaomi Redmi K20 Pro (M1903F11A) and Mi 9T Pro (M1903F11G). It replaces OEM part BP40 and fits the same connector and housing slot as the original cell. Voltage is 3.85V nominal — match your original label before installing.

• Redmi K20 Pro and Mi 9T Pro shared platform: Both the Redmi K20 Pro and Mi 9T Pro run the same Snapdragon 855 board with an identical battery bay, connector pinout, and BMS handshake. One cell covers both SKUs without any hardware modification.
• Bench tested on actual hardware: We cycled this cell through charge, full discharge, and fast-charge re-entry on the K20 Pro. The BMS accepted the handshake on the first cycle, and the protection circuit triggered correctly at the low-voltage cutoff threshold without a hard lockout.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging for one complete discharge-charge cycle. The fuel gauge IC on these devices calibrates its coulomb counter against the cell's discharge curve — letting it run a single slow cycle before fast charging prevents early percentage drift and inaccurate readings.

Sudden shutdown at 20–30% on the replacement BP40 cell

This is a voltage-cliff failure, not a capacity problem. Under peak modem or display load, the cell voltage drops sharply — the BMS interprets this as a critically low threshold and cuts power before the percentage gauge catches up. It happens most often on fresh cells because the fuel gauge IC is still calibrated to the old cell's internal resistance curve. Run one full slow discharge to 3% and a full charge to 100% without fast charging, and the fuel gauge recalibrates to the new cell. After that cycle, shutdowns at 20–30% stop in most cases.

USB-PD or Mi Turbo Charge not activating after swap

Fast charge protocols on the K20 Pro require the charge IC to verify the battery's BMS response before stepping voltage up. On the first charge cycle with a new cell, the IC may default to 5V standard charging until it has completed one full handshake cycle. Leave the phone on the charger for a full charge to 100% without interruption on that first session. If fast charge still does not activate by the second cycle, check that the battery connector is fully seated — a partially engaged pin on row 2 breaks the BMS communication line and keeps the IC locked at 5V.