Xiaomi Redmi K20 BP41 Replacement Battery 3.85V 3900mAh

Xiaomi Redmi K20 / Mi 9T — 3.85V Li-Polymer Replacement Battery (BP41)

The BP41 is a 3.85V, 3900mAh Li-Polymer cell that fits the Redmi K20, Mi 9T, and the M1903F10G and MZB7757IN variants. It replaces the original cell when capacity has dropped from repeated charge cycles or the battery has failed outright. Voltage and capacity match OEM spec exactly.

• Redmi K20 and Mi 9T platform fit: Both devices share the same chassis, connector pinout, and BMS handshake — the BP41 was spec'd across the entire platform. The fuel gauge IC on both reads cell chemistry the same way, so the replacement cell pairs without any hardware modification.
• Bench tested on actual hardware: We ran the BP41 through full charge and discharge cycles on a Redmi K20 unit. The BMS accepted the cell without fault codes, charge termination fired correctly at 4.35V, and the coulomb counter tracked discharge within normal tolerance.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state.

Sudden shutdown at 20–30% on the BP41 after installation

This happens because the fuel gauge IC is still calibrated to the old cell's discharge curve. When the new cell hits a voltage cliff under screen or modem load, the system reads it as a sudden drop and shuts down — even though the percentage shown looks safe. The coulomb counter hasn't yet learned where the new cell's usable voltage floor sits. One full discharge to auto-shutdown followed by a full charge at standard rate resets the calibration and eliminates the false shutdowns.

USB fast charge not activating after fitting the BP41

On the first cycle after a cell swap, the charge IC sometimes refuses to negotiate the proprietary fast-charge protocol because it detects an unknown cell impedance profile. This is a protective hold, not a fault. Charge once at standard 5V/1A to completion, then reconnect the fast charger — the IC re-evaluates impedance on the second session and typically accepts the protocol. If it still won't engage, confirm the cable supports the Xiaomi fast-charge standard, not just USB-PD generic.