Redmi K20 BP41 Compatible Battery 3.85V 3900mAh

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

The BP41 is the OEM-matched Li-Polymer cell for the Redmi K20 and Mi 9T smartphones. It runs at 3.85V nominal and holds 3900mAh (15.02Wh). This replacement fits the M1903F10G and MZB7757IN board revisions as well as the full K20 and Mi 9T hardware lines.

• K20 / Mi 9T platform fit: The K20 and Mi 9T share identical mainboard layouts, flex connector pinouts, and BMS handshake protocols — one cell covers both. The BP41 connector seats the same way across all regional SKUs, and the NTC thermistor line maps correctly to the charge IC on each board revision.
• Bench tested on actual hardware: We cycled this cell on a Mi 9T mainboard and confirmed the BMS accepted the cell without fault flags, the charge IC stepped through trickle, CC, and CV phases cleanly, and the fuel gauge IC registered full capacity after one complete cycle.
• First-cycle fast charge protocol: On first use after installation, disable Turbo Charge for one full discharge-to-charge cycle. The fuel gauge IC recalibrates its coulomb counter against the new cell's discharge curve. Running high-current fast charging into an uncalibrated cell causes the charge IC to cut short at an incorrect state-of-charge threshold.

Sudden shutdown at 20–30% on the Redmi K20 after a cell swap

This is a voltage cliff, not a calibration error. A fresh Li-Polymer cell has a steeper discharge curve than the aged cell the fuel gauge IC was trained on, so the reported percentage still reads 25% when the actual cell voltage drops below 3.5V under modem and display load. The phone's power IC hits its low-voltage cutoff before the OS can warn you. One full discharge-charge cycle — taken all the way to automatic power-off, then charged uninterrupted to 100% — lets the coulomb counter rebuild its curve map against the new cell. After that cycle, shutdowns at falsely high percentages stop.

Device will not power on after the replacement cell sat in storage

Li-Polymer cells self-discharge during storage, and below approximately 2.5V per cell the BMS trips a deep-discharge lockout to prevent further drain. The phone shows nothing — no charging indicator, no vibration — because the BMS is blocking current to the mainboard entirely. Connect the phone to a 5V wall adapter (not a PC USB port, which may not source enough current) and leave it untouched for 15–20 minutes. The charge IC applies a trickle current that slowly walks the cell voltage back above the BMS re-enable threshold, after which normal charging resumes and the boot sequence unlocks.