Redmi K50 BM5F Replacement Battery 3.87V 5400mAh

Redmi K50 Series — 3.87V Li-Polymer Replacement Battery (BM5F)

The BM5F is a 3.87V, 5400mAh (20.9Wh) Li-Polymer cell for the Redmi K50, K50 5G, and model 22041211AC. It replaces the original battery when capacity has degraded to the point where the phone can no longer hold charge through a normal day. Dimensions are 89.30 × 65.15 × 5.20mm — verify these against your existing cell before installation.

• K50 and K50 5G compatibility: Both models share the same battery bay dimensions, BM5F part number, and power management interface. The 3.87V nominal voltage rail matches the PMIC spec across the entire K50 platform, so no adapter or connector modification is needed.
• Bench tested on actual hardware: We ran the BM5F through charge and discharge cycles on the K50 platform. The BMS accepted the cell without error flags, and the charge IC stepped through the correct constant-current and constant-voltage phases without thermal events.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate coulomb count against the new cell's actual discharge curve before high-current charging is applied to an uncalibrated cell.

Sudden shutdown at 20–30% on a replacement cell

This is a voltage cliff issue, not a capacity problem. When the K50's modem transmits or the display peaks, it pulls a burst of current the new cell can't sustain at that state of charge — voltage drops below the PMIC cutoff threshold and the phone shuts off. The fuel gauge IC is still calibrated to the old cell's discharge curve, so the percentage reading doesn't match the new cell's actual voltage profile. Run one full discharge to near-zero and a full charge without interruption; the coulomb counter resets and the shutdowns typically stop.

USB-PD or proprietary fast charge not triggering after cell swap

On the first cycle after installation, the charge IC may not negotiate the fast charge protocol with the new BMS. This shows up as the phone pulling only 5W when it previously fast charged at higher wattage. The BMS on a fresh cell starts with conservative charge limits until it completes one baseline cycle. Complete one standard charge from low to 100% at normal speed — fast charge will resume on subsequent cycles once the BMS has cleared its first-cycle flag.