Redmi Note 5 BN45 Replacement Battery 3.85V 3900mAh

Redmi Note 5 — 3.85V Li-Polymer Replacement Battery (BN45)

The BN45 is a 3900mAh Li-Polymer cell that powers the Redmi Note 5 and Note 5 Standard Edition Global variants, including Dual SIM TD-LTE configurations. It operates at 3.85V nominal and delivers 15.02Wh. Replace this cell when the original degrades, causes unexpected shutdowns, or no longer sustains a full day of use.

• Note 5 platform compatibility: All listed Note 5 variants share the same battery bay dimensions, connector pinout, and BMS handshake requirements, so one cell covers the full range — 84.94 × 63.90 × 3.92mm with no modification needed.
• Bench tested on actual hardware: We ran the BN45 through full charge and load cycles on a Note 5 unit. The BMS accepted the cell without error flags, and the charge IC stepped through CC/CV phases cleanly with no thermal trip.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes into an uncalibrated state.

Sudden shutdown at 20–30% on the replacement cell

A new Li-Polymer cell has a steeper voltage curve at the low end than an aged cell the fuel gauge IC was originally calibrated against. Under modem or display load, voltage sags past the cutoff threshold before the percentage counter reaches zero. The phone reads 25% but the cell voltage is already below 3.5V under load. Run two full discharge-charge cycles without fast charging — the coulomb counter will remap to the new curve and shutdowns will stop.

USB fast charge not activating on the first cycle after swap

After a cell replacement, the charge IC defaults to a lower trickle rate on the first session while it verifies the new cell's internal resistance and temperature response. Xiaomi's proprietary fast charge handshake will not engage until the BMS confirms the cell is within its safe operating window. Plug in using the original charger and cable — third-party adapters can stall the handshake entirely. After one full standard charge cycle, fast charging resumes automatically at the correct current.