Xiaomi Redmi Note 4 BN41 Replacement Battery 3.85V 4000mAh

Xiaomi Redmi Note 4 — 3.85V Li-Polymer Replacement Battery (BN41)

This 3.85V, 4000mAh Li-Polymer battery replaces the original BN41 cell in the Xiaomi Redmi Note 4. It fits the Redmi Note 4 specifically — not the Note 4X or Note 4 Global, which use a different cell footprint. Dimensions are 79.30 x 66.30 x 4.20mm, matching the original bay without modification.

• Redmi Note 4 compatibility: The BN41 connector and flex tab position are unique to the MediaTek Helio X20 variant of the Redmi Note 4. Snapdragon variants use the BN42. Confirm your board revision before ordering — the BMS handshake is board-specific and cross-fitting will prevent charging.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on a Redmi Note 4 unit. The BMS accepted the new cell without error codes, charge termination occurred correctly at 4.35V, and the protection circuit tripped as expected at low-voltage cutoff.
• 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 discharge curve against the new cell before high-current charging pushes load into an uncalibrated state.

Sudden shutdown at 20–30% on the Redmi Note 4 after a cell swap

The Redmi Note 4 uses a coulomb-counter fuel gauge IC that was calibrated to the discharge curve of the original aged cell. A new 4000mAh cell has a steeper voltage cliff under combined modem, display, and CPU load than the worn cell the IC learned from. The gauge reports 25% remaining, but the actual cell voltage under load drops below the hardware cutoff threshold — typically around 3.2V — and the phone shuts off to protect the circuit. One complete drain-to-shutdown and full recharge cycle resets the coulomb counter and eliminates the premature cutoff.

USB fast charge not working after fitting the BN41 replacement

Xiaomi's proprietary fast charge protocol requires the charge IC to read the BMS impedance and respond within a handshake window on the first connection. A fresh cell with higher internal impedance than an aged one can cause the charge IC to fall back to 5V standard charging on the first cycle. This is not a fault with the battery or the port. Run one full standard-charge cycle to let the charge IC characterise the new cell, then reconnect with the original Xiaomi charger — fast charge typically re-enables at that point.