Xiaomi BN40 Hongmi 4 Compatible Battery 3.85V 4100mAh

Xiaomi Redmi 4X / Hongmi 4 Premium Edition — 3.85V Li-Polymer Replacement Battery (BN40)

This 3.85V, 4100mAh lithium-polymer cell (BN40) replaces the factory battery in the Xiaomi Hongmi 4 Premium Edition, Hongmi 4X, and Redmi 4X variants. It fits the dual-SIM TD-LTE and Standard Edition models. Dimensions are 80.40 × 59.40 × 4.70 mm — measure your original before fitting if you are unsure which variant you have.

• Hongmi 4 and Redmi 4X compatibility: These models share the same battery bay geometry, connector pinout, and BMS handshake protocol. The BN40 part number covers all variants in this cluster — the 4X Standard and Premium differ in RAM and storage, not the power rail.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Redmi 4X platform. The BMS accepted the cell on first connect, reported voltage correctly at the OS level, and held the charge IC in CV phase without premature 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 map its coulomb counter against the new cell's actual discharge curve before high-current charging is introduced into an uncalibrated reference.

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

This happens when the fuel gauge IC is still calibrated to the old cell's discharge curve. The new cell hits a voltage cliff — typically around 3.55–3.60V under modem or display load — faster than the IC expects. The OS reads 25% remaining but the cell voltage drops below the shutdown threshold under load. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the coulomb counter and eliminates this behaviour in most cases.

Xiaomi fast charging not activating after BN40 replacement

Xiaomi's proprietary fast charge protocol requires a BMS handshake on the first full cycle before the charge IC increases current above standard 5V input. On a freshly installed cell, the BMS may present elevated impedance until it completes its first calibration pass. If fast charge is not triggering, charge the device at standard rate to 100%, allow it to cool fully, then reconnect the charger — the charge IC should then negotiate the higher voltage and current on the next session. Confirm activation by checking the lock screen charge indicator, which should read "Fast Charging" at a charger output of 9V/2A.