Hongmi BN40 Replacement Battery 3.85V 4100mAh Li-Polymer

Hongmi 4 Premium Edition — 3.85V Li-Polymer Replacement Battery (BN40)

This is a 3.85V, 4100mAh Li-Polymer cell built to the BN40 specification. It fits the Hongmi 4 Premium Edition, 4X Premium Edition, Redmi 4X Premium Edition Dual SIM TD-LTE, and Redmi 4X Standard Edition. The physical dimensions are 80.40 × 59.40 × 4.70 mm — measure your existing cell before ordering if you are unsure which variant you have.

• Cross-model fit — BN40 platform: These models share the same 3.85V power rail, connector pinout, and BMS communication protocol. The BN40 part number is the OEM identifier across the entire variant group, so one cell covers all listed configurations without modification.
• Bench tested on actual hardware: We cycled this cell on a Redmi 4X unit and monitored BMS handshake, charge termination voltage, and cutoff behaviour. The BMS accepted the cell cleanly, terminated at 4.35V, and the low-voltage cutoff triggered correctly at 3.0V under load.
• First-cycle calibration — fuel gauge recalibration: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes into an uncalibrated state.

Why the Redmi 4X reports wrong battery percentage after a BN40 cell swap

The fuel gauge IC in these devices uses a coulomb counter calibrated to the original cell's internal resistance and discharge curve. When a new cell goes in, the stored curve no longer matches the physical cell, so the IC reads percentage from stale reference data. This causes the display to show inflated or deflated figures until the IC relearns the curve. One full discharge to automatic shutoff, followed by a full uninterrupted charge to 100%, forces the coulomb counter to rebuild its reference map against the replacement cell.

Sudden shutdown at 20–30% remaining on the new cell

This is a voltage cliff failure — under combined modem transmission and screen load, the cell voltage drops sharply below the BMS cutoff threshold even though the fuel gauge still reads 20–30%. It happens when the gauge IC has not yet calibrated to the new cell and overestimates remaining capacity. The fix is the same recalibration cycle: run the device to shutoff, charge fully without interruption. After two full cycles the gauge IC will align the reported percentage with actual cell voltage, and shutoffs at 20–30% will stop.