Xiaomi Redmi 4 BN42 Replacement Battery 3.85V 4000mAh

Xiaomi Redmi 4 — 3.85V Li-Polymer Replacement Battery (BN42)

This is a 3.85V, 4000mAh Li-Polymer replacement cell for the Xiaomi Redmi 4 (Hongmi 4), including the Standard Edition, Standard Edition Dual SIM, and Standard Edition Dual SIM TD-LTE variants. It replaces the original BN42 cell when capacity fade causes shortened screen-on time or forces overnight charging just to reach a full day. Rated at 15.4Wh, it matches OEM voltage and capacity specifications exactly.

• Redmi 4 Standard Edition compatibility: All variants listed — Standard, Dual SIM, and TD-LTE — use the same BN42 cell with the same connector pinout, BMS handshake protocol, and physical footprint. One battery covers the entire Redmi 4 Standard Edition line without modification.
• Bench tested on actual hardware: We ran full charge-discharge cycles through a Redmi 4 unit and confirmed the BMS accepted the cell without triggering fault flags. The fuel gauge IC completed its first read pass without error codes and charge termination occurred at the correct cutoff voltage.
• Fuel gauge recalibration on first use: On first installation, disable fast charging and run one complete discharge down to automatic shutdown, then charge to 100% on standard 5V/1A input. This lets the coulomb counter recalibrate against the new cell's actual discharge curve before high-current charging is introduced.

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

The Redmi 4 uses a coulomb counter in the PMIC to estimate state-of-charge. That counter was calibrated against the original BN42 cell's discharge curve over hundreds of cycles. A fresh replacement cell has a different internal resistance profile, so the existing calibration data no longer maps accurately to real voltage. The phone may display 40% while the cell is already below 3.5V under load. One full discharge-charge cycle on standard input resets the learning registers and restores accurate percentage reporting.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem, display, or both draw a current spike the new cell cannot sustain at that state-of-charge without dropping below the BMS low-voltage cutoff threshold — typically 3.0V per cell. The fuel gauge still shows 20–30%, but the cell voltage collapses under load faster than the counter updates. It is not a faulty battery. Run the recalibration cycle first: discharge to auto-shutdown, then charge uninterrupted to 100% at 5V/1A before returning to normal use.