Xiaomi Redmi 9s BN55 Replacement Battery 3.85V 4800mAh

Xiaomi Redmi 9s — 3.85V Li-Polymer Replacement Battery (BN55)

This is a 3.85V, 4800mAh Li-Polymer replacement battery for the Xiaomi Redmi 9s smartphone. It carries OEM part number BN55 and fits the Redmi 9s directly. Swap it in when the original cell can no longer hold a full day of charge after repeated cycle degradation.

• Redmi 9s fit: The Redmi 9s uses a dedicated connector layout and BMS handshake tied to the BN55 cell spec. Substituting a cell with a different impedance profile breaks the fuel gauge IC's learned discharge curve, causing erratic percentage readings. This cell matches the voltage rail and connector pinout the Redmi 9s charge IC expects.
• Bench tested on actual hardware: We ran this cell through a full charge-discharge cycle on the Redmi 9s and monitored the BMS response at low-state-of-charge. The protection circuit engaged correctly at the low-voltage cutoff threshold, and the charge IC accepted the cell without error flags on the first cycle.
• Fuel gauge recalibration tip: On first use after installation, disable fast charging in the phone's settings and let the battery complete one full discharge-to-charge cycle at standard current. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes into an uncalibrated coulomb counter.

Why the Redmi 9s reports wrong battery percentage after a cell swap

The Redmi 9s uses a fuel gauge IC that builds a discharge model based on the original cell's internal resistance and capacity curve. When a new cell goes in, that stored model no longer matches the physical cell, so the IC reports percentage based on stale data. You'll see the percentage jump or drop suddenly — especially between 30% and 10% — until the IC re-learns. One complete discharge below 5% followed by a full charge at standard current resets the coulomb counter and restores accurate reporting.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or screen draws a short burst of high current and the cell voltage drops below the BMS cutoff threshold faster than the fuel gauge IC expects. The IC still reads 25% state-of-charge, but the actual cell voltage collapses under load to below 3.2V, tripping the protection circuit. It is more common in the first few cycles on a new cell before the BMS has profiled the cell's internal resistance. After two or three full cycles, the shutdown threshold stabilises — if it persists, check that battery saver mode is off, as it can mask the underlying voltage sag rather than fix it.