Redmi 8 BN51 Replacement Battery 3.85V 4900mAh

Redmi 8 Series — 3.85V Li-Polymer Replacement Battery (BN51)

The BN51 is the OEM-spec Li-Polymer cell for the Redmi 8 and Redmi 8A smartphones. It runs at 3.85V and holds 4900mAh (18.87Wh). Compatible models include M1908C3IE, M1908C3IC, and over a dozen regional variants that share the same connector, BMS handshake, and physical footprint.

• Redmi 8 and 8A shared platform: Both models draw from the same hardware platform — identical voltage rail, the same 5-pin connector, and a shared BMS communication protocol. One cell covers both devices without modification.
• Bench tested on actual hardware: We cycled this cell on a Redmi 8 unit and monitored BMS handshake, charge acceptance at 10W, and cutoff behaviour at low state-of-charge. The BMS accepted the cell on first insertion and reported capacity within normal range after one full cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle. This allows the fuel gauge IC to map the new cell's discharge curve before high-current charging pushes amperage into an uncalibrated cell.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff issue, not a capacity defect. The fuel gauge IC on the Redmi 8 was calibrated against the discharge curve of the original, aged cell — not a fresh 4900mAh one. When the new cell's voltage drops steeply under modem or display load at lower state-of-charge, the IC misreads it as critically low and triggers shutdown. One full discharge down to automatic cutoff, followed by an uninterrupted full charge, resets the coulomb counter and eliminates most unexpected shutdowns.

Fast charging not working on the first cycle after replacement

The Redmi 8 uses a proprietary charge negotiation step before stepping up to 10W. On a freshly installed cell, the charge IC sometimes defaults to standard 5W on the first cycle while it validates the new BMS. This is not a fault with the replacement cell. Unplug and reconnect the charger after the phone has been on for two minutes — the charge IC typically renegotiates on the second handshake. Confirm fast charge is active by checking Settings → Battery → show charging speed, which should read above 1.0A at 5V.