BN51 Xiaomi Redmi 8 Compatible Battery 3.85V 4900mAh

Xiaomi Redmi 8 / Redmi 8A — 3.85V Li-Polymer Replacement Battery (BN51)

This is a 3.85V, 4900mAh Li-Polymer cell built to the BN51 specification. It fits the Xiaomi Redmi 8, Redmi 8A, and variants including M1908C3IE and M1908C3IC. Swap it in when the original cell no longer holds a useful charge or causes unexpected shutdowns.

• Redmi 8 and 8A shared platform: Both devices run the same Snapdragon 439 voltage rail at 3.85V nominal and use an identical BMS connector pinout — same fuel gauge handshake, same charge IC. One cell covers both models without any modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Redmi 8 mainboard. The BMS accepted the new cell, charge termination triggered correctly at full capacity, and the fuel gauge IC initialised without error codes.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current fast charging pushes current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the Redmi 8 after a cell swap

The Redmi 8's fuel gauge IC holds a discharge curve from the old, degraded cell. When a fresh 4900mAh cell goes in, that stored curve no longer matches the new cell's actual voltage profile. Under modem load or screen-on bursts, the phone reads a voltage drop it interprets as near-empty and triggers a protective shutdown — even with significant charge remaining. The fix is one full discharge to auto-off followed by a full charge to 100% at standard rate. After that single cycle, the coulomb counter recalibrates against the new cell and reported percentage stabilises.

USB fast charge not activating on the first cycle after replacement

Xiaomi's proprietary fast charge protocol requires a BMS handshake between the charger and the charge IC on the new cell. On the first cycle, some replacement cells present a higher internal impedance than the aged original — the charger detects this and falls back to standard 5V charging as a precaution. This is normal behaviour, not a fault. Run that first full cycle at standard rate, then reconnect the Xiaomi fast charger. The handshake completes correctly once the BMS has completed one full charge cycle and the cell impedance settles.