Xiaomi Black Shark BSO1FA Replacement Battery 3.85V 3900mAh

Xiaomi Black Shark Series — 3.85V Li-Polymer Replacement Battery (BSO1FA)

This 3.85V, 3900mAh Li-Polymer cell replaces the original BSO1FA battery in the Xiaomi Black Shark gaming smartphone, including the Black Shark Dual SIM and SKR-A0 variants. It matches the OEM voltage rail and physical footprint — 86.64 x 62.74 x 4.08mm — so it seats correctly in the factory battery bay without modification.

• Black Shark SKR-A0 and Dual SIM compatibility: These variants share the same BSO1FA connector pinout and BMS handshake protocol. Swapping between them does not require a different cell — the voltage rail and physical dimensions are identical across the lineup.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on Black Shark hardware. The BMS accepted charge current without flagging a fault, and the protection circuit triggered correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current charging pushes amps into an uncalibrated cell.

Sudden shutdown at 20–30% on the Black Shark after a cell swap

The Black Shark's Snapdragon platform and high-refresh display pull current spikes that expose a voltage cliff in a degraded or freshly installed cell. When the cell voltage drops below roughly 3.5V under that load burst, the BMS trips the protection circuit before the OS registers empty. The phone reads 25% and shuts down — not because the cell is empty, but because it cannot hold voltage under peak draw. One full discharge-charge cycle with fast charging off lets the fuel gauge IC recalibrate its cutoff prediction to the new cell's actual curve.

USB fast charge not negotiating after replacement

After a cell swap, the charge IC sometimes refuses to negotiate the proprietary fast-charge protocol on the first plug-in — defaulting to standard 5V charging instead. This happens because the BMS on the new cell has not completed its first initialisation handshake with the phone's charge controller. Plug in, let it charge to 100% at standard rate, then unplug and restart the device. On the next charge cycle, fast charging should re-negotiate correctly at the rated voltage.