BBK VIVO Y9S Compatible Battery B-K3 3.85V 4200mAh

BBK VIVO Y9S / V1945A / V1945T — 3.85V Li-Polymer Replacement Battery (B-K3)

This is a 3.85V, 4200mAh (16.17Wh) Li-Polymer cell built to replace the B-K3 battery in the VIVO Y9S, V1945A, and V1945T smartphones. It matches the original voltage rail and connector, so the phone's charge IC and BMS communicate with the replacement without modification. Physical dimensions are 78.38 × 61.85 × 5.10mm — verify clearance before installation if your unit has aftermarket adhesive residue on the battery tray.

• VIVO Y9S, V1945A, V1945T fitment: All three model numbers share the same battery bay geometry, flex connector pinout, and BMS handshake protocol. One cell covers all three variants without adapter or wiring change.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a VIVO Y9S unit. The BMS accepted the cell, charge current ramped correctly, and the protection circuit tripped at the expected low-voltage threshold without nuisance cutoffs.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one complete 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 state.

Sudden shutdown at 20–30% on the VIVO Y9S after a cell swap

The VIVO Y9S uses a coulomb counter-based fuel gauge that was calibrated to the original cell's internal resistance curve. A new cell has a different voltage-versus-state-of-charge profile, so the OS reads 25% remaining while the actual cell voltage has already dropped below the modem's minimum operating threshold. The phone cuts out before the display ever reaches 0%. One full discharge cycle — letting the phone shut itself off naturally, then charging to 100% uninterrupted — resets the coulomb counter reference points and eliminates the early shutdown.

USB fast charge not engaging on the first cycle after replacement

VIVO's proprietary fast charge handshake involves the charge IC querying the BMS before it steps up current. On a freshly installed cell, the BMS may return a state-of-charge value that causes the charge IC to default to standard 5V/1A input until one cycle completes. This is a software-side safety gate, not a hardware fault with the replacement cell. Charge once at standard speed to full, then disconnect and reconnect — the handshake will re-negotiate and fast charging should activate at that point.