Vivo Y9S Replacement Battery B-K3 3.85V 4200mAh

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

This is a 4200mAh 3.85V lithium-polymer replacement battery for the Vivo Y9S, V1945A, and V1945T smartphones. It carries OEM part number B-K3 and matches the original cell's footprint at 78.38 × 61.85 × 5.10mm. Install it when the original cell no longer holds a charge or shuts the phone down unexpectedly.

• Y9S / V1945A / V1945T platform fit: All three model variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell covers the full range without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a V1945 unit and confirmed the BMS accepted the handshake, reported state-of-charge correctly, and held cutoff voltage at the expected 4.40V charge ceiling.
• First-cycle fast-charge tip: On first use after installation, disable fast charging and run one complete discharge-charge cycle on standard 5W. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell.

Sudden shutdown at 20–30% on the replacement cell

A new cell has a slightly different voltage-versus-capacity curve than the aged cell the fuel gauge IC was calibrated against. When the phone reaches the steep voltage cliff under modem or display load, the IC misreads available capacity and the protection circuit trips before the gauge reaches zero. The fix is a full calibration cycle: drain the phone to automatic shutdown, then charge uninterrupted to 100% with fast charging off. After one complete cycle the coulomb counter resets its reference points and the shutdown behaviour stops.

Phone won't power on after the replacement cell sat in storage

Li-polymer cells self-discharge during storage. If the cell drops below approximately 2.5V per cell, the BMS enters lockout mode and blocks output to protect the cell from permanent damage. Plugging straight into a fast charger will not recover it — the BMS rejects the high-current handshake when cell voltage is this low. Connect to a standard 5W charger and leave it for 20–30 minutes; the charge IC trickle-charges the cell back above the BMS unlock threshold, after which the phone will power on normally. Target recovery voltage is 3.0V before attempting a full charge cycle.