About this replacement
Vivo S9 / V2072A — 3.89V Li-Polymer Replacement Battery (B-P9)
This 3.89V, 3900mAh Li-Polymer cell replaces the original B-P9 battery in the Vivo S9 and V2072A smartphones. It fits the same physical footprint at 83.40 × 61.90 × 4.22mm and connects to the existing flex cable without modification. Capacity is 15.17Wh, matching the factory specification.
- S9 and V2072A fitment: Both model numbers refer to the same hardware revision — same battery bay dimensions, same connector pinout, and same BMS handshake protocol. One cell covers both.
- Bench tested on actual hardware: We cycled the B-P9 replacement through charge and discharge on the S9 mainboard. The BMS accepted the cell without error flags, and the charge IC reached full charge cutoff at the correct terminal voltage.
- 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 the new cell's discharge curve before high-current charging pushes current into an uncalibrated state.
Sudden shutdown at 20–30% after fitting the B-P9 replacement
This happens because the Vivo S9 fuel gauge IC is still running the discharge curve from the old, degraded cell. The new cell holds a higher voltage at low state-of-charge than the IC expects, so the phone reads 25% while the actual cell voltage is already near the load-collapse threshold. Under modem or display load, voltage dips briefly below the system cutoff and the phone shuts off. One full discharge to automatic shutdown, followed by a full charge without interruption, forces the coulomb counter to resync to the new cell curve and clears this behaviour.
USB-PD fast charge not activating on the first cycle after replacement
After a cell swap, the BMS on the replacement unit starts in a conservative state. The Vivo S9 charge IC queries the BMS before stepping up to fast charge current, and if the BMS has not completed its first charge cycle, it responds with a lower negotiated current limit. Fast charging typically activates normally from the second charge cycle onward. If fast charge still does not engage after two full cycles, check that the charger is the original Vivo adapter — third-party adapters may not assert the correct voltage handshake to unlock the fast charge rail.