Vivo X27 Replacement Battery B-G5 3.85V 3900mAh

Vivo X27 / X27A Series — 3.85V Li-Polymer Replacement Battery (B-G5)

This is a 3.85V, 3900mAh lithium-polymer replacement cell for the Vivo X27, X27A, V1829A, and V1829 smartphones. It replaces the original B-G5 battery when the existing cell has degraded, lost capacity, or stopped holding a charge. Dimensions are 72.85 × 62.48 × 5.05mm — match these before ordering if your unit has been previously repaired.

• X27 and V1829 series fit: The X27, X27A, V1829A, and V1829 all share the same battery bay geometry, connector pinout, and BMS handshake protocol — that is why one cell covers all four model numbers. Swapping between these variants requires no adapter or wiring change.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a V1829A unit. The BMS accepted the cell without a fault flag, charge current ramped correctly through CC-CV stages, and the protection circuit tripped cleanly at the expected low-voltage cutoff threshold.
• Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one complete discharge down to auto-shutdown, then charge to 100% on standard current. This gives the fuel gauge IC one full reference cycle against 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

This is a voltage cliff, not a capacity fault. Under heavy load — 5G modem active, high-brightness display, background sync — the cell's internal resistance causes voltage to sag below the BMS cutoff threshold while the fuel gauge still reads 20–30%. The phone shuts down to protect the cell even though reported charge remains. The fix is one full discharge-charge calibration cycle: run the phone from 100% to shutdown under normal use, then charge uninterrupted to 100%. After that cycle, the fuel gauge IC recalibrates its coulomb counter to the actual discharge curve of the new cell.

USB fast charge not activating after installation

Vivo's proprietary fast charge protocol requires the charge IC to negotiate with the BMS on the first session. If the new cell arrives partially discharged, the charge IC may default to standard 5V/1A input rather than entering high-current mode. Connect the original Vivo charger — third-party USB-PD adapters will not trigger the proprietary handshake. If fast charge still does not activate, discharge the phone to auto-shutdown and attempt a fresh full charge from below 5%.