Vivo X9i B-B9 Compatible Battery 3.85V 3150mAh

Vivo X9i — 3.85V Li-Polymer Replacement Battery (B-B9)

This is a 3150mAh (12.13Wh) Li-Polymer cell at 3.85V, built to the same dimensions as the original B-B9 unit fitted in the Vivo X9i. It replaces a swollen, degraded, or non-charging original cell in the X9i mid-range Android handset. Capacity figures come from the product data — 3150mAh is what this cell is rated at, not an estimate.

• X9i fitment: The X9i uses a sealed rear panel with a specific cell footprint — 82.90 x 62.40 x 3.40mm. This cell matches those dimensions exactly. The connector orientation and tab length match the original BMS harness routing, so no forced bending at the flex point.
• Bench tested on actual hardware: We ran charge and discharge cycles through the X9i charge IC and confirmed the BMS handshake completes correctly. The protection circuit trips at the expected undervoltage threshold and resets cleanly on reconnection to a charger.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one full discharge-charge cycle. The X9i's fuel gauge IC is calibrated against the old cell's discharge curve. Running one slow cycle against the new cell gives the coulomb counter accurate data before high-current charging is applied.

Sudden shutdown at 20–30% on the X9i after a cell swap

A new cell has a discharge curve the fuel gauge IC hasn't mapped yet. When the X9i drives the modem or display at high brightness, the instantaneous current draw causes a voltage sag the IC didn't anticipate. The phone reads a safe percentage but the cell voltage collapses below the shutdown threshold under load. One full slow discharge down to 3.0V followed by a full slow charge resets the coulomb counter and eliminates most of these shutdowns.

Vivo's fast charge protocol not activating after battery replacement

The X9i uses a proprietary charge protocol negotiated between the adapter and the charge IC — not standard USB-PD. On the first cycle with a new cell, the charge IC may default to a conservative 5V/1A profile while it verifies the new cell's impedance is within range. This is normal behaviour, not a fault. After one complete charge cycle the IC re-negotiates the higher-current profile and fast charging resumes. If it still fails after two cycles, verify the charge port pins are clean and the original fast charge adapter is in use.