About this replacement
Vivo NEO 3 / R831K — 3.85V Li-Polymer Replacement Battery (B-M8)
This is a 3.85V, 4250mAh (16.36Wh) Li-Polymer replacement battery for the Vivo NEO 3 and R831K smartphones. It carries OEM part number B-M8 and matches the original cell's dimensions at 85.00 × 63.70 × 4.50mm. Install it when the original cell no longer holds a charge or causes unexpected shutdowns.
- NEO 3 and R831K compatibility: Both models share the same battery bay dimensions, voltage rail, and B-M8 connector layout. The BMS handshake protocol is identical across this pair, so one cell covers both devices without modification.
- Bench tested on actual hardware: We ran this cell through a full charge and discharge cycle on the NEO 3. The BMS accepted the charge handshake cleanly, voltage stayed stable across the discharge curve, and no protection trips fired under screen-plus-modem load.
- Fuel gauge recalibration on first use: On first installation, disable fast charging and run one complete discharge-to-charge cycle at standard speed. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes power into an uncalibrated cell.
Why the NEO 3 reports wrong battery percentage after a cell swap
The NEO 3 uses a fuel gauge IC that builds its charge model against the original cell's discharge curve over time. Swap the cell and the IC is still referencing an old curve that no longer matches the new chemistry. The result is a percentage readout that jumps, stalls, or reads full while the phone is already low on charge. One complete slow discharge down to automatic shutdown, followed by a full charge to 100%, resets the coulomb counter and brings the gauge back in line.
Sudden shutdown at 20–30% on the replacement cell
This happens when the fuel gauge IC has not yet calibrated to the new cell's voltage cliff. Under combined modem and display load, the cell voltage drops sharply at a point the gauge still thinks is safe — the phone hits the hardware undervoltage cutoff before the OS can react. It is not a faulty cell. Charge the phone to 100%, let it discharge in normal use to automatic shutdown without interruption, then charge back to 100% — the IC recalibrates the cliff point to around 3.4–3.5V on this cell.