Vivo NEX B-F6 Replacement Battery 3.85V 3400mAh

Vivo NEX Series — 3.85V Li-Polymer Replacement Battery (B-F6)

This is a 3400mAh, 3.85V Li-Polymer replacement battery for the Vivo NEX, NEX Dual Display, and V1821 smartphones. It replaces the original B-F6 cell when the installed battery no longer holds adequate charge. Dimensions are 77.30 × 64.68 × 3.86mm — verify clearance before ordering if your unit has any aftermarket modifications.

• NEX, NEX Dual Display, and V1821 compatibility: All three models share the same battery bay geometry, connector pinout, and BMS handshake protocol, which is why a single B-F6 cell covers the full range. The fuel gauge IC on each variant communicates over the same interface, so no firmware conflict occurs during cell recognition.
• Bench tested on actual hardware: We ran the B-F6 cell through charge and discharge cycles on the NEX platform, monitoring BMS handshake, charge acceptance, and cutoff behaviour. The BMS accepted the cell without fault flags and terminated charge correctly at full voltage.
• First-cycle fast-charge tip: On first use after installation, disable fast charging and complete one full discharge-charge cycle at standard rate. This lets the fuel gauge IC recalibrate its coulomb counter against the new cell's actual discharge curve before high-current fast charging applies load to an uncalibrated cell.

Why the Vivo NEX reports wrong battery percentage after a cell swap

The fuel gauge IC on the NEX builds its percentage model from accumulated charge and discharge data tied to the original cell's impedance profile. When a new cell goes in, the IC still references that old model, so it miscalculates state-of-charge from the first boot. The result is a percentage readout that jumps, stalls, or reads high until the IC collects fresh data. One complete discharge to around 3.2V followed by a full charge at standard rate resets the coulomb counter and brings the readout back into alignment.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage-cliff failure, not a capacity problem. Under high-draw conditions — active modem, screen at full brightness, or background sync — the cell voltage drops faster than the fuel gauge IC predicts, crossing the BMS cutoff threshold while the display still shows charge remaining. The IC's stored discharge curve doesn't match the new cell yet, so it doesn't anticipate the drop. Complete two full standard-rate charge cycles and the fuel gauge recalibrates; shutdowns at false percentages typically stop once the IC has accurate voltage-to-capacity mapping for the new cell.