B-F6 BBK VIVO NEX Replacement Battery 3.85V 3400mAh

BBK VIVO NEX / Nex Dual Display — 3.85V Li-Polymer Replacement Battery (B-F6)

The B-F6 is a 3400mAh (13.09Wh) Li-Polymer cell for the VIVO NEX, VIVO Nex Dual Display, and V1821 smartphones. It sits at 3.85V nominal, matching the original voltage rail that feeds the processor, display stack, and modem. Install this when the original cell can no longer hold charge or the phone shuts down unexpectedly.

• VIVO NEX, Nex Dual Display, V1821 compatibility: All three models share the same battery bay geometry (77.30 × 64.68 × 3.86mm), the same 3.85V rail, and the same B-F6 connector pinout — which is why one cell covers all three variants without modification.
• Bench tested on actual hardware: We cycled this cell on a VIVO NEX chassis and confirmed the BMS accepted the new cell without lockout, charge current ramped correctly through CC/CV stages, and the protection circuit triggered cleanly at the low-voltage cutoff threshold.
• Fast charge recalibration on first cycle: On first use after installation, disable fast charging and run one full discharge-charge cycle at standard rate. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging pushes current into an uncalibrated coulomb counter.

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

The VIVO NEX fuel gauge IC retains the discharge curve from the old cell in memory. When it calls 25% remaining, the new cell's actual open-circuit voltage may already be near the modem's brownout floor. Under LTE or display load, voltage sags below the cutoff threshold and the phone shuts off — even though the OS reported charge remaining. One complete discharge to auto-off followed by a full charge to 100% resets the coulomb counter and re-anchors the gauge to the new cell's real curve. After that cycle, percentage readings stabilise.

VIVO NEX won't power on after sitting in storage with a flat battery

Li-Polymer cells that drain below roughly 2.5V trigger a BMS lockout to prevent damage — the phone shows nothing when you press the power button, and a standard charger may not initiate a charge session at all. Connect the phone to a low-current USB source (a 5W brick, not a fast charger) and leave it for 15–20 minutes without pressing anything. The BMS needs a trickle to recover the cell voltage above the reinitialisation threshold before normal charging can begin. Once the charging indicator appears, you can switch to your regular charger.