Vivo Y51S Replacement Battery B-N5 3.87V 4400mAh

Vivo Y51S / Y70S Series — 3.87V Li-Polymer Replacement Battery (B-N5)

This 3.87V, 4400mAh Li-Polymer cell replaces the original B-N5 battery in the Vivo Y51S, Y70S, V2002A, and V2002. It fits devices that share the same connector, BMS pinout, and physical envelope — 90.00 × 63.92 × 4.30mm. When the original cell degrades and the phone can no longer hold charge through a normal day, this is the direct swap.

• Y51S / Y70S platform compatibility: These models run the same PCB layout and battery connector spec. The B-N5 part number covers both because Vivo used the same voltage rail, BMS handshake, and physical footprint across the V2002 variant lineup.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Y51S unit. The BMS accepted charge current without triggering a fault, and the protection circuit tripped correctly at the low-voltage cutoff threshold — no runaway, no false full-charge flag on the first cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before the coulomb counter starts reporting to the OS — skipping this step causes percentage jumps early on.

Sudden shutdown at 20–30% on the Y51S after a cell swap

This happens because the fuel gauge IC is still running the discharge curve from the old, degraded cell. The new cell has a steeper voltage cliff at low state-of-charge than the worn cell did, so the phone reads 25% while the actual cell voltage has already dropped below what the modem and display load can sustain. The BMS trips to protect the cell before the OS sees zero percent. One full discharge cycle — down to automatic shutdown, then a full charge without interruption — forces the coulomb counter to resync against the new cell's actual voltage curve.

USB fast charge not activating after replacement

After fitting a new B-N5 cell, some Y51S units revert to standard 5V charging and do not negotiate the higher voltage tier. The charge IC re-evaluates BMS handshake data on the first session and may default to a safe low-current mode until the cell has completed one full cycle. Plug in using the original Vivo adapter, let the phone charge uninterrupted to 100%, then unplug and reboot. Fast charge typically resumes from the second session once the BMS has logged an initial full-cycle signature.