BBK VIVO Y31 Replacement Battery B-77 3.8V 2000mAh

BBK VIVO Y31 / Y31A / Y31L — 3.8V Li-ion Replacement Battery (B-77)

The B-77 is a 3.8V Li-ion cell rated at 2000mAh (7.6Wh), built as a direct swap for the VIVO Y31, Y31A, and Y31L smartphones. It replaces a degraded or failed original cell when the phone can no longer hold adequate charge through a normal day. Dimensions are 76.00 × 52.00 × 4.50mm — the same form factor as the OEM cell.

• VIVO Y31 / Y31A / Y31L platform fit: All three variants share the same battery bay dimensions and connector pinout, which is why a single B-77 cell covers them. The BMS handshake voltage and charge termination thresholds are consistent across the Y31 lineup, so the fuel gauge IC reads correctly on all three models after one calibration cycle.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the Y31 platform and monitored BMS cutoff behaviour at both ends of the voltage range. Charge termination triggered cleanly at 4.35V and the low-voltage cutoff held at 3.0V without triggering a premature shutdown.
• Fuel gauge recalibration on first install: On first use after fitting, disable fast charging and run one complete discharge-to-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before the coulomb counter locks in its reference points — skipping this step causes erratic percentage readings for several days.

Why the VIVO Y31 reports wrong battery percentage after a cell swap

The Y31 uses a coulomb counter IC that builds its charge model against the original cell's impedance and discharge curve. When a new cell goes in, the stored reference data no longer matches the physical cell — the IC is still predicting behaviour from the old battery. This mismatch typically shows as the percentage reading jumping or sticking at a fixed value. One full discharge down to automatic shutdown, followed by an uninterrupted charge to 100%, forces the IC to rewrite its reference and restores accurate reporting.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff issue, not a capacity issue. Under peak load — active LTE modem, screen at full brightness, or a background sync burst — the cell voltage drops faster than the fuel gauge tracks, crossing the BMS cutoff threshold before the displayed percentage reaches zero. It most commonly appears in the first few cycles before the coulomb counter recalibrates. Run two full discharge-charge cycles without fast charging enabled, then confirm the cell holds above 3.5V under load by checking the battery stats in the VIVO diagnostics menu.