Vivo Y31 B-77 Compatible Battery 3.8V 2000mAh

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 to replace the original battery in the Vivo Y31, Y31A, and Y31L. These are mid-range Android smartphones that share an identical battery bay, connector, and BMS handshake across all three variants. If your Y31 is shutting down unexpectedly, holding a fraction of its original charge, or refusing to power on, this cell addresses the failed original unit.

• Y31 / Y31A / Y31L compatibility: All three model variants use the same physical cell format — 76.00 × 52.00 × 4.50mm — with an identical flex connector and the same BMS communication protocol. The charge IC on all three boards negotiates with the cell's BMS in the same way, so no firmware conflict occurs when swapping to this replacement cell.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on Y31 hardware. The BMS reported correct voltage handshake at 3.8V nominal, the charge IC accepted the cell without fault codes, and the protection circuit tripped correctly at the low-voltage cutoff threshold during deep discharge simulation.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current charging is applied to an uncalibrated cell.

Sudden shutdown at 20–30% on the Vivo Y31

This happens when the cell's voltage drops sharply under load — typically during an active call, screen-on navigation, or mobile data use — before the reported percentage reaches zero. The fuel gauge IC predicted remaining capacity based on the old cell's degraded discharge curve, so the percentage shown is simply wrong. The modem and display draw enough current to pull cell voltage below the BMS cutoff threshold in milliseconds. One full slow discharge-to-charge cycle after fitting the replacement cell resets the coulomb counter and eliminates this behaviour.

Phone won't power on after the B-77 sat in storage

Li-ion cells self-discharge in storage, and if the B-77 has dropped below approximately 2.5V per cell, the BMS enters lockout mode and blocks all charge current as a safety measure. The phone will not respond to the power button and may show no charging indicator at all. Connect the phone to a low-current charger — a standard 5W USB adapter, not a fast charger — and leave it for 20–30 minutes without attempting to power on. Once the cell recovers above the BMS re-enable threshold, the charge IC will resume normal operation and the phone will power on.