About this replacement
Vivo Y78+ / Y78 Plus — 3.89V Li-Polymer Replacement Battery (B-Z3)
The B-Z3 is the OEM-matched Li-Polymer cell for the Vivo Y78+ and Y78 Plus. It runs at 3.89V and delivers 4500mAh (17.51Wh) — the same capacity as the factory-fitted unit. If your Y78+ no longer holds charge through a full day, this cell replaces the degraded original directly.
- Y78+ and Y78 Plus compatibility: Both model names refer to the same hardware platform. They share the same battery bay dimensions, flex connector, and BMS handshake protocol, so one cell covers both listings without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Y78+ platform. The BMS accepted the cell on the first charge, thermal sensors reported normal, and the protection circuit tripped correctly at low-voltage cutoff — no fault codes logged.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current fast charging is introduced to an uncalibrated cell.
Sudden shutdown at 20–30% on the replacement cell
This is the most common complaint after a cell swap on the Y78+. The fuel gauge IC still holds the discharge curve of the old degraded cell, so it miscalculates the remaining charge in the new one. Under modem or display load, the cell voltage drops faster than the IC expects, and the phone hits its emergency cutoff before the reported percentage reaches zero. One full discharge-charge cycle at standard rate forces the IC to rebuild its curve against the new cell. After that cycle, shutdowns at 20–30% typically stop.
USB-PD fast charge not triggering on the first cycle after replacement
After fitting a new B-Z3, some Y78+ units default to standard 5V charging on the first cycle and refuse to negotiate fast charge. The charge IC runs a BMS handshake on first contact with a new cell — if the cell voltage is low from storage, the IC holds the charger at trickle current until the cell clears roughly 3.0V per cell. Once that threshold is passed, the fast-charge protocol re-negotiates automatically. If fast charge still does not activate after a full first cycle, check that the USB-C port and cable both support the Vivo FlashCharge spec — the bottleneck is often the cable, not the battery.