Vivo B-S9 IQOO Neo 5s Compatible Battery 7.78V 2100mAh

Vivo iQOO Neo 5s — 7.78V Li-Polymer Replacement Battery (B-S9)

This is a 7.78V, 2100mAh Li-Polymer battery for the Vivo iQOO Neo 5s, iQOO 9 SE 5G, and I2109 variants. It replaces part number B-S9 when the original cell has degraded past usable capacity. At 16.34Wh and 84.10 × 62.80 × 5.10mm, it matches the factory footprint and connector position.

• iQOO Neo 5s and 9 SE 5G platform fit: These models share the same dual-cell 7.78V architecture, connector pinout, and BMS handshake protocol — one cell part number covers the full range without modification to the flex cable or adhesive mounting points.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Neo 5s mainboard. The BMS accepted the new cell without error flags, and the charge IC negotiated correctly at both standard and fast-charge current levels after the first full cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard current. This lets the coulomb counter recalibrate its discharge curve against the new cell before high-current charging is applied to an uncalibrated state-of-charge model.

Why the iQOO Neo 5s shuts down suddenly at 20–30% after a cell swap

The Neo 5s uses a coulomb-counter fuel gauge that was calibrated against the original cell's discharge voltage curve. A new cell has a slightly different curve — flatter at mid-charge, steeper near depletion. Under modem or display load, voltage can drop sharply below the BMS cutoff threshold even when the OS still shows 25%. The phone reads that as a valid charge level until the voltage cliff hits. One full discharge-to-shutdown cycle, followed by an uninterrupted charge to 100%, resets the coulomb counter baseline and eliminates most false-cutoff events.

Fast charging not activating after replacement on the Neo 5s

Vivo's fast-charge protocol requires a BMS handshake that some replacement cells do not complete on the first cycle. The charge IC falls back to 5V standard input rather than triggering the higher-voltage fast-charge mode. This is not a fault — it is a protocol fallback. Run one standard charge cycle to 100%, then reboot the device before reconnecting the fast charger. The charge IC renegotiates the handshake and fast charging activates from the second cycle onward. Confirm the mode is active by checking charge current in the battery settings menu.