F-Fook F669 Compatible Battery 3.7V 1200mAh Li-ion

F-Fook F669 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh Li-ion cell built to replace the original battery in the F-Fook F669 mobile phone. It restores power to a device that no longer holds a charge or shuts down unexpectedly. Dimensions are 53.00 × 33.90 × 5.70mm — confirm against your existing cell before installing.

• F-Fook F669 fitment: The F669 uses a compact single-cell 3.7V nominal Li-ion pack. This replacement matches the voltage rail and physical footprint required by the phone's charge IC and connector orientation.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS held cutoff correctly at both ends — no false full-charge termination and no undervoltage lockout during discharge under simulated load.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging begins on an uncalibrated state-of-charge register.

Why the F669 reports wrong battery percentage after a cell swap

The F669's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The phone reads voltage and maps it against the old curve, so percentages appear incorrect or jump around. One full discharge-to-charge cycle forces the coulomb counter to reset and relearn the new cell's profile.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — typically during a call, screen-on, or data activity — faster than the fuel gauge IC expects. The phone's power management IC sees voltage fall below its cutoff threshold before the displayed percentage reaches zero. It is a calibration gap, not a faulty cell. Run one full cycle without fast charging and let the gauge IC recalibrate; the cutoff should stabilise closer to the 5–10% mark where cell voltage genuinely reaches 3.0V.