About this replacement
Asus F402C Series — 7.4V Li-Polymer Replacement Battery (C21-X402)
This is a 7.4V, 5100mAh Li-Polymer replacement battery for the Asus F402C notebook. It fits the F402C, F402CA-WX102H, X402CA-WX094D, X402CA-1D, and over 20 additional F402/X402CA variants. When the original C21-X402 cell degrades and no longer holds charge, this unit restores portable power to the machine.
- F402 and X402CA platform fit: These models share the same battery bay dimensions, 7.4V power rail, and C21-X402 connector. The BMS handshake uses the same two-wire communication protocol across the entire F402/X402 series, so one cell covers the full range.
- Bench tested on actual hardware: We ran this cell through full charge and load cycles on an F402CA unit. The BMS completed charge termination cleanly at 8.4V and held protection thresholds correctly under sustained CPU and display load.
- Post-install calibration on the F402C: After fitting this battery, run one full discharge down to the BIOS hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.
BIOS reporting poor battery health immediately after fitting the C21-X402
The F402C BIOS reads health data from EEPROM registers written by the original factory cell. When a new cell is installed, those registers don't match the new chemistry profile, so the BIOS flags the battery as degraded or unknown. This is a firmware read issue, not a fault with the replacement cell. Running one full discharge-to-hibernate and uninterrupted recharge gives the fuel gauge IC enough data to rewrite accurate state-of-charge values. After two full cycles, the health warning clears on most F402CA units.
Laptop shuts down at 20–30% charge shown on the F402C
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The OS shows remaining charge based on stale EEPROM data, so the shutdown hits before the displayed percentage reaches zero. Under full CPU load plus backlit display, the cell voltage drops faster than the uncalibrated gauge predicts, and the BIOS pulls the plug to protect the circuit. Run two complete discharge-to-hibernate cycles and the gauge recalibrates — shutdowns at false percentages stop once the IC maps the real voltage cliff, which on this cell sits at approximately 6.0V under load.