About this replacement
Asus Vivobook X442 Series — 7.6V Li-Polymer Replacement Battery (C21N1638)
This 7.6V, 4750mAh (36.1Wh) Li-Polymer battery replaces OEM part C21N1638 (also 0B200-02630000) in the Asus Vivobook X442 lineup. It fits the VivoBook X442UR, VivoBook A480U, VivoBook F442U, and over 90 additional Vivobook variants sharing this cell format. The physical dimensions are 177.60 × 80.65 × 9.54mm — verify clearance before ordering if your chassis has been previously serviced.
- Vivobook X442 series fitment: These models share a common 7.6V two-cell Li-Polymer rail with the same connector pinout and BMS handshake protocol. The C21N1638 part number spans the entire X442 generation, so one cell covers the full model spread without wiring adapters or firmware workarounds.
- Bench tested on actual hardware: We ran this cell through charge, load, and cutoff cycles on a Vivobook X442UR. The BMS responded correctly at both charge termination and low-voltage cutoff. The EC reported state-of-charge without errors once the battery learn cycle completed.
- Post-install calibration on the X442: After fitting this cell, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This resets the Vivobook's BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap — skipping this step leaves the fuel gauge reading stale data from the old cell.
BIOS reporting poor battery health immediately after fitting the C21N1638
The Vivobook EC reads health data from EEPROM registers that the old cell wrote over months of use. A new cell arrives with different EEPROM baseline values, so the BIOS flags it as degraded before any real-world data exists. This is not a fault with the replacement cell — it is a calibration mismatch. Run the full discharge-to-hibernate and uninterrupted full-charge cycle described above. After two to three complete cycles, the EC rewrites its health baseline against the new cell's actual charge curve.
Vivobook X442 shutting down at 20–30% charge shown
This happens when the fuel gauge IC is still calibrated to the old cell's voltage-to-capacity curve. The new cell hits a voltage cliff under full CPU plus display load at a point the gauge still maps to 20–30%. The laptop interprets the sudden voltage sag as an emergency and cuts power before the gauge reaches zero. Discharge the battery fully until the laptop hibernates on its own, then charge to 100% without interruption — repeat this twice and the fuel gauge IC recalibrates its curve to match the new cell's actual discharge profile.