About this replacement
Asus ZenBook UX21 Series — 7.4V Li-Polymer Replacement Battery (C23-UX21)
This 7.4V, 4500mAh (33.3Wh) Li-Polymer battery replaces the original C23-UX21 cell in the Asus ZenBook UX21, UX21A, and UX21E ultrabooks. It fits the slim 5.80mm profile of the UX21 chassis without modification. Install it when the original cell no longer holds charge or fails to power the machine.
- UX21 and UX21A/UX21E compatibility: These models share the same 7.4V two-cell Li-Polymer architecture, identical connector pinout, and the same BMS handshake protocol — so one cell fits all three variants without rewiring or adapter plates.
- Bench tested on actual hardware: We cycled this cell on a UX21E, confirmed BMS communication with the EC, and verified charge acceptance from 0V recovery through to full 8.4V cutoff. Discharge curves matched rated capacity within 3%.
- Post-install calibration on the UX21: After installing, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This triggers the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.
Why the UX21 BIOS flags a new battery as poor health
The UX21 EC reads health data stored in the old cell's EEPROM and compares it against the new cell's reported values. A fresh cell has no charge history logged, so the EC interprets the blank data as degraded capacity. This triggers a "battery health poor" flag in the BIOS even though the cell is new. One full discharge-to-hibernate followed by an uninterrupted charge to 100% writes fresh calibration data to the EC and clears the flag.
ZenBook UX21 shutting down at 20–30% charge shown
This happens when the fuel gauge IC still holds calibration data from the old degraded cell. The IC predicts remaining capacity based on past voltage curves, and when the new cell hits a slightly different voltage slope, the gauge loses accuracy and calls shutdown too early. It is not a fault with the replacement cell — it is a calibration mismatch. Run two full discharge-to-hibernate and recharge cycles; the fuel gauge IC re-maps its model against the new cell's voltage curve and the early shutdowns stop.