About this replacement
Asus Zenbook 3 UX390UA — 7.7V Li-Polymer Replacement Battery (C23N1606)
This is a 7.7V, 5050mAh (38.89Wh) Li-Polymer replacement battery for the Asus Zenbook 3 series, including the UX390UA, UX390, and UX390U. It fits the ultra-slim chassis where the original C23N1606 cell sits flush along the base. It restores portable operation to units where the original cell has degraded or swollen.
- UX390 series compatibility: The UX390UA, UX390U, and base UX390 all share the same battery bay geometry, connector pinout, and 7.7V charging rail. The BMS on each model communicates over the same SMBus lines, so the cell negotiates charge parameters without modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the UX390UA platform. The BMS initialised correctly, accepted a full charge to 8.6V, and the fuel gauge IC registered capacity without throwing a protection fault.
- Post-install discharge cycle on the UX390UA: After fitting, run one full discharge to the automatic hibernate cutoff, then charge uninterrupted to 100%. The Zenbook 3 BIOS runs a battery learn cycle on this sequence — skipping it leaves the health indicator showing poor or unknown after every cell swap.
BIOS reporting poor battery health immediately after fitting the C23N1606
The Zenbook 3 BIOS reads health data from EEPROM registers in the battery's fuel gauge IC. A new cell ships with factory EEPROM values that don't match the charge history the BIOS expects to see. This mismatch triggers a "poor health" or "replace battery" flag even on a brand-new cell. Run one full discharge to hibernate cutoff followed by an uninterrupted charge to 100% — this writes fresh learned data to the fuel gauge IC and clears the warning. After two or three cycles the BIOS health reading stabilises.
Zenbook 3 shutting down at 20–30% charge shown on the indicator
This is a voltage cliff issue, not a fuel gauge error. Under full CPU and display load the cell voltage drops faster than the fuel gauge IC tracks, and the system hits the low-voltage cutoff before the displayed percentage reaches zero. It occurs most often after several shallow-charge cycles on a new cell before the fuel gauge is calibrated. Run a full discharge to hibernate cutoff — the system will shut down at approximately 3.0V per cell — then charge to 100% without interruption. Repeat this once more and the gauge recalibrates its empty-voltage reference.