About this replacement
Asus Zenbook UX305UA — 11.55V Li-Polymer Replacement Battery (C31N1539)
This is a 11.55V, 4850mAh (56.02Wh) Li-Polymer replacement battery for the Asus Zenbook UX305UA and UX305UAB ultrabooks. It uses OEM part numbers C31N1539 and 0B200-01180200. Install it when the original cell no longer holds a charge or the system reports degraded battery health.
- UX305UA and UX305UAB compatibility: Both variants share the same motherboard battery connector, voltage rail, and BMS handshake protocol, which is why a single cell covers both. The 3-cell Li-Polymer configuration matches the original charge controller's expected impedance and EEPROM communication profile.
- Bench tested on actual hardware: We cycled this cell on a UX305UA unit and confirmed the BMS initialised correctly, charge acceptance reached full capacity without cutoff errors, and the system reported normal battery status within two charge cycles.
- Post-install calibration for the UX305UA: After fitting this cell, run one full discharge to the hibernate cutoff point, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning the firmware typically throws after any cell swap.
BIOS reporting poor battery health immediately after fitting the new cell
The Zenbook UX305UA stores battery health data in EEPROM on the old cell. When you fit a new cell, the BIOS reads residual or zeroed EEPROM values and flags the battery as degraded or unknown — even though the cell is new. This is a firmware read issue, not a hardware fault. Running two full charge and discharge cycles allows the battery learn cycle to overwrite the stale EEPROM data. After the second full cycle, the BIOS health indicator should return to normal.
Laptop shutting down unexpectedly at 20–30% charge shown
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual discharge curve. The IC maps voltage to percentage using data from the old, degraded cell — so the estimated percentage drops faster than the real state of charge. Under combined CPU and display load, the voltage reading hits the BMS cutoff threshold while the gauge still shows 20–30% remaining. Run two full discharge-to-hibernate then full-charge cycles to let the fuel gauge IC recalibrate. After calibration the reported percentage will align with actual cell voltage, and the unexpected shutdowns will stop.