About this replacement
Asus BU201 Series — 7.6V Li-ion Replacement Battery (B21N1404)
This 7.6V 4200mAh (31.92Wh) Li-ion battery replaces the OEM B21N1404 cell in the Asus BU201, BU201L, BU201LA, and Pro Advanced BU201 ultrabooks. It fits the original bay without modification and uses the same connector pinout as the factory unit. Capacity figures are from the product specification — not extrapolated from web sources.
- BU201 series compatibility: The BU201, BU201L, and BU201LA share an identical battery bay, connector, and BMS handshake protocol — all three draw from the same 7.6V cell platform, which is why a single OEM part number covers the full lineup.
- Bench tested on actual hardware: We ran the B21N1404 replacement through charge and discharge cycles on a BU201LA unit. The BMS accepted charge from cold without tripping, and the BIOS recognised the cell as a valid pack on first boot.
- Post-install calibration on the BU201: After fitting this battery, run one full discharge to 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 swap on the BU201
The BU201 BIOS reads EEPROM data stored in the battery pack to assess health. When a new cell arrives, that EEPROM carries factory baseline figures that the BIOS compares against its own learned cycle history — a mismatch triggers the health warning. This is not a fault with the replacement cell. Running one full discharge-to-hibernate followed by an uninterrupted full charge forces the BIOS to write a fresh learn cycle to the new pack's EEPROM and clears the warning on the next reboot.
BU201 shutting down at 20–30% charge shown on the gauge
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual discharge curve. The gauge under-reports remaining capacity, so the BIOS triggers a low-battery shutdown before the cell is genuinely depleted. Two to three full discharge-and-charge cycles give the fuel gauge IC enough data to map the new cell accurately. After calibration, the shutdown threshold should align with a cell voltage at or below 6.8V under load.