About this replacement
Asus BU400A / Pro BU400 Series — 7.4V Li-Polymer Replacement Battery (C22-B400A)
This is a 7.4V, 6750mAh (49.95Wh) lithium-polymer replacement battery for the Asus BU400A, BU400V, Pro BU400, and Pro BU400A ultrabook series. It replaces OEM part numbers C22-B400A, 0B200-00160000, and C22-BU400A. Fit these models as a direct swap when the original cell no longer holds a charge or fails to register correctly with the system.
- BU400 and Pro BU400 platform compatibility: The BU400A and BU400V share the same battery bay geometry, connector pinout, and BMS handshake protocol. One cell covers both lines without adapter or modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a BU400A unit. The BMS negotiated correctly, charge termination triggered at the expected voltage ceiling, and protection cutoff engaged at the low-voltage threshold without fault codes.
- First-cycle reset on the BU400A: After installation, run one full discharge down to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that almost always appears after a cell swap on this platform.
Why the BU400A BIOS reports poor battery health after a cell swap
The BU400A stores battery health data in EEPROM on the original cell. When you fit a new cell, the BIOS reads that EEPROM baseline and flags a mismatch against the new chemistry profile. This is not a fault with the replacement — it is the firmware reacting to unfamiliar cycle count and capacity data. Running the full learn cycle described above writes fresh calibration data and clears the warning. After two or three full cycles, the BIOS health indicator stabilises.
Laptop shuts down with 20–30% charge still showing on screen
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual discharge curve. The old curve predicts voltage drop incorrectly, so the system hits the low-voltage protection threshold well before the displayed percentage reaches zero. The fix is the same learn cycle: discharge fully to hibernate-cutoff, then charge to 100% without interruption. After two full calibration cycles, the gauge tracks accurately and the early shutdowns stop.