About this replacement
Asus BU403UA / PU403UA Series — 11.1V Li-Polymer Replacement Battery (B31N1507)
This 11.1V, 4050mAh Li-Polymer battery replaces the OEM cell in the Asus BU403UA, PU403UA, PU403UF, and P5430UA ultrabooks. It matches the original pack's connector, BMS handshake protocol, and physical footprint at 186.46 × 99.10 × 8.00mm. Capacity is rated at 44.96Wh — identical to the factory specification.
- BU403UA and PU403UA platform compatibility: These models share the same 11.1V three-cell Li-Polymer architecture, identical battery bay dimensions, and the same BMS communication protocol over SMBus. One cell fits the entire family without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a BU403UA unit. The BMS accepted the pack without fault codes, balanced all three cells within 10mV at top-of-charge, and held the full 11.1V nominal rail under CPU and display load.
- First-cycle calibration after install: After fitting this cell, run the laptop on battery until Windows forces hibernate, then charge uninterrupted to 100% without interruption. This single cycle resets the BIOS battery-learn register and clears the inaccurate health warning that appears after every cell swap on this platform.
Why the BU403UA shuts down at 20–30% shown on the fuel gauge
The fuel gauge IC on the BU403UA references discharge curves stored from the previous cell. When a new cell is installed, those curves no longer match the actual chemistry, so the gauge underestimates remaining charge. The system triggers a low-battery shutdown before the cell is genuinely depleted. Two to three full discharge-to-hibernate and charge-to-100% cycles recalibrate the gauge IC against the new cell's actual voltage-to-capacity curve. After calibration, the shutdown point drops back to the correct 3% to 5% threshold.
BIOS reporting battery health as poor or unknown immediately after replacement
The BIOS pulls health data from EEPROM registers written by the original factory cell. A replacement cell ships with its own EEPROM data, which can read as mismatched or unrecognised on first boot. This is not a fault — it is a data conflict between old and new cell records. Run one full discharge-to-hibernate cycle followed by a full uninterrupted charge to 100%, then reboot. The BIOS battery-learn cycle rewrites the health register against the new cell and the poor-health flag clears.