Asus ExpertBook P2 Replacement Battery B31N1909 11.4V 4155mAh

Asus ExpertBook P2 P2451FA Series — 11.4V Li-Polymer Replacement Battery (B31N1909)

This 11.4V 4155mAh Li-Polymer battery replaces the OEM B31N1909 cell in the Asus ExpertBook P2 P2451FA and P2451FB series laptops. It fits a wide range of P2451 variants including the EK0261T, EK0229T, and EK0261 models. Voltage, connector, and BMS handshake match the original specification.

• P2451FA and P2451FB platform fit: These models share the same three-cell Li-Polymer configuration, 11.4V voltage rail, and B31N1909 part number. The connector pinout and BMS communication protocol are identical across the P2451 line, so one cell covers the full range.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a P2451FA unit. The BMS initialised without error, charge termination triggered correctly at full capacity, and the protection circuit responded normally to load spikes during CPU stress testing.
• Post-install calibration on the P2451 series: After fitting this cell, run one full discharge until the laptop hibernates, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle on the P2451 platform and clears the inaccurate health warning that appears after every cell swap.

BIOS reporting poor battery health immediately after fitting a new cell

The P2451FA BIOS reads health data from the battery's EEPROM, which stores cycle count and rated capacity from the previous cell. When a new cell is installed, the BIOS compares current charge data against stale EEPROM values and flags the battery as degraded before a single cycle has run. This is a firmware read issue, not a fault with the replacement cell. Running the full discharge-to-hibernate and uninterrupted charge sequence forces the BIOS to rewrite the learn cycle data against the new cell's actual capacity.

Laptop shutting down at 20–30% charge shown on the P2451FA

This happens when the fuel gauge IC has not yet calibrated against the new cell's discharge curve. The OS reads a percentage based on voltage thresholds mapped to the old cell, so the reported figure and the actual remaining capacity diverge sharply under full CPU and display load. At that load level, the cell voltage drops faster than the gauge predicts, and the BMS cuts power to protect the cell before the OS can display a low-battery warning. Two to three full discharge and charge cycles recalibrate the fuel gauge IC to the new cell — after that, shutdowns at reported 20–30% stop.