About this replacement
Asus VivoBook 15 X505B Series — 11.52V Li-Polymer Replacement Battery (B31N1631)
This is an 11.52V, 3500mAh (40.32Wh) lithium-polymer battery for the Asus VivoBook 15 X505B, X505Z, F505Z, X505BA, and over 200 related VivoBook 15 variants. It replaces part number B31N1631 and its cross-references (0B200-02510200E, 0B200-02510500, 0B200-02510000). Fit it when the original cell no longer holds charge or fails to power the laptop through basic daily tasks.
- VivoBook 15 X505 and F505 platform fit: These models share a common 3-cell Li-Polymer architecture running an 11.52V nominal rail. The connector pinout and BMS handshake protocol are identical across the X505B, X505Z, F505Z, and X505BA lines, which is why one part number covers all of them.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the X505 platform. The BMS communicated correctly with the EC, charge current tapered as expected at high state-of-charge, and no fault codes triggered during the test sequence.
- Post-install calibration on the X505 platform: After fitting this cell, run the laptop down to hibernate-cutoff under normal use, then charge uninterrupted to 100%. This forces the VivoBook's fuel gauge IC to recalibrate against the new cell's actual capacity and clears the inaccurate battery health warning that appears in BIOS after every cell swap.
BIOS reporting poor battery health immediately after fitting the new cell
The VivoBook 15 BIOS reads health data from EEPROM registers that were written by the old cell during its lifetime. When a new cell goes in, those registers still hold the degraded values from the previous pack. The EC flags this as a fault before the new cell has completed a single cycle. Run one full discharge-to-hibernate followed by an uninterrupted charge to 100% — this triggers the battery learn cycle and rewrites the relevant registers with data from the new cell.
Laptop shuts down at 20–30% charge shown on screen
This happens when the fuel gauge IC has not yet calibrated against the new cell's voltage curve. Under combined CPU and display load, the cell voltage drops faster than the uncalibrated gauge expects, and the EC triggers an emergency shutdown before the displayed percentage reaches zero. It is a software-side miscalibration, not a fault with the cell itself. Complete two full discharge-to-hibernate and full-charge cycles — after that the gauge tracks the real voltage curve and shutdowns at low-but-not-zero readings stop.