About this replacement
Asus VivoBook 14 X421 Series — 11.55V Li-ion Replacement Battery (B31N1911)
This is an 11.55V Li-ion replacement battery rated at 3450mAh (39.85Wh) for the Asus VivoBook 14 X421 and related models including V4050FA, A413FF, and X413FF. It replaces OEM part numbers B31N1911, C31N1911, and B0B200-03580300. Swap it in when the original cell no longer holds enough charge for portable use away from the AC adapter.
- X421 and V4050FA platform fit: These models share the same 11.55V three-cell Li-ion architecture, connector pinout, and BMS handshake protocol. One replacement part covers the full group because the charge controller uses the same EEPROM communication spec across all variants in this lineup.
- Bench tested on actual hardware: We ran this cell through full charge-discharge cycles on an X421 unit. The BMS negotiated correctly with the Asus charge IC, current tapered as expected at the 4.35V per-cell threshold, and the fuel gauge reported accurate state-of-charge without error flags.
- Post-install calibration on X421: After fitting, run one full discharge down to the 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 — it is a firmware behaviour, not a fault with the replacement cell.
Why the X421 BIOS flags a new battery as poor health immediately after installation
The Asus BIOS stores learned capacity data from the previous cell in its battery EEPROM registers. When a new cell goes in, those stored values no longer match the actual chemistry, so the firmware flags a mismatch and reports poor or unknown health. This is not a fault with the replacement — it is the BIOS reacting to stale reference data. Running one full discharge-to-hibernate followed by a complete uninterrupted charge gives the learn cycle enough data to overwrite the old EEPROM values. After one or two cycles the health indicator returns to normal.
X421 shutting down at 20–30% battery shown on screen
This happens when the cell voltage drops sharply under combined CPU and display load — a voltage cliff that triggers the BMS protection cutoff before the OS fuel gauge reaches zero. The fuel gauge IC is reading stored state-of-charge data, but the actual cell voltage has already fallen below the safe discharge threshold. On a degraded original battery this is a cell problem; on a freshly installed replacement it means the fuel gauge has not yet calibrated against the new cell's discharge curve. Run two full discharge-to-hibernate cycles, charging to 100% each time, and confirm the cell resting voltage reads 12.4–12.6V at full charge before the next use.