About this replacement
Asus VivoBook X540S Series — 11.25V Li-ion Replacement Battery (A31N1519)
This is an 11.25V, 2200mAh (24.75Wh) Li-ion replacement battery for the Asus VivoBook X540S and related X540 series laptops. It fits the X540LA, X540SA, X540LJ, and over 300 additional X540 variants that share the A31N1519 platform. Install it when the original cell no longer holds a charge or the laptop will not run unplugged.
- X540 platform compatibility: The X540LA, X540SA, X540LJ, and X540S all share the same 11.25V three-cell Li-ion architecture, connector pinout, and BMS handshake protocol — which is why a single OEM part number (A31N1519) covers the entire lineup.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an X540 unit, confirming the BMS handshake completes correctly, charge termination triggers at full capacity, and the protection circuit trips at the expected low-voltage cutoff.
- First-cycle calibration on X540 hardware: After fitting this cell, run the laptop on battery until it reaches hibernate cutoff — do not interrupt it. Then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell and clears the inaccurate health warning that appears after every cell swap on this platform.
BIOS reporting battery health as poor immediately after fitting the new cell
The X540 BIOS stores charge history and wear data from the previous cell in its own memory — it does not automatically reset when a new battery is installed. Until the learn cycle runs, the BIOS compares new-cell performance against the old cell's degraded baseline and flags health as poor or critical. This is not a fault with the replacement cell. Run one full discharge to hibernate cutoff followed by an uninterrupted charge to 100% and the health indicator will update correctly.
Laptop shutting down at 20–30% battery shown on the gauge
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge IC uses stored data from the old degraded cell to predict remaining capacity, so it reads the remaining charge incorrectly and triggers shutdown well before the cell is actually empty. It typically corrects itself after two to three full discharge-to-hibernate and charge-to-100% cycles. If shutdowns persist past three cycles, check that the BIOS is not enforcing a low-voltage cutoff above 10.5V — that threshold is adjustable on some X540 firmware builds.