Asus VivoBook S500 Replacement Battery C31-X502 11.1V 4000mAh

Asus VivoBook S500 Series — 11.1V Li-Polymer Replacement Battery (C31-X502)

This 11.1V, 4000mAh (44.4Wh) Li-Polymer battery replaces the original C31-X502 cell in the Asus VivoBook S500, S500C, S500CA, and related variants. It fits the slim 304.10 x 77.07 x 7.94mm bay and connects directly to the existing cable harness. When the factory cell degrades, this restores full untethered operation to the ultrabook.

• VivoBook S500 series fit: The S500, S500C, and S500CA share the same battery bay dimensions, voltage rail, and connector pinout. The BMS on all three accepts the C31-X502 cell ID, so no firmware conflict occurs on insertion.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the S500CA. The BMS accepted the pack immediately, charge current ramped normally, and the protection circuit tripped cleanly at the low-voltage cutoff threshold without triggering a hard shutdown.
• First-cycle calibration after swap: After installing, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This forces the fuel gauge IC to relearn the cell's actual capacity and clears the inaccurate health warning the BIOS displays after every cell replacement.

BIOS reporting poor battery health immediately after replacement

The VivoBook S500 BIOS reads health data from EEPROM stored by the previous cell. When a new pack is fitted, that stale EEPROM data still registers, so the BIOS flags the battery as worn or unknown before a single charge cycle runs. This is not a fault with the replacement cell. Run one complete discharge-to-hibernate followed by a full uninterrupted charge. After that cycle, the BIOS recalculates health against the new cell's actual capacity and the warning clears.

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 discharge curve. The software reads voltage and predicts remaining capacity using data mapped to the old, degraded cell — so it misses the voltage cliff where the new cell still has usable charge. Under combined CPU and display load, the real voltage drops faster than the stale map predicts, and the system cuts power before the gauge reaches zero. Run two full discharge-to-hibernate and charge-to-100% cycles to remap the curve, and the cutoff point should settle to below 5%.