About this replacement
Asus VivoBook S300 / S400 Series — 11.1V Li-Polymer Replacement Battery (C31-X402)
This is an 11.1V, 4000mAh (44.4Wh) Li-Polymer battery for the Asus VivoBook S300, S400, S400C, and S400CA. It replaces OEM part numbers C31-X402, 0B110-00210000, C21-X401, and C31X402. If your original cell is swelling, failing to charge, or no longer holding power, this is a direct cell swap.
- VivoBook S300 / S400 platform fit: These models share the same 11.1V three-cell Li-Polymer battery bay, connector pinout, and BMS communication protocol. The S400C and S400CA variants use the same pack form factor — 302 x 69.42 x 7.82mm — with the same fuel gauge IC handshake, so one SKU covers the full lineup.
- Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on a VivoBook S400 unit. The BMS accepted the pack without fault codes, the charge controller stepped through its CC/CV stages cleanly, and the fuel gauge IC registered the cell correctly within two full cycles.
- Post-install discharge cycle on VivoBook S300 / S400: After fitting this battery, run one full discharge down to hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears on the first boot after any cell swap.
BIOS reporting poor battery health immediately after fitting a new cell
The VivoBook BIOS stores battery health data in EEPROM on the old pack. When a new cell is fitted, the BIOS reads zero cycle history and flags the pack as degraded or unknown. This is not a fault with the replacement cell — it is the BIOS comparing fresh EEPROM data against its stored baseline. Run one complete discharge-to-hibernate followed by a full uninterrupted charge. After two to three cycles, the fuel gauge IC recalibrates and the BIOS health status updates to reflect the actual cell condition.
VivoBook S300 / S400 shutting down at 20–30% charge shown
This happens when the fuel gauge IC is still calibrated against the old, degraded cell's voltage curve. The display shows 25% remaining while the actual cell voltage has already dropped below the BMS cutoff threshold under combined CPU and display load. The laptop cuts power before the OS can trigger a graceful shutdown. Run two full discharge-to-hibernate cycles — the fuel gauge IC maps the new cell's voltage curve and the reported percentage aligns with the real state of charge, typically resolving the early cutoff after cycle two.