About this replacement
Asus UX431 / VivoBook S14 Series — 7.7V Li-Polymer Replacement Battery (C21N1833)
This is a 7.7V, 6000mAh (46.2Wh) Li-Polymer replacement battery for the Asus UX431 and VivoBook S14 ultrabook range. It fits the UX431FA, UX431FA-AM022T, S431FL-AM041T, and over 139 additional variants sharing the same OEM part number C21N1833 or 0B200-03340000. The slim 5mm profile matches the original cell envelope for the UX431's thin chassis.
- UX431 and VivoBook S14 platform fit: These models share a common power rail, connector pinout, and BMS handshake protocol tied to the C21N1833 spec. The replacement cell carries the same communication data the EC chip expects, so the system recognises it without rejecting it at the firmware level.
- Bench tested on actual hardware: We ran this cell on a UX431FA and monitored BMS communication through the charge and discharge cycle. The fuel gauge IC picked up the new cell without error codes, and the charge controller stepped through CC/CV stages correctly to full capacity.
- Post-swap calibration on the UX431: After fitting, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after swapping cells in this platform.
BIOS reporting battery health as poor immediately after fitting the new cell
The UX431 BIOS reads health status from EEPROM data stored on the battery's fuel gauge IC. A new cell ships with factory-default EEPROM values that don't match the system's learned baseline from the old battery. The EC chip flags this as degraded health — it's a data mismatch, not a cell fault. Running a single full discharge-to-hibernate followed by an uninterrupted charge to 100% forces the learn cycle to rewrite those values against the new cell.
UX431 shutting down hard at 20–30% charge shown on screen
This happens when the cell voltage drops below the BMS cutoff threshold under combined CPU and display load before the OS fuel gauge reaches 0%. The fuel gauge IC is reading state-of-charge from a calibration curve built against the old cell — it hasn't recalculated the voltage cliff for the new one yet. The result is an abrupt shutdown while Windows still shows charge remaining. After two full discharge-and-charge calibration cycles, the gauge IC recalibrates the curve and the cutoff aligns with the displayed percentage.