About this replacement
Asus VivoBook S14 S433FA Series — 11.55V Li-Polymer Replacement Battery (C31N1821)
This is a 11.55V, 4150mAh (47.93Wh) Li-Polymer battery for the Asus VivoBook S14 S433FA-EB491 and a wide range of S433 series variants. It replaces OEM part numbers C31N1821, C31POJH, and 0B200-03210100. The three-cell flat-pack form factor fits flush inside the S14 chassis without modification.
- S433 series fitment: The S433FA, S433EA, and S433IA all run the same 11.55V three-cell Li-Polymer rail with an identical connector and BMS handshake protocol. That shared architecture is why one cell covers all those variants — the BIOS battery interface communicates the same way across the lineup.
- Bench tested on actual hardware: We ran this cell on an S433FA unit and confirmed the BMS initialised without fault codes. Charge acceptance was clean from 0V recovery through to the BIOS charge cutoff threshold, and the EC reported cell voltage within normal tolerance on all three cells.
- First-cycle reset on the S433 platform: After fitting, let the laptop discharge fully until it hibernates on its own, then charge it in one uninterrupted session to 100%. This forces the BIOS battery learn cycle to recalibrate against the new cell and clears the inaccurate health warning the EC throws after any cell swap.
BIOS reporting poor battery health immediately after fitting a new cell
The S433's embedded controller stores learned charge data from the previous cell in EEPROM. When a new cell goes in, that old data still sits in memory and the EC flags a health mismatch — sometimes showing "poor" status or an incorrect Wh rating before a single charge cycle runs. This is not a fault with the replacement cell. Running one full discharge-to-hibernate followed by an uninterrupted charge to 100% triggers the BIOS learn cycle and overwrites the stale EEPROM values. After two to three full cycles, the health readout in Windows Battery Report will normalise.
VivoBook S14 shuts off abruptly at 20–30% shown on the fuel gauge
This symptom points to a voltage cliff under combined CPU and display load — the cell can't sustain the voltage rail when current draw peaks, even though the fuel gauge still shows charge remaining. The fuel gauge IC on the S433 platform reads state-of-charge against a stored discharge curve, and if that curve hasn't been calibrated against the new cell, it reports capacity that isn't actually deliverable under load. Run two full calibration cycles — discharge to hibernate cutoff, then charge uninterrupted to 100% — and the fuel gauge IC will map its curve to the actual cell. Shutdowns at a false 20–30% should stop once the IC has accurate data to work from.