About this replacement
Asus UX333FN / UX333FA Series — 11.55V Li-Polymer Replacement Battery (C31N1815)
This is an 11.55V, 4250mAh (49.09Wh) lithium-polymer replacement battery for the Asus VivoBook 13 UX333 series ultrabook. It fits the UX333FN-A4115T, UX333FA-A3064T, UX333FA-A4136T, UX333FN-A4131T, and 82 additional UX333 variants. OEM part numbers C31N1815 and 0B200-03150000 both match this cell.
- UX333 platform compatibility: Every UX333FA and UX333FN variant runs the same 11.55V three-cell Li-Polymer architecture. They share an identical connector pinout, BMS handshake protocol, and EEPROM communication line — so one replacement cell covers the full model range without modification.
- Bench tested on actual hardware: We installed this cell on a UX333FN unit and cycled it through full charge-to-discharge sequences. The BMS authenticated correctly on first insertion, charge current ramped normally through CC/CV phases, and no cutoff faults were logged during discharge under combined CPU and display load.
- First-cycle calibration after install: After fitting the new cell, run the laptop down to automatic hibernate cutoff — do not force-shutdown — then charge uninterrupted to 100% without use. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after a cell swap.
BIOS reporting battery health as poor immediately after replacement
The UX333 BIOS reads health data stored in the battery's EEPROM — not the live cell voltage. When you swap the cell, the EEPROM on the new pack reports factory baseline data that doesn't match the charge history the BIOS tracked on the old pack. This mismatch triggers a "poor health" or "replace battery" flag even though the cell is new. Running one full learn cycle — full discharge to hibernate, then uninterrupted charge to 100% — writes new baseline data and clears the flag. If the warning persists past two full cycles, check BIOS version; some UX333 firmware revisions require a manual reset under Power Settings.
Laptop shuts down at 20–30% charge shown on screen
This happens when the fuel gauge IC still holds calibration data from the degraded original cell. The percentage reading becomes detached from actual cell voltage, so the system shows 25% while the cell has already dropped below the 9.8V minimum threshold under CPU and display load. The laptop shuts down to protect the cell, not because the replacement is faulty. Fix it by completing two full discharge-to-hibernate cycles without interruption — each cycle forces the fuel gauge IC to recalibrate its empty-point reference against the new cell chemistry. After the second cycle, the displayed percentage and actual voltage align within the normal ±3% tolerance.