About this replacement
Asus UX31 / UX31A Series — 7.4V Li-Polymer Replacement Battery (C22-UX31)
This is a 7.4V, 6800mAh (50.32Wh) Li-Polymer battery for the Asus UX31 and UX31A ultrabook. It replaces the OEM C22-UX31 cell when the original pack no longer holds a usable charge. Fits the UX31, UX31A, and UX31A-R4004H among 53 confirmed models in the series.
- UX31 and UX31A compatibility: Both chassis use the same 7.4V two-cell Li-Polymer pack with an identical connector pinout and BMS handshake. The BIOS on both models reads cell state through the same SMBus protocol, so the same replacement cell satisfies both platforms without modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a UX31A unit. The BMS accepted the new cell without error flags, reported correct voltage at each charge stage, and hit cutoff cleanly at the low-voltage threshold without tripping any unexpected protection events.
- Post-install recalibration on the UX31: After fitting, run one full discharge down to the hibernate cutoff, then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell and clears the inaccurate health warning that appears after every cell swap on this platform.
BIOS reporting battery health as poor immediately after replacement
The UX31 BIOS stores battery health data from the previous cell in its EEPROM. When a new cell goes in, the BIOS compares the fresh cell against degraded historical data and flags the health as poor — even though the new cell is fine. This is not a fault with the replacement battery. One full discharge-to-hibernate followed by a full uninterrupted charge resets the learn cycle. After two to three full cycles, the BIOS recalibrates and the health warning clears.
UX31 shutting down at 20–30% charge remaining after battery swap
This happens because the fuel gauge IC is still calibrated against the old, degraded cell. It miscalculates the remaining capacity and sends a shutdown signal before the new cell is actually depleted. The cell voltage is hitting what the IC incorrectly reads as the low-voltage cliff. Run two to three full discharge-to-hibernate and full-recharge cycles to let the gauge IC map itself to the new cell's actual voltage curve. After calibration, the shutdowns stop and the percentage readout stabilises — confirm the fix by checking that resting voltage at reported 20% reads above 7.0V.