About this replacement
Asus M5 / S5 Series — 11.1V Li-ion Replacement Battery (A31-S5 / A32-S5)
This is an 11.1V 2200mAh Li-ion replacement battery for the Asus M5, S5, M5000, and S5000 notebook series. It cross-references OEM part numbers A31-S5, A32-S5, A31-W5F, A32-W5F, and a range of 70-N and 90-N Asus codes. Capacity is 24.42Wh — matching the original cell specification.
- M5 / S5 series compatibility: These models share a common 11.1V three-cell architecture and the same physical connector pinout. The BMS handshake uses the same SMBus protocol across the M5, S5, M5000, and S5000, so one cell works across all variants without firmware differences.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on an M5-series unit. The BMS accepted the new cell cleanly, balancing all three cell groups without triggering an over-voltage fault. Charge current tracked normally from 0% to cutoff.
- Post-install calibration on the Asus M5: After fitting this battery, run a single 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 first boot after any cell swap.
BIOS reporting poor battery health immediately after fitting a new cell
The Asus M5 BIOS reads health data stored in the battery's EEPROM. When a new cell arrives, that EEPROM data does not match the charge history the BIOS expects, so it flags the battery as degraded or unknown. This is a calibration state, not a fault with the cell. Running one full discharge-to-hibernate then a complete uninterrupted charge gives the BIOS enough data to rewrite its estimate. After one or two cycles, the health reading normalises.
Laptop shutting down at 20–30% remaining after battery swap
This happens because the fuel gauge IC is still using a voltage-to-capacity curve calibrated to the old, degraded cell. The new cell has a steeper voltage cliff under combined CPU and display load, so the BIOS triggers a hard shutdown before the gauge reaches zero. It is not a defective cell. Discharge fully to hibernate-cutoff twice in a row — this forces the fuel gauge IC to recalibrate its curve against the actual chemistry of the new cell. After recalibration, the cutoff point drops back to the correct level near 3.0V per cell.