Asus M5 11.1V Replacement Battery A31-S5 4400mAh

Asus M5 / S5 Series — 11.1V Li-ion Replacement Battery (A31-S5)

This is an 11.1V, 4400mAh (48.84Wh) Li-ion replacement battery for the Asus M5, S5, M5000, and S5000 series notebooks. It slots into the original battery bay and reconnects to the same three-cell power circuit the laptop expects. Capacity figure comes from product data — 4400mAh at the rated voltage rail.

• M5 / S5 platform compatibility: These models share the same three-cell 11.1V architecture and use identical OEM part numbers across the A31-S5 and A32-S5 lines. The connector pinout, cell count, and BMS handshake are consistent across M5, S5, M5000, and S5000 variants, so one cell fits all four.
• Bench tested on actual hardware: We ran the cell through charge and load cycles on an M5-series board. The BMS negotiated correctly, charge termination triggered at full capacity, and the protection circuit tripped cleanly at the low-voltage cutoff threshold without locking the pack.
• Post-install calibration on the Asus M5: After fitting this cell, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. The BIOS battery learn cycle needs this pass to map the new cell's actual capacity curve — skipping it leaves the fuel gauge reading off for weeks.

BIOS reporting poor battery health immediately after swapping the A31-S5

The Asus BIOS reads health data from the EEPROM embedded in the old cell during every boot. When you swap to a new cell, the EEPROM counter resets and the BIOS flags it as degraded or unknown — not because the cell is faulty, but because the learned data no longer exists. Running a full discharge-to-hibernate followed by an uninterrupted charge to 100% forces the system to rewrite the learn-cycle data against the new cell. After two or three full cycles, the BIOS health indicator clears. No firmware update or BIOS reset is needed.

Laptop cuts off at 20–30% charge remaining under CPU and display load

This is a voltage cliff, not a capacity fault. Under simultaneous CPU and backlit display load, current draw spikes and cell voltage drops faster than the fuel gauge IC recalculates. If the gauge hasn't been calibrated to the new cell, it over-reports remaining charge right up to the voltage cutoff threshold. The fix is to complete the calibration cycle — a full discharge to hibernate-cutoff, then a full uninterrupted charge. After two cycles the fuel gauge IC has enough data to track the voltage slope accurately, and the premature shutdowns stop. Target a resting cell voltage of 12.3–12.5V at full charge as confirmation.