Thunderobot MixBook Air L140BAT-4 Replacement Battery 7.7V

Thunderobot MixBook Air — 7.7V Li-Polymer Replacement Battery (L140BAT-4)

This is a 7.7V, 9300mAh (71.61Wh) Li-Polymer replacement battery for the Thunderobot MixBook Air laptop. It fits directly using the OEM part number L140BAT-4 and restores portable power to the notebook. Install it when the original cell no longer holds a usable charge or the system reports degraded battery health.

• MixBook Air fitment: The MixBook Air uses a slim 5.40mm Li-Polymer cell with a specific connector orientation and BMS handshake tied to the L140BAT-4 part number. Any cell outside that spec will either fail to communicate with the BIOS or trigger an immediate health warning on first boot.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the bench, monitoring BMS communication, cutoff voltage behaviour, and charge acceptance across the full capacity range. The cell completed the cycle without thermal event or premature cutoff.
• Post-install calibration on the MixBook Air: After fitting this battery, run the laptop down to hibernate-cutoff under normal load, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.

Why the MixBook Air BIOS flags the replacement cell as poor health on first boot

The MixBook Air stores battery health data in EEPROM on the original cell. When a new cell goes in, that EEPROM data no longer matches, and the BIOS reads the mismatch as a fault. The laptop did not fail — the health metric is simply calibrated against the old cell's stored data. One full discharge-to-hibernate followed by an uninterrupted charge cycle rewrites the learn data against the new cell and clears the warning.

MixBook Air shutting down at 20–30% charge shown on screen

This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge loses accuracy at the low end and the system hits the hardware voltage floor before the displayed percentage reaches zero. It is not a fault with the replacement cell. Run two or three full discharge-to-hibernate and full-charge cycles — by the third cycle the fuel gauge IC recalibrates and shutdowns stop occurring above 5%.