C31N2005 Asus ChromeBook CX9 Compatible Battery 11.55V 4200mAh

Asus ChromeBook CX9 / ExpertBook B3 Flip — 11.55V Li-Polymer Replacement Battery (C31N2005)

This is an 11.55V, 4200mAh (48.51Wh) Li-Polymer replacement battery for the Asus ChromeBook CX9 and ExpertBook B3 Flip series, including the B3402FBA and B3402FEA. It fits where the original C31N2005 cell sits and restores unplugged operation when the factory battery has degraded or failed. Voltage and capacity match the original spec exactly.

• ChromeBook CX9 and ExpertBook B3 Flip platform fit: Both the CX9 and B3402 series run the same 3-cell 11.55V battery rail with a shared connector pinout and BMS handshake protocol. That is why one cell covers both lines — the charge controller negotiates identically across the platform.
• Bench tested on actual hardware: We cycled this cell through charge, load discharge, and thermal checks. The BMS communicated correctly with the charge IC, current tapered as expected at top-of-charge, and no protection trips occurred under sustained display-plus-CPU load.
• Post-install calibration on the CX9: After fitting this cell, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that consistently appears after a cell swap on this platform.

Why the CX9 reports poor battery health immediately after a new cell install

The CX9 BIOS stores learned charge parameters from the previous cell in EEPROM. When a new cell goes in, those cached values no longer match actual cell behaviour — so the firmware flags a health warning before it has run a single calibration cycle. This is not a fault with the replacement cell. The health indicator reads against stored history, not live chemistry. One complete discharge-to-hibernate followed by an uninterrupted full charge gives the BIOS enough data to overwrite the stale EEPROM values and report correctly.

CX9 shutting down at 20–30% shown on the fuel gauge

This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge miscalculates remaining capacity, and when the real cell voltage drops under full CPU-plus-display load, the system hits undervoltage protection before the displayed percentage reaches zero. It looks like an early shutdown but is a calibration lag. Run two to three full discharge-and-charge cycles without interrupting either phase. After that, the gauge IC maps the new cell's discharge curve accurately and the cutoff will align with the percentage shown — typically stabilising at or below 5%.