About this replacement
Asus VivoBook A412FA / A412UA Series — 7.6V Li-Polymer Replacement Battery (C21N1818)
This is a 7.6V 4100mAh (31.16Wh) Li-Polymer battery for the Asus VivoBook 14 A412FA, A412UA, A412UB, and related 14-inch notebooks. OEM part numbers covered include C21N1818, C21N1818-1, 0B200-03190600, and 0B200-03280500 among others. It fits the two-cell bay used across the A412 platform.
- A412 and A509 platform fit: These models share the same two-cell 7.6V Li-Polymer configuration, connector pinout, and BMS handshake protocol. The same battery communicates correctly with the EC firmware across all A412 and A509 variants listed under this SKU.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an A412FA chassis. The BMS handshook within the first charge cycle, the EC recognised the cell, and charge termination triggered correctly at 8.6V full state. No fault codes were thrown.
- First-cycle learn reset on VivoBook firmware: After installing, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell. Skip this step and the health indicator will likely show a warning or inaccurate state-of-charge for weeks.
BIOS reporting poor battery health immediately after fitting a new cell
The A412 EC reads health data from the old cell's EEPROM during every boot. When a new cell goes in, the EC has no charge history to reference and flags the battery as degraded or unknown. This is a firmware behaviour, not a fault with the replacement cell. Running one full discharge-to-hibernate then a full uninterrupted charge gives the EC enough data to recalibrate. After two to three cycles, the health report typically corrects itself.
Laptop shuts down at 20–30% shown on the fuel gauge after swap
This happens because the fuel gauge IC is still calibrated to the old cell's voltage curve. A new Li-Polymer cell holds voltage differently under combined CPU and display load, and the gauge miscalculates the remaining capacity. The cell hits the low-voltage cliff before the OS expects it, triggering an immediate shutdown. Run two full discharge-to-hibernate cycles followed by full charges — the fuel gauge IC re-maps its curve against the new cell and the readings stabilise around 3.6V per cell at the low end.