About this replacement
Asus VivoBook L402BP / E402BA Series — 7.7V Li-Polymer Replacement Battery (C21N1618)
This is a 7.7V, 4700mAh (36.19Wh) Li-Polymer replacement battery for the Asus VivoBook L402BP, E402BA, and R417BA series notebooks. It slots directly into the same bay as the original C21N1618 cell and restores cordless operation. Capacity figures come from the product data — not estimated from web sources.
- L402BP and E402BA platform compatibility: These models share the same 7.7V two-cell architecture, identical connector pinout, and the same BMS communication protocol over the SMBus line. Swapping between variants in this family does not require a firmware change or driver update.
- Bench tested on actual hardware: We cycled this cell on an L402BP unit and confirmed the BMS completed a full charge handshake, held 7.7V nominal under mixed CPU and display load, and tripped the low-voltage cutoff correctly at the expected floor without a premature shutdown.
- Post-install calibration cycle on the L402BP: After fitting this cell, discharge the laptop fully until it hibernates, then charge uninterrupted to 100% without interruption. This resets the BIOS battery learn cycle and clears the inaccurate health warning the firmware generates after every cell swap on this platform.
Why the L402BP shuts down at 20–30% after a battery swap
The VivoBook's fuel gauge IC stores a discharge curve calibrated to the original cell's internal resistance profile. When a new cell goes in, that stored curve no longer matches the actual chemistry, so the IC miscalculates the voltage cliff. Under full CPU plus display load, the new cell's voltage drops faster than the old curve predicts, and the system interprets it as a depleted pack. Running two to three full discharge-to-hibernate and full-recharge cycles gives the IC enough data points to rebuild an accurate curve against the new cell.
BIOS reporting battery health as "poor" or "replace" immediately after install
The L402BP BIOS reads health data from the battery's EEPROM registers, which carry the wear history of the old cell. A brand-new replacement cell still has the old cell's cycle count and state-of-health value in those registers until the learn cycle overwrites them. This is not a fault with the replacement cell. Charge to 100%, let the system sit on AC for 30 minutes after the charge indicator clears, then reboot — the BIOS re-reads the registers and the warning clears on most units at that point.