About this replacement
Asus VivoBook S14 S406UA Series — 7.7V Li-Polymer Replacement Battery (C21N1701)
This is a 7.7V, 5000mAh (38.5Wh) Li-Polymer replacement battery for the Asus VivoBook S14 S406UA series. It fits models including the S406UA-BV021T, S406UA-BM231T, S406UA-GS8202T, and V406UA7100, along with 89 additional variants. OEM part numbers C21N1701, C21PQC5, and 0B200-02640000 all cross-reference to this cell.
- S406UA platform compatibility: These models share a unified 7.7V two-cell Li-Polymer architecture with the same connector pinout and BMS handshake protocol. A single replacement covers the full S406UA production run without modification.
- Bench tested on actual hardware: We ran this cell on an S406UA board and confirmed the BMS initialised correctly on first boot — charge acceptance started immediately, and the fuel gauge IC began tracking current draw without fault codes.
- Post-install calibration cycle: After fitting, run the laptop down to hibernate cutoff on a real workload — not idle — then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell and clears the inaccurate health warning that appears after every cell swap.
BIOS reporting battery health as poor immediately after replacement
The S406UA stores battery health data in EEPROM on the original cell. When a new cell goes in, the BIOS reads stale cycle-count and wear data that doesn't match the fresh chemistry. The system flags the battery as degraded before it has run a single cycle. One full discharge-to-hibernate followed by an uninterrupted charge to 100% rewrites that data and clears the warning in most cases.
Laptop shutting down at 20–30% shown on the gauge after swap
This happens when the fuel gauge IC hasn't calibrated against the new cell's actual voltage curve. The gauge predicts a safe cutoff, but the cell hits a voltage cliff under combined CPU and display load before the predicted threshold. The laptop cuts power to protect itself — the displayed percentage is simply wrong at that point. Run two full discharge-to-recharge cycles under normal use and the gauge will track accurately against the real cell floor.