About this replacement
Asus VivoBook S451 Series — 7.5V Li-Polymer Replacement Battery (C21N1335)
This is a 7.5V, 5050mAh (37.88Wh) lithium-polymer battery for the Asus VivoBook S451 family of 14-inch ultrabook laptops. It fits the S451, S451LA, S451LB, S451LN, and over a dozen other S451 variants. It matches the OEM cell dimensions at 196.70 x 82.50 x 9.15mm for a direct internal fit.
- S451 family compatibility: Every S451 variant in this lineup runs the same 7.5V two-cell configuration with the same connector pinout and BMS handshake protocol. One battery covers the entire range without wiring adapters or firmware workarounds.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an S451LB unit. The BMS communicated correctly with the EC at first contact, charge current stepped down normally as the cell approached capacity, and the fuel gauge IC tracked state of charge without erratic jumps.
- First-install calibration on S451 units: After fitting this battery, 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 complete against the new cell and clears the inaccurate health warning that almost always appears after a cell swap on this platform.
BIOS reporting battery health as poor immediately after fitting the new cell
The S451 stores capacity and cycle data in EEPROM on the battery's protection circuit. When you swap cells, the BIOS reads stale data from the old cell's final state and flags the new battery as degraded before it has run a single cycle. This is not a fault with the replacement cell. Run one full discharge to hibernate cutoff, then charge uninterrupted to 100% — the BIOS learn cycle will update the EEPROM health data and the warning clears on the next boot.
VivoBook S451 shutting down abruptly at 20–30% charge shown
This happens when the cell voltage drops below the BMS cutoff threshold under combined CPU and display load before the fuel gauge IC reaches its low-battery warning point. The fuel gauge IC on the S451 calibrates its voltage-to-percentage curve over several full cycles. On a brand-new cell, this curve is not yet accurate and the laptop hits voltage cliff before the OS can issue a shutdown warning. Run three full charge and discharge cycles to allow the fuel gauge IC to build an accurate capacity map. After calibration, the reported percentage and actual cutoff voltage should align within a few percent.