About this replacement
Asus E402S / E502S Series — 7.6V Li-Polymer Replacement Battery (B21N1505)
This is a 7.6V, 4100mAh (31.16Wh) Li-Polymer replacement battery for the Asus VivoBook E402S, E402SA, E502S, E402WA, and over 260 compatible models. It replaces the original B21N1505 cell when the factory battery no longer holds a charge or fails to register in the system. The connector, BMS communication lines, and physical dimensions match the OEM spec.
- E402 and E502 platform fit: These models share the same two-cell Li-Polymer pack architecture, 7.6V nominal voltage rail, and B21N1505 connector pinout. The BMS handshake protocol is identical across E402S, E402SA, E502S, and E402WA variants, so one cell services the full range.
- Bench tested on actual hardware: We ran this battery through charge and load cycles on an E402SA. The BMS reported state-of-charge correctly, protection circuits tripped at expected voltage thresholds, and the pack reached full capacity without thermal event.
- BIOS learn cycle after install: After fitting this cell, run the laptop down to hibernate-cutoff on battery only, 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 on this platform.
Why the E402S shuts down at 20–30% after a battery swap
The E402S fuel gauge IC builds a charge map against the original cell over hundreds of cycles. When a new cell goes in, that map is stale and the reported percentage drifts sharply from actual cell voltage. Under combined CPU and display load, the laptop hits the low-voltage cutoff before the gauge reaches zero. The fix is two to three full discharge-to-hibernate and uninterrupted charge cycles. After that, the fuel gauge IC recalibrates against the new cell's actual voltage curve and shutdowns stop.
BIOS reporting battery health as "poor" or "replace now" after installing a new cell
The BIOS pulls health data from EEPROM values written by the old cell's embedded controller — not from measuring the new cell directly. A fresh replacement will often show "poor" or "0% health" on first boot simply because those stored values are out of range for an unfamiliar cell. Run the Windows Battery Report (powercfg /batteryreport) to confirm actual charge capacity versus design capacity. Once you complete the full discharge-recharge learn cycle, the BIOS recalculates health from live data and the warning clears.