Asus VivoBook E402S Replacement Battery B21N1505 7.6V 4100mAh
Available by SPECIAL ORDER. Delivery for this product typically takes 2 weeks.
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Asus VivoBook E402S Replacement Battery B21N1505 7.6V 4100mAh - is backordered and will ship as soon as it is back in stock.
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Battery Care Tips
Battery Care Tips
🔹 Getting Started
Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.
🔹 Keep It Healthy
Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.
Delivery and Shipping
Delivery and Shipping
🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.
Disclaimer
Disclaimer
⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.
🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.
Asus VivoBook E402S Replacement Battery B21N1505 7.6V 4100mAh - is backordered and will ship as soon as it is back in stock.
Voltage
7.6V
Amp
4100mAh
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.
Compatible Models
Replaces Part Numbers
Technical Specifications
Product Highlights
- Brand: Asus
- Manufacturer: CS
- Series: Standard
- Color: Black
- Product Type: Li-Polymer
- Battery Type: Li-Polymer
- Warranty: 12 Months
- Bulk Orders: sales@batteryweb.com
Frequently Asked Questions
My E402S shows the wrong Wh rating in system info after fitting the new battery — is the cell faulty?
The Wh figure shown in Windows or HWiNFO pulls from the EEPROM embedded in the battery controller, not from a live measurement of the new cell. The rated chemistry value stored there can differ from the actual cell's nominal capacity by a small margin until the learn cycle runs. Do one full discharge to hibernate-cutoff, then charge uninterrupted to 100%, and check the reported figure again — it will align to the correct 31.16Wh once the fuel gauge IC has a complete data set.
The E402S fuel gauge jumps around wildly for the first few charges — is the battery defective?
This is fuel gauge IC calibration behaviour, not a cell defect. The IC on the E402S motherboard builds its charge model from accumulated cycle data tied to the old cell. With a new cell installed, it has no baseline and interpolates badly for the first two to three cycles. Run two full discharge-to-hibernate cycles with uninterrupted charges to 100% each time. After that the gauge IC locks onto the new cell's voltage curve and the percentage readout stabilises.
The new battery charges to 100% once, but on the next cycle it stops at 80% and won't go higher — what causes that?
The E402S BIOS includes a charge-limit feature that activates if it detects a pattern it flags as a stressed cell — sometimes triggered incorrectly after a swap because the EEPROM health data from the old cell carries over. Enter the BIOS (F2 at boot), navigate to Advanced > Battery Health Charging, and confirm the mode is set to Full Capacity rather than Balanced or Maximum Lifespan. If the setting is already on Full Capacity, run the battery down to hibernate-cutoff and back to 100% once — this resets the charge-limit flag and the next cycle should reach 100%.
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