Asus X421 Replacement Battery 11.55V 3450mAh B31N1911
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Asus X421 Replacement Battery 11.55V 3450mAh B31N1911 - 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.
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Disclaimer
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🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.
Asus X421 Replacement Battery 11.55V 3450mAh B31N1911 - is backordered and will ship as soon as it is back in stock.
Voltage
11.55V
Amp
3450mAh
Asus VivoBook 14 X421 Series — 11.55V Li-ion Replacement Battery (B31N1911)
This is an 11.55V Li-ion replacement battery rated at 3450mAh (39.85Wh) for the Asus VivoBook 14 X421 and related models including V4050FA, A413FF, and X413FF. It replaces OEM part numbers B31N1911, C31N1911, and B0B200-03580300. Swap it in when the original cell no longer holds enough charge for portable use away from the AC adapter.
- X421 and V4050FA platform fit: These models share the same 11.55V three-cell Li-ion architecture, connector pinout, and BMS handshake protocol. One replacement part covers the full group because the charge controller uses the same EEPROM communication spec across all variants in this lineup.
- Bench tested on actual hardware: We ran this cell through full charge-discharge cycles on an X421 unit. The BMS negotiated correctly with the Asus charge IC, current tapered as expected at the 4.35V per-cell threshold, and the fuel gauge reported accurate state-of-charge without error flags.
- Post-install calibration on X421: After fitting, run one full discharge down to the hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap — it is a firmware behaviour, not a fault with the replacement cell.
Why the X421 BIOS flags a new battery as poor health immediately after installation
The Asus BIOS stores learned capacity data from the previous cell in its battery EEPROM registers. When a new cell goes in, those stored values no longer match the actual chemistry, so the firmware flags a mismatch and reports poor or unknown health. This is not a fault with the replacement — it is the BIOS reacting to stale reference data. Running one full discharge-to-hibernate followed by a complete uninterrupted charge gives the learn cycle enough data to overwrite the old EEPROM values. After one or two cycles the health indicator returns to normal.
X421 shutting down at 20–30% battery shown on screen
This happens when the cell voltage drops sharply under combined CPU and display load — a voltage cliff that triggers the BMS protection cutoff before the OS fuel gauge reaches zero. The fuel gauge IC is reading stored state-of-charge data, but the actual cell voltage has already fallen below the safe discharge threshold. On a degraded original battery this is a cell problem; on a freshly installed replacement it means the fuel gauge has not yet calibrated against the new cell's discharge curve. Run two full discharge-to-hibernate cycles, charging to 100% each time, and confirm the cell resting voltage reads 12.4–12.6V at full charge before the next use.
Compatible Models
Replaces Part Numbers
Technical Specifications
Product Highlights
- Brand: Asus
- Manufacturer: CS
- Series: Standard
- Color: Black
- Product Type: Li-ion
- Battery Type: Li-ion
- Warranty: 12 Months
- Bulk Orders: sales@batteryweb.com
Frequently Asked Questions
Windows is showing the Asus X421 battery as 0% or "plugged in, not charging" right after I installed the new cell — what's wrong?
The fuel gauge IC is still reading the old cell's EEPROM data and hasn't mapped the new cell's charge curve yet. Plug in the AC adapter, let it charge uninterrupted to 100%, then discharge fully to hibernate cutoff — repeat this once more. After two complete cycles the fuel gauge re-maps to the new cell and the 0% or stuck reading clears. Confirm the battery icon shows a rising percentage within the first 10 minutes of charging; if it does, the BMS handshake is working correctly.
My X421 says the battery is only 32Wh in system info, but the replacement is rated 39.85Wh — is something wrong with the cell?
Nothing is wrong. The figure Windows or HWiNFO displays is pulled from the EEPROM data carried over from the old cell, not measured from the new one. The BIOS learn cycle hasn't updated those registers yet. Run one full discharge to hibernate, then charge to 100% without interruption — the reported Wh value will update to reflect the actual new cell chemistry after the learn cycle completes.
The X421 charge stops at 80% and won't go higher — is the replacement battery defective?
This is almost always the Asus Battery Health Charging feature, not a fault with the cell. Asus BIOS ships with a charge limit mode that caps charging at 80% to reduce long-term cell stress when the laptop sits plugged in all day. Open the MyAsus app or Asus Battery Health Charging utility, set the mode to "Full Capacity Mode," and the charge will proceed to 100%. The replacement cell itself has no 80% ceiling — the BIOS firmware is enforcing it.
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