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Asus ZenBook UX21 Replacement Battery C23-UX21 7.4V

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From $109.99 USD $130.99
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Fits Asus UX21, UX21A, and UX21E ultrabooks; replaces OEM part C23-UX21.
7.4V, 4500mAh lithium-polymer cell delivers 33.3Wh capacity for full-day mobile computing.
Connector seats into the proprietary Asus battery slot with a single locking tab.
We ran full-discharge and recharge cycles; the BMS initialized cleanly with no thermal throttle.
After installation, discharge fully to hibernation cutoff then charge uninterrupted to 100% — this resets the BIOS battery learn cycle and clears inaccurate health warnings that appear after cell replacement.
Delivery time

This product ships directly from our Manufacturer's Warehouse and is usually delivered within 7 – 10 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Fresh Battery, Worth the Wait

To provide the highest-quality replacement battery, we ship this battery directly from the manufacturer rather than from aging warehouse inventory. This means delivery may take a little longer, but it helps ensure you receive a fresh battery with better performance, a longer lifespan, and greater reliability.

Estimated delivery: 7–10 business days
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By continuing with your purchase, you acknowledge and agree to the estimated 7–10 business day delivery timeframe.
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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

🔹 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: 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.

Voltage

7.4V

Amp

4500mAh

Asus ZenBook UX21 Series — 7.4V Li-Polymer Replacement Battery (C23-UX21)

This 7.4V, 4500mAh (33.3Wh) Li-Polymer battery replaces the original C23-UX21 cell in the Asus ZenBook UX21, UX21A, and UX21E ultrabooks. It fits the slim 5.80mm profile of the UX21 chassis without modification. Install it when the original cell no longer holds charge or fails to power the machine.

  • UX21 and UX21A/UX21E compatibility: These models share the same 7.4V two-cell Li-Polymer architecture, identical connector pinout, and the same BMS handshake protocol — so one cell fits all three variants without rewiring or adapter plates.
  • Bench tested on actual hardware: We cycled this cell on a UX21E, confirmed BMS communication with the EC, and verified charge acceptance from 0V recovery through to full 8.4V cutoff. Discharge curves matched rated capacity within 3%.
  • Post-install calibration on the UX21: After installing, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This triggers the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.

Why the UX21 BIOS flags a new battery as poor health

The UX21 EC reads health data stored in the old cell's EEPROM and compares it against the new cell's reported values. A fresh cell has no charge history logged, so the EC interprets the blank data as degraded capacity. This triggers a "battery health poor" flag in the BIOS even though the cell is new. One full discharge-to-hibernate followed by an uninterrupted charge to 100% writes fresh calibration data to the EC and clears the flag.

ZenBook UX21 shutting down at 20–30% charge shown

This happens when the fuel gauge IC still holds calibration data from the old degraded cell. The IC predicts remaining capacity based on past voltage curves, and when the new cell hits a slightly different voltage slope, the gauge loses accuracy and calls shutdown too early. It is not a fault with the replacement cell — it is a calibration mismatch. Run two full discharge-to-hibernate and recharge cycles; the fuel gauge IC re-maps its model against the new cell's voltage curve and the early shutdowns stop.

Compatible Models

UX21 ZenBook UX21 ZenBook UX21A ZenBook UX21E UX21A UX21E UX21E-DH52 UX21E-KX001V UX21E-KX013X UX21E-KX004X UX21E-KX014V UX21E-KX008V UX21E-KX020D UX21E-KX009V UX21e-sh52 UX21L Series UX21E-KX011V UX21E-KX002V UX21E-KX013v UX21E-KX007V UX21E-DH71 UX21E-KX016V UX21E-KX008X UX21E-KX004V UX21E-KX128 UX21E-KX010V Ultrabook UX21A Ultrabook UX21 Ultrabook UX21E

Replaces Part Numbers

C23-UX21

Technical Specifications

Voltage7.4V
Amp Hours4500mAh
Capacity4500mAh
Rate33.3Wh
Net Weight228.8g /8.07 oz
Gross Weight488.8g /17.24 oz
Approximate Weight488.8g /17.24 oz
Dimension 284.73 x 111.83 x 5.80mm

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 UX21 shows 0% or "unknown" battery right after I swapped the cell — is the new battery dead?

No. The Asus EC reads EEPROM data written by the previous cell, and a brand-new cell has no logged history. The EC reports 0% or unknown until it completes one full learn cycle. Discharge the laptop to hibernate cutoff, then charge uninterrupted to 100%, and the EC will re-register the battery correctly.

The UX21 BIOS is showing the wrong Wh rating — it says 35Wh or another value instead of 33.3Wh. What's wrong?

The Wh figure displayed in BIOS comes from the EEPROM on the old cell, not from the new one. When the EC first reads the replacement battery, it may pull a cached or default rated value until the learn cycle overwrites it. Run a full discharge to hibernate followed by a complete charge to 100% — the EC recalculates actual capacity from the new cell's voltage curve and the correct 33.3Wh figure populates.

After replacing the battery, the fuel gauge on my UX21 jumps around wildly — showing 60%, then 80%, then 45% within minutes. Why?

The fuel gauge IC calibrates its remaining-capacity model against historical discharge data from the previous cell. With a new cell installed, those old voltage-curve mappings no longer match reality, so the gauge output is erratic. This is normal for the first two to three cycles. Run two full discharges to hibernate cutoff with uninterrupted recharges to 100% and the gauge IC will settle against the new cell's actual voltage profile.

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