Asus VivoBook X756UJ Replacement Battery 7.6V 5000mAh C21N1515
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Asus VivoBook X756UJ Replacement Battery 7.6V 5000mAh C21N1515 - 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 X756UJ Replacement Battery 7.6V 5000mAh C21N1515 - is backordered and will ship as soon as it is back in stock.
Voltage
7.6V
Amp
5000mAh
Asus X756UJ Series — 7.6V Li-Polymer Replacement Battery (C21N1515)
This 7.6V 5000mAh (38Wh) Li-Polymer battery replaces the original C21N1515 cell in the Asus VivoBook X756 series, including the X756UJ, X756UA, X756UX, and X756UB. It fits the standard battery bay without modification and connects via the OEM multi-pin connector. Capacity is sourced from the product specification, not extrapolated from teardowns.
- X756 series compatibility: The X756 lineup shares a common chassis width and the same 7.6V two-cell Li-Polymer architecture across UJ, UA, UX, and UB variants. All use the C21N1515 part number and the same BMS communication protocol over the SMBus pins in the connector. One battery covers the full range.
- Bench tested on actual hardware: We ran this cell on an X756UA and monitored BMS handshake through POST. The BIOS detected the pack on first boot, SMBus communication initialised without error, and charge current ramp-up followed the expected CC/CV profile through to full capacity.
- Post-install calibration on the X756: After installing, run one full discharge to hibernate cutoff, then charge uninterrupted to 100% without unplugging. The X756 BIOS uses this cycle to re-learn the cell's capacity endpoints — skipping it leaves the fuel gauge reading up to 30% off and may trigger a false health warning in MyASUS or Windows Battery Report.
BIOS reporting poor battery health immediately after fitting the C21N1515 replacement
The X756 stores battery health data in EEPROM on the original pack. When a new cell is installed, the BIOS reads residual or zeroed EEPROM values and flags the battery as degraded before a single charge cycle completes. This is not a fault with the replacement — it reflects stale data, not measured capacity. Running one full discharge-to-hibernate followed by an uninterrupted full charge clears the learn cycle and resets the health flag. After two calibration cycles, Windows Battery Report should show rated capacity within 5% of 38Wh.
X756 shutting down abruptly at 20–30% charge remaining
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge maps the old cell's discharge profile, so it misjudges where the voltage cliff sits on the replacement chemistry. Under combined CPU and display load, actual cell voltage drops below the BMS cutoff threshold while the OS still shows charge remaining. The fix is a deliberate full-load discharge — avoid putting the laptop to sleep near 30% until two full cycles are complete. After calibration, the cutoff should track consistently to 3.0V per cell.
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
MyASUS shows the new C21N1515 battery as "poor health" straight out of the box — is the cell faulty?
No — the X756 BIOS reads health data stored in the EEPROM of the old pack, and a fresh cell has no matching history. The system flags it as degraded because it has no learned data to reference, not because the cell measured poorly. Run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. After that cycle, the BIOS re-evaluates and the health flag clears.
Windows Battery Report is showing the X756 battery's full charge capacity as far lower than 38Wh — what's wrong?
The fuel gauge IC reports capacity based on what it has measured against the installed cell, and it hasn't run a full learning cycle against the new chemistry yet. The EEPROM-derived Wh figure shown in Battery Report reflects the old cell's rated data, not a live measurement of the replacement. Two full discharge-and-charge cycles let the gauge IC recalculate against the actual cell. After that, reported full charge capacity should align closely with the 38Wh spec.
The X756 won't charge past 80% after fitting the replacement battery — is the new cell defective?
Not necessarily — Asus laptops include a BIOS-level charge limit setting that caps charging at 80% to reduce cell wear during long periods on AC power. Check MyASUS or the ASUS Battery Health Charging utility under Windows and confirm the setting is on "Full Capacity Mode" rather than "Balanced" or "Maximum Lifespan." Switch it to Full Capacity Mode, unplug, and reconnect the charger. Charging should then proceed to 100%.
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