Asus VivoBook S14 S406UA Replacement Battery C21N1701 7.7V
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Asus VivoBook S14 S406UA Replacement Battery C21N1701 7.7V - 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 S14 S406UA Replacement Battery C21N1701 7.7V - is backordered and will ship as soon as it is back in stock.
Voltage
7.7V
Amp
5000mAh
Asus VivoBook S14 S406UA Series — 7.7V Li-Polymer Replacement Battery (C21N1701)
This is a 7.7V, 5000mAh (38.5Wh) Li-Polymer replacement battery for the Asus VivoBook S14 S406UA series. It fits models including the S406UA-BV021T, S406UA-BM231T, S406UA-GS8202T, and V406UA7100, along with 89 additional variants. OEM part numbers C21N1701, C21PQC5, and 0B200-02640000 all cross-reference to this cell.
- S406UA platform compatibility: These models share a unified 7.7V two-cell Li-Polymer architecture with the same connector pinout and BMS handshake protocol. A single replacement covers the full S406UA production run without modification.
- Bench tested on actual hardware: We ran this cell on an S406UA board and confirmed the BMS initialised correctly on first boot — charge acceptance started immediately, and the fuel gauge IC began tracking current draw without fault codes.
- Post-install calibration cycle: After fitting, run the laptop down to hibernate cutoff on a real workload — not idle — 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.
BIOS reporting battery health as poor immediately after replacement
The S406UA stores battery health data in EEPROM on the original cell. When a new cell goes in, the BIOS reads stale cycle-count and wear data that doesn't match the fresh chemistry. The system flags the battery as degraded before it has run a single cycle. One full discharge-to-hibernate followed by an uninterrupted charge to 100% rewrites that data and clears the warning in most cases.
Laptop shutting down at 20–30% shown on the gauge after swap
This happens when the fuel gauge IC hasn't calibrated against the new cell's actual voltage curve. The gauge predicts a safe cutoff, but the cell hits a voltage cliff under combined CPU and display load before the predicted threshold. The laptop cuts power to protect itself — the displayed percentage is simply wrong at that point. Run two full discharge-to-recharge cycles under normal use and the gauge will track accurately against the real cell floor.
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
Why does Windows show the new battery as 0% or "plugged in, not charging" right after I installed it?
The fuel gauge IC on the S406UA hasn't mapped its charge curve to the new cell yet. It reads the cell voltage as out of range or unknown and holds the charge indicator at 0% until it gets a reference point. Plug in the charger, leave it connected for a full uninterrupted charge to 100%, and the gauge IC will lock onto the new cell's voltage floor and ceiling. After one complete cycle the percentage display normalises.
System info shows the battery Wh rating as lower than what's printed on the cell — is the battery wrong?
No. The Wh figure Windows reports is pulled from the EEPROM data the firmware wrote during the learn cycle — not from the physical cell. A fresh cell with no completed cycles reports conservative rated capacity until the BIOS battery learn cycle runs and rewrites that value. Complete one full discharge-to-hibernate then a full charge; the reported Wh will update to match the actual 38.5Wh rating.
The S406UA charges the new battery but stops at 80% and won't go higher — what's blocking it?
This is a BIOS-controlled charge limit, not a fault with the cell. Asus firmware on some S406UA builds ships with a battery health charging mode enabled that caps charge at 80% to reduce cell stress during AC-tethered use. Go into MyASUS or the BIOS power settings, find "Battery Health Charging" or "Maximum Battery Life," and switch it to "Full Capacity Mode." The battery will then charge to 100%.
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