About this replacement
Asus X556UA Series — 7.6V Li-Polymer Replacement Battery (C21N1509)
This is a 7.6V, 5000mAh (38Wh) Li-Polymer replacement battery for the Asus VivoBook X556 series, including the X556UA, X556UB, X556UF, and X556UJ. It slots into the same bay as the original C21N1509 cell and restores cordless operation when the factory battery can no longer hold a usable charge. Voltage and connector match the original spec exactly.
- X556 series compatibility: The X556UA, UB, UF, and UJ share a common battery bay, connector pinout, and BMS handshake — all confirmed against the C21N1509 spec. The same 7.6V two-cell Li-Polymer configuration runs across the entire sub-series, so one cell fits all of them.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on an X556 board and confirmed the BMS handshake completed without error codes. The BIOS picked up the cell, reported capacity, and charge current stepped down correctly at the top of the charge curve.
- First-cycle reset on the X556: After fitting, let the battery discharge fully until the laptop hibernates, then charge it uninterrupted to 100%. This forces the BIOS battery learn cycle to recalibrate against the new cell and clears the false "poor health" warning that appears after every cell swap on this platform.
Why the X556 BIOS flags a new battery as poor health
The X556 BIOS stores battery health data from the previous cell in EEPROM. When a new cell is installed, that stored data no longer matches the new chemistry profile — so the system flags poor health immediately, even on a brand-new battery. This is a calibration state, not a fault with the replacement cell. Running one full discharge-to-hibernate followed by a complete uninterrupted charge overwrites the EEPROM reference point and resolves the warning.
Laptop shuts down at 20–30% charge shown on screen
This happens when the fuel gauge IC has not yet calibrated to the new cell — it reports a percentage, but the underlying voltage curve it's reading from is still mapped to the old, degraded cell. Under full CPU and display load, the actual cell voltage drops faster than the gauge expects and hits the cutoff threshold early. The fix is two to three full discharge-and-charge cycles, which give the fuel gauge IC enough data to remap its curve to the new cell. After calibration, the gauge and actual remaining capacity align at a consistent voltage floor around 6.0V under load.