About this replacement
Asus X456 Series — 7.6V Li-Polymer Replacement Battery (C21N1508)
This 7.6V, 4800mAh Li-Polymer battery replaces the original C21N1508 cell in Asus X456 series laptops, including the X456UJ, X456UV, and X456UF. It fits the physical bay and connector without modification. Capacity is 36.48Wh — matching the original specification.
- X456 series fit: The X456UJ, X456UV, and X456UF share the same 7.6V two-cell Li-Polymer architecture, the same connector pinout, and the same BMS communication protocol — that is why one part number covers the full range.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on an X456UJ board. The BMS negotiated charge current correctly, the fuel gauge IC registered the new cell, and the protection circuit tripped at the expected low-voltage threshold.
- First cycle after installation: After fitting this battery, run the laptop on battery power until it hibernates on its own, 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.
Why the X456 shuts down at 20–30% charge shown after a battery swap
The X456 uses a fuel gauge IC that builds its capacity model from repeated charge and discharge cycles. When a new cell goes in, the IC still references the old cell's learned curve. Under full CPU and display load, actual cell voltage drops faster than the IC predicts. The system hits the hardware low-voltage cutoff while the gauge still shows 20–30% remaining. Two to three full discharge-to-hibernate, charge-to-100% cycles recalibrate the IC against the new cell and the early shutdowns stop.
BIOS reports battery health as "poor" or capacity as 0% immediately after fitting a new cell
The BIOS reads health data from EEPROM registers embedded in the battery's BMS — not from live voltage. When the old cell's EEPROM data differs from the new cell's reported parameters, the BIOS flags a mismatch and displays a poor health or unknown warning. This is not a fault with the replacement cell. Run one complete learn cycle — discharge to hibernate cutoff, then charge uninterrupted to 100% — and the BIOS will update its health register to reflect the new cell's actual state.