About this replacement
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.