About this replacement
Asus ZenBook Flip S UX370UA Series — 7.7V Li-Polymer Replacement Battery (C21N1624)
This 7.7V, 4950mAh (38.12Wh) lithium-polymer battery replaces the OEM C21N1624 cell in the Asus ZenBook Flip S UX370UA convertible laptop. It fits the UX370UA-C4061T, UX370UA-C4202T, UX370UA-C4219T, UX370UA-XB74T-BL, and over 30 additional UX370UA variants. The slim 5.16mm profile matches the original pack dimensions required by the ZenBook Flip S chassis.
- UX370UA platform fit: All listed UX370UA variants share the same 7.7V two-cell architecture, the same C21N1624 connector pinout, and the same BMS communication protocol — so one cell works across the full model range without modification.
- Bench tested on actual hardware: We ran this cell on a UX370UA board and confirmed the BMS handshake completed correctly. The charge controller accepted the new cell without fault codes, and the protection circuit responded to overcharge and short conditions within spec.
- Post-install calibration for UX370UA: After fitting this cell, run the laptop down to hibernate cutoff on battery alone — no USB-C power connected — then charge uninterrupted to 100%. This forces the ASUS 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.
BIOS reporting battery health as poor immediately after swap on UX370UA
The ZenBook Flip S stores charge history and health data in the battery pack's EEPROM, not in the motherboard firmware. When a new cell goes in, the BIOS reads blank or mismatched EEPROM data and flags the battery as degraded before it has ever been used. This is not a fault with the replacement cell — it is a calibration gap. Running one full discharge-to-hibernate cycle followed by an uninterrupted charge to 100% writes fresh cycle data to the EEPROM and clears the warning. After two or three cycles the health indicator will reflect the actual cell state.
ZenBook Flip S cutting off at 20–30% charge shown on screen
When the CPU and display are both under load simultaneously, the UX370UA draws peak current that forces the cell voltage to drop sharply — a voltage cliff that the BMS reads as a critically low state and triggers an immediate shutdown. This happens most often when the original cell has aged past 70% capacity, but a poorly calibrated fuel gauge IC on a fresh replacement can cause the same symptom. The fuel gauge IC maps the percentage readout against a stored discharge curve; if that curve doesn't match the new cell's chemistry, the percentage shown is meaningless. Run the calibration cycle described above — discharge fully to hibernate, charge to 100% without interruption — then check that the system reads a stable voltage above 7.0V at 20% before assuming the cell is at fault.