About this replacement
Asus Transformer AiO P1801 Tablet PC — 7.4V Li-Polymer Replacement Battery (C21-P1801)
This 7.4V, 5000mAh (37Wh) Li-Polymer cell replaces the original C21-P1801 battery in the Asus Transformer AiO P1801 hybrid tablet-desktop. It fits the detachable touchscreen tablet unit, not the docking base. Confirmed compatible with P1801, AP180C, P18Y23, and P1801-B040K variants.
- P1801 tablet unit compatibility: The AiO P1801 splits into a Windows 8 all-in-one desktop and a detachable Android tablet. This battery powers only the tablet portion. All listed variants — P1801, AP180C, P18Y23, P1801-B040K — share the same tablet unit, the same C21-P1801 connector pinout, and the same BMS handshake voltage thresholds.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the P1801 tablet unit. The BMS accepted the cell without error flags, charge current ramped correctly through CC/CV phases, and the protection circuit triggered cleanly at low-voltage cutoff.
- Post-install calibration on the P1801: After fitting this cell, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. The P1801's fuel gauge IC calibrates against the new cell's actual capacity — skipping this step leaves the percentage readout unreliable for the first several use cycles.
Why the P1801 tablet shows "poor battery health" immediately after a cell swap
The P1801 stores historical charge data in EEPROM on the battery's protection circuit board. When a new cell goes in, the system reads that EEPROM as blank or mismatched and flags poor health — even though the cell is brand new. This is a data state, not a hardware fault. Running a full discharge-to-cutoff followed by an uninterrupted charge to 100% resets the battery learn cycle and clears the warning. After two or three complete cycles, the OS health indicator updates to reflect the actual cell condition.
P1801 tablet shutting down at 20–30% shown on the gauge
This happens when the cell voltage drops below the BMS cutoff threshold before the fuel gauge reaches zero. The gauge IC calculated remaining capacity based on old cycle data — it expected the cell to hold voltage longer than it actually did. The fix is a full calibration cycle: discharge to automatic hibernate, do not force-shutdown, then charge to 100% without interruption. After two full cycles the gauge IC re-maps the voltage curve against the new cell, and the shutdown-at-30% behaviour stops.