About this replacement
Asus Eee PC S101 — 7.4V Li-Polymer Replacement Battery (AP22-U1001)
This is a 7.4V, 4900mAh (36.26Wh) Li-Polymer replacement battery for the Asus Eee PC S101 netbook. It uses OEM part number AP22-U1001 and also cross-references B2, YS1, and 890AAQ566970. The battery slots into the S101 and EPCS101-BPN003X without modification.
- Eee PC S101 and EPCS101-BPN003X compatibility: Both units run the same 7.4V battery rail, use the same connector pinout, and communicate with the same BMS handshake protocol — one cell fits both without adaptation.
- Bench tested on actual hardware: We ran this cell through charge, discharge, and BMS interrogation on the S101 platform. The protection circuit responded correctly to overvoltage and cutoff triggers, and BIOS recognised the battery without errors on a fresh boot.
- First-cycle calibration on the S101: After fitting this cell, run it down until the system hibernates, then charge it uninterrupted to 100%. This forces the S101's fuel gauge IC to relearn the new cell's capacity curve and clears the inaccurate health warning that appears after every cell swap.
BIOS reporting poor battery health immediately after fitting the new cell
The S101's BIOS reads health data from EEPROM stored on the old cell's protection circuit. When a new cell is installed, the EEPROM contains no learned data, so BIOS flags it as degraded or unknown — not because the cell is faulty. Running one full discharge-to-hibernate followed by an uninterrupted charge to 100% writes fresh calibration data. After one or two cycles, BIOS health status updates to reflect actual capacity. No firmware update or BIOS reset is required.
S101 shutting down without warning at 20–30% charge shown
This happens when the cell voltage drops faster than the fuel gauge IC predicts under combined CPU and display load. The gauge was calibrated to the old cell's voltage curve, so it shows 25% remaining while actual cell voltage has already hit the cutoff threshold. The system shuts down to protect the cell — it is not a firmware fault or a faulty replacement. Run two full discharge-to-hibernate cycles and the fuel gauge IC will remap the cutoff point against the new cell, typically stabilising around 3.5V per cell before shutdown.