About this replacement
IBM ThinkPad i1400 2621-XXX Series — 10.8V Li-ion Replacement Battery (02K6601)
This is a 10.8V, 6600mAh (71.28Wh) Li-ion replacement battery for the IBM ThinkPad i1400 2621-XXX and 2651-XXX series laptops, along with the ThinkPad i1420 and i1421. It replaces OEM part numbers 02K6601, 02K6602, 02K6603, 02K6632, 02K6633, 02K6648, and FRU 02K6647. Install it when the original cell no longer holds a charge or fails to register with the BIOS.
- ThinkPad i1400 and i1420 series compatibility: These models share the same 10.8V three-cell voltage rail, connector pinout, and BMS handshake protocol, which is why a single replacement cell covers all listed variants.
- Bench tested on actual hardware: We ran this cell through a full charge and discharge cycle on a ThinkPad i1400 unit. The BMS communicated correctly with the system, the BIOS recognised the pack without error codes, and charge current behaved as expected across all stages.
- Post-install discharge cycle on ThinkPad units: After installing, run one full discharge down to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that typically appears after every cell swap on this platform.
BIOS reporting poor battery health immediately after swap on ThinkPad i1400
The ThinkPad BIOS reads EEPROM data from the battery pack to assess health. When a new cell is installed, the EEPROM values do not yet match the machine's charge history, so the BIOS flags the pack as degraded. This is not a fault with the replacement cell. Running one complete discharge-to-hibernate followed by an uninterrupted full charge gives the fuel gauge IC the reference points it needs to reset the health status correctly.
Laptop shuts down at 20–30% battery remaining shown on screen
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge loses track of the real state of charge, and the cell voltage drops below the BMS cutoff threshold before the display percentage reaches zero. The fix is two to three full calibration cycles — discharge fully to hibernate, then charge to 100% without interruption each time. After the third cycle, the fuel gauge IC will track the new cell accurately and shutdowns at false percentages will stop.