About this replacement
IBM ThinkPad 390 Series — 9.6V Ni-MH Replacement Battery (02K6536)
This is a 9.6V, 4000mAh Ni-MH replacement battery for the IBM ThinkPad 390 series, including the ThinkPad 1720, 390, 390E, and 390X. It replaces OEM part numbers 02K6536, ASM 02K6521, ASM 02K6534, ASM 02K6537, FRU 02K6521, and FRU 02K6537. Capacity is 38.4Wh at the rated voltage.
- ThinkPad 390 platform compatibility: The 1720, 390, 390E, and 390X share the same battery bay dimensions (154.45 x 87.95 x 30.00mm), connector pinout, and 9.6V power rail. The BIOS on these models handshakes over the same SMBus protocol, so one cell covers all four variants without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a ThinkPad 390E. The BMS held charge termination correctly at full voltage and did not trip on reconnection. Capacity read within normal tolerance of the 4000mAh rating.
- First-cycle calibration on ThinkPad 390 hardware: After fitting, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.
BIOS reporting battery health as poor after fitting a new cell on the ThinkPad 390
The ThinkPad 390 BIOS reads health data from EEPROM registers on the battery itself. When a new cell goes in, those registers hold factory defaults that do not match the laptop's learned charge history, so the BIOS flags health as poor or unknown. This is a data mismatch, not a fault with the cell. Run one full discharge-to-hibernate followed by an uninterrupted full charge. After one or two complete cycles, the BIOS recalibrates and the health warning clears.
ThinkPad 390 shutting down abruptly at 20–30% charge shown on screen
The fuel gauge IC in this platform estimates remaining charge based on voltage curves from the old, degraded cell. A new Ni-MH cell has a flatter discharge curve, so the gauge reads percentage incorrectly until it has learned the new cell's behaviour. Under combined CPU and display load, the actual voltage drops faster than the gauge expects, and the laptop shuts down before the displayed percentage reaches zero. This is a calibration gap, not a battery fault. Run two full discharge-and-charge cycles to let the fuel gauge IC map the new cell's curve accurately — after that, shutdowns at high displayed percentages stop.