About this replacement
IBM ThinkPad 370 / 750 Series — 9.6V Ni-MH Replacement Battery (66G2821)
This is a 9.6V, 4000mAh Ni-MH replacement battery for the IBM ThinkPad 370, 370C, 750, and 750C notebook computers. It matches the original cell format and connector used across this ThinkPad generation. OEM part numbers covered include 66G2821, 66G2846, 66G5063, ASM 66G2817, FRU 66G2820, and FRU 66G5063.
- ThinkPad 370 and 750 platform fit: IBM used a shared battery architecture across the 370 and 750 lines — same 9.6V rail, same physical housing, same connector pinout. One cell covers both platforms without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on ThinkPad-class hardware and confirmed the BMS accepted the new cell without fault flags. Charge current stabilised correctly and cutoff triggered at the expected voltage ceiling.
- Ni-MH conditioning after installation: After fitting, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. Ni-MH cells are sensitive to shallow cycling — this first full cycle resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on these ThinkPads.
Why the ThinkPad 370 BIOS reports poor battery health after a fresh cell swap
The ThinkPad 370 stores battery health data in EEPROM on the system board, not on the battery itself. When you swap the cell, that stored data still reflects the old, degraded pack. The BIOS compares current charge readings against its saved baseline and flags a mismatch as poor health. Running one complete discharge-to-hibernate followed by a full uninterrupted charge gives the BIOS enough data to rewrite that baseline against the new cell's actual capacity.
Fuel gauge reading wildly off for the first few charges
The fuel gauge IC on these ThinkPads calibrates against charge and discharge curves it has learned over time. A new Ni-MH cell has different internal resistance and voltage characteristics than the worn cell it replaces, so the IC's model is temporarily wrong. This shows up as jumpy percentage readings or a gauge that drops from 60% to 5% with no warning. Run two to three full discharge-and-charge cycles without interruption and the fuel gauge IC will recalibrate — readings typically stabilise by the third cycle, with voltage settling above 9.0V at mid-charge.