IBM ThinkPad 755CB Replacement Battery 8.4V 4000mAh 29H9033

IBM ThinkPad 755CB Series — 8.4V Li-ion Replacement Battery (29H9033)

This 8.4V, 4000mAh (33.6Wh) Li-ion battery replaces the original cell in the IBM ThinkPad 755CB and related 755-series notebooks. It fits the 755CB, 755CD, 755CDV, and 755CE, along with eighteen additional ThinkPad 755 variants. OEM part numbers covered include 29H9033, 29H9233, 41H9720, 84G6440, 85G1510, and 85G1513.

• ThinkPad 755 series compatibility: All 755-series models in this fit list share the same 8.4V two-cell Li-ion voltage rail, physical form factor, and connector pinout. The BMS communicates over the same SMBus protocol across these variants, so one cell works across the full range without modification.
• Bench tested on actual hardware: We ran this cell through charge, discharge, and recharge cycles on a 755-series unit. The BMS completed handshake without error codes, accepted a full charge to 8.4V, and held voltage through discharge without premature cutoff.
• First-cycle calibration on the 755 series: After fitting this battery, run the laptop down to hibernate cutoff under normal load — do not force-power-off. Then charge uninterrupted to 100%. This single discharge-charge cycle resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after any cell swap on ThinkPad 755 hardware.

BIOS reporting battery health as poor immediately after swapping the 755 cell

The ThinkPad 755 BIOS reads health data stored in the battery's EEPROM. When a new cell arrives, that EEPROM contains factory default values that do not match the laptop's learned discharge profile, so the BIOS flags the battery as degraded. This is a data mismatch — not a fault with the cell. Run one full discharge-to-hibernate cycle followed by an uninterrupted charge to 8.4V. After that cycle, the BIOS rewrites its learned data against the new cell and the health warning clears.

Fuel gauge jumping erratically or reading 0% with a fully charged new cell

The fuel gauge IC on ThinkPad 755 hardware estimates state-of-charge by tracking voltage curves learned from the previous cell. A replacement cell has different internal resistance and a fresh electrochemical baseline, so the IC's stored curves no longer map accurately — the result is wild percentage swings or a flat 0% reading despite a full charge. This is not a fault; the IC simply needs calibration cycles against the new cell's actual behaviour. Run two to three full discharge-and-charge cycles under normal CPU and display load. By the third cycle the gauge IC recalibrates and percentage readings stabilise.