About this replacement
Lenovo ThinkPad E40 / E50 Series — 11.1V Li-ion Replacement Battery (42T4235)
This is an 11.1V, 6600mAh (73.26Wh) Li-ion battery for the Lenovo ThinkPad E40, E50, and ThinkPad Edge 14" family. It cross-references over 80 OEM part numbers including 42T4731, 42T4757, 45N1000, and 0A36303. The connector, BMS handshake protocol, and cell count all match the original Lenovo specification for these platforms.
- ThinkPad E40 / E50 platform fit: These models share a common 6-cell Li-ion architecture, identical connector pinout, and the same BMS communication line used by the ThinkPad EC firmware. One battery covers the full E40, E50, and ThinkPad Edge 0578 series because Lenovo kept the power subsystem consistent across that generation.
- Bench tested on actual hardware: We cycled this cell on a ThinkPad E40 and monitored BMS communication through the EC. The protection circuit responded correctly to charge termination, cell balancing, and over-discharge cutoff at the expected low-voltage threshold — no false fault flags during the test run.
- Post-install calibration on ThinkPad EC: After fitting this battery, run one full discharge to hibernate cutoff, then charge uninterrupted to 100%. This forces the ThinkPad EC to complete a battery learn cycle against the new cell data and clears the inaccurate health warning that appears after any cell swap.
Why the ThinkPad E40 BIOS reports poor battery health after a new cell install
The ThinkPad EC stores learned charge data from the previous cell in EEPROM. When a new battery arrives, that stored data no longer matches the actual cell chemistry, so the EC flags it as degraded. This is an EEPROM mismatch, not a fault with the replacement cell itself. Running one full discharge-to-hibernate followed by an uninterrupted charge to 100% overwrites the old learn data. After one or two full cycles, the health indicator corrects itself.
ThinkPad E40 shutting down abruptly at 20–30% charge shown
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The displayed percentage stays optimistic until the cell voltage drops sharply under combined CPU and display load — at that point the EC triggers an emergency cutoff before the gauge catches up. The fix is to complete two or three full discharge-to-recharge cycles so the fuel gauge IC maps the real voltage-to-capacity curve. Once calibrated, the system will begin stepping down load and warning correctly before the cell reaches its 2.5V per-cell cutoff floor.