About this replacement
Lenovo ThinkPad E40 / E50 Series — 11.1V Li-ion Replacement Battery (42T4235)
This is an 11.1V, 4400mAh (48.84Wh) Li-ion battery for the Lenovo ThinkPad E40, E50, and ThinkPad Edge 14" series laptops. It replaces a wide range of Lenovo FRU and ASM part numbers including 42T4235, 42T4731, 45N1000, and 0A36303. If the original cell is no longer holding a charge or the BIOS is flagging battery health warnings, this is the direct swap.
- ThinkPad E40 / E50 / Edge 14" platform: These models share a common battery bay form factor, connector pinout, and SMBus BMS handshake protocol. The cells communicate charge state and health data over the same two-wire bus, so the battery fits and communicates correctly across all listed variants without adapter or modification.
- Bench tested on actual hardware: We ran this cell on a ThinkPad E40 chassis and confirmed the BMS handshake completed within the first charge cycle. Charge current held steady through the CC phase, transitioned to CV correctly at 12.6V, and the BIOS accepted the cell without throwing a hardware error on boot.
- First-cycle calibration after swap: After installing this battery, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This forces the BIOS battery learn cycle to reset against the new cell's chemistry, clearing the inaccurate health warning that appears after every cell swap on ThinkPad E-series hardware.
BIOS reporting poor health or unknown status after installing a new cell
ThinkPad firmware reads health data from EEPROM embedded in the battery's BMS circuit. When a new cell arrives, the EEPROM holds factory default values that don't match the BIOS's learned capacity profile from the previous battery. The mismatch triggers a "poor health" or "unknown" flag in Lenovo Vantage or the BIOS hardware summary. Running a full discharge-to-hibernate then a single uninterrupted charge cycle overwrites the learned profile and clears the flag on the next boot.
Laptop shuts down at 20–30% charge shown on screen
This is a voltage cliff failure — the fuel gauge IC is reading state-of-charge against a capacity curve calibrated to the old, degraded cell. Under full CPU and display load, the new cell's actual discharge curve drops faster than the outdated profile predicts, so the system hits the low-voltage cutoff before the gauge reaches 0%. The fix is calibration, not a hardware fault. Discharge fully to hibernate-cutoff, charge to 100% without interruption, and repeat once more if the cutoff persists. After two cycles the fuel gauge IC re-maps against the new cell and the shutdown threshold aligns correctly near 5–8V per cell group.