About this replacement
Lenovo ThinkPad T420s / T420si — 11.1V Li-ion Replacement Battery (0A36287)
This 11.1V, 3600mAh (39.96Wh) Li-ion battery fits the Lenovo ThinkPad T420s and T420si business laptops. It replaces OEM part numbers including 0A36287, 42T4844, 42T4845, 42T4846, 42T4847, 45N1036, and 45N1037. The T420s uses a slim 13.10mm profile battery — confirm your existing part number before ordering.
- T420s and T420si compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The T420si is the Intel integrated graphics variant — the battery is electrically identical across both chassis. Cross-reference against the OEM part number on your original cell to confirm fit.
- Bench tested on actual hardware: We ran this cell on a T420s under combined CPU and display load. The BMS held the discharge curve without premature cutoff and accepted a full charge cycle without triggering the BIOS charge-fault flag.
- BIOS learn cycle after install: After fitting this battery, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle. Skipping it causes the health warning that appears in Lenovo Vantage or Power Manager after every cell swap — it is a calibration state, not a battery fault.
Why the T420s BIOS reports poor health immediately after a new battery install
The ThinkPad BIOS reads health data from the battery's EEPROM on first contact. A new cell ships with factory default EEPROM values that do not match Lenovo's expected charge history profile. The BIOS interprets this mismatch as a degraded cell and flags it accordingly. One full discharge-to-hibernate followed by an uninterrupted charge to 100% writes updated data and clears the warning in Lenovo Vantage and the BIOS battery status screen.
T420s shutting down at 20–30% charge shown on the fuel gauge
This is a voltage cliff — under combined CPU and display load, the cell voltage drops faster than the fuel gauge IC can track. The IC was calibrated against the old, degraded cell and its discharge curve no longer maps accurately to the new chemistry. The laptop hits the BIOS low-voltage cutoff before the displayed percentage reaches zero. Run two to three full discharge-and-charge cycles to recalibrate the fuel gauge IC against the new cell; after calibration the shutdown threshold should align with a displayed level below 5%.