About this replacement
Lenovo ThinkPad T400s Series — 11.1V Li-ion Replacement Battery (51J0497)
This 11.1V, 4400mAh (48.84Wh) Li-ion battery replaces the original cell in the Lenovo ThinkPad T400s and its sub-variants, including the T400s 2801, 2808, and 2809. It slots into the standard T400s battery bay and connects via the original multi-pin interface. Cross-references OEM part numbers 42T4688, 42T4689, 42T4690, and 42T4691.
- T400s platform fit: All listed T400s variants share the same 11.1V three-cell rail, battery bay dimensions, and BMS handshake protocol — which is why one part number covers the full sub-variant range without modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a T400s unit, confirmed BMS communication across all charge states, and verified the pack holds voltage through CPU and display load without triggering an early cutoff.
- Post-install calibration on the T400s: After fitting, 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 actual capacity and clears the inaccurate health warning that appears after every cell swap.
BIOS reporting poor battery health immediately after replacement
The ThinkPad BIOS stores battery health data in EEPROM on the battery's internal circuit board. When a new cell is installed, the BIOS reads stale or mismatched EEPROM data and flags the pack as degraded before it has run a single cycle. This is not a fault with the replacement cell — it is a calibration state. Running one full discharge-to-hibernate followed by an uninterrupted charge to 100% triggers the BIOS learn cycle and rewrites the health register. After two to three full cycles, the health indicator should read normal.
T400s shutting down at 20–30% charge shown on the gauge
This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The gauge reports 25% remaining while the cell is already near its low-voltage cutoff under combined CPU and display load — the sudden draw exposes the mismatch and triggers an immediate shutdown. The fix is the same calibration cycle: discharge fully to hibernate, then charge to 100% without interruption. After two full cycles, the fuel gauge IC recalibrates its endpoint references and the shutdowns stop. If the problem persists after three cycles, check that the BIOS battery charge threshold is not set below 95% in Lenovo Power Manager.