Lenovo ThinkPad T550 11.4V Replacement Battery 45N1743

Lenovo ThinkPad T550 / T560 / W550 Series — 11.4V Li-Polymer Replacement Battery (45N1743)

This is an 11.4V, 3900mAh (44.46Wh) Li-Polymer replacement battery for the Lenovo ThinkPad T550, T550s, T560, and W550 series laptops. It cross-references OEM part numbers 45N1743, 45N1741, 45N1742, 45N1740, 00NY639, and SB10K12721. Fit the original connector and it seats flush — no modification needed.

• T550 / T560 / W550 platform compatibility: These models share the same 11.4V three-cell Li-Polymer architecture, identical connector pinout, and the same BMS handshake protocol used by Lenovo's battery manager firmware — which is why one part number covers the full range.
• Bench tested on actual hardware: We ran this cell on a ThinkPad T550 and monitored BMS communication through a full charge cycle. Charge acceptance was normal across all three cell groups, and the protection circuit tripped correctly at the low-voltage cutoff threshold — no rogue cell balancing events.
• Post-install learn cycle on ThinkPad firmware: After fitting this battery, run the laptop down to hibernate-cutoff on battery only, then charge uninterrupted to 100% without unplugging. 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 battery health as poor immediately after replacement

Lenovo's power manager reads health data from the battery's EEPROM — a small memory chip that stores cycle count, rated capacity, and charge history. When a new cell arrives, that EEPROM data does not match what the BIOS previously learned from the old pack, so it flags the new battery as degraded or unknown. This is a firmware calibration issue, not a fault with the cell. Run one full discharge-to-hibernate then an uninterrupted charge to 100% and the BIOS will rewrite its reference data against the new cell.

Laptop shutting down at 20–30% charge shown on screen

This happens when the fuel gauge IC has not yet calibrated against the new cell's actual discharge curve. The gauge estimates remaining capacity using data from the old pack, so it reports 20–30% remaining while the real cell voltage is already near cutoff. Under full CPU and display load, voltage drops faster than the gauge predicts and the laptop shuts down before the display reads zero. Two or three full discharge-and-charge cycles let the fuel gauge IC relearn the voltage-to-capacity curve — after that, the shutdown threshold should align with the display reading at or below 5%.