About this replacement
Lenovo ThinkPad X13S GEN 1 — 7.74V Li-Polymer Replacement Battery (L21M4P76)
This 7.74V, 6400mAh (49.54Wh) lithium-polymer cell replaces the original battery in the Lenovo ThinkPad X13S GEN 1 series, including the 21BX000EHV, 21BX000GSC, 21BX000HMB, and 21BX000HSC variants. It matches OEM part numbers L21M4P76, L21L4P76, L21C4P76, 5B10W51878, and SB10W51879 among others. The ThinkPad X13S is an ARM-based ultralight business laptop, so the battery must communicate correctly with the BIOS firmware — a mis-matched cell trips health warnings immediately.
- X13S GEN 1 platform compatibility: Every model in the 21BX prefix range shares the same 7.74V two-cell Li-Polymer architecture, the same physical connector pinout, and the same BMS handshake protocol. That is why one cell covers 20-plus sub-models without modification.
- Bench tested on actual hardware: We ran this cell on a ThinkPad X13S GEN 1 unit and confirmed full BIOS recognition, correct Wh reporting in Lenovo Vantage, and clean charge cycling through the onboard BMS without overcurrent trips.
- Post-install calibration on the X13S: After fitting this cell, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100% without interruption. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears on the status bar after every cell swap.
Why the ThinkPad X13S BIOS reports poor battery health right after a new cell is installed
The X13S stores charge history and degradation data in EEPROM on the battery's embedded controller. When a new cell arrives, that EEPROM either holds factory defaults or shows a mismatch against what the BIOS last recorded. The BIOS reads this as a fault, not a fresh battery, and flags health as "poor" or "unknown" in Lenovo Vantage. Running the learn cycle — one full discharge to hibernate threshold, then a complete uninterrupted charge to 100% — forces the BIOS to rewrite the EEPROM baseline against the new cell's actual chemistry.
ThinkPad X13S 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 was tuned to the old, degraded cell, so it loses accuracy in the lower range and calls shutdown well before true empty. It is not a cell defect. Complete two to three full discharge-and-charge cycles and the IC re-maps the curve to the new cell. After the third cycle, the reported percentage at shutdown should align to below 5% rather than cutting at 20–30%.