About this replacement
Lenovo ThinkPad X40 / X41 Series — 3V Lithium CMOS Backup Battery (92P1004)
This is a 3V, 200mAh lithium coin cell that replaces the CMOS backup battery in the Lenovo ThinkPad X40, X41, and X41 Tablet series. It powers the RTC circuit and SRAM when the laptop is off or unplugged, keeping BIOS settings, system time, and hardware configuration intact. Without a functional cell, those settings reset to factory defaults every time mains power is removed.
- ThinkPad X40 / X41 platform fit: The X40, X41, and X41 Tablet models (including 1866 and 1867 variants) share the same CMOS socket, connector footprint, and retention voltage threshold. One replacement cell covers the full group.
- Bench tested on actual hardware: We confirmed open-circuit voltage at 3.0V and verified the BMS handoff — the RTC circuit latched correctly and held settings through a full mains disconnect cycle without resetting.
- Post-install clock correction: After fitting the new cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value after any power interruption, and that default date persists into the OS until manually corrected in firmware.
BIOS clock resetting to 2000 after every power cycle
The ThinkPad X40 RTC circuit requires a minimum retention voltage of 2.8V from the CMOS cell to hold the clock register between power cycles. A depleted cell drops below that threshold the moment mains power is removed, and the RTC resets to its default epoch — typically January 1, 2000. The symptom appears on boot as a wrong system date, sometimes accompanied by a low-battery or date/time warning prompt. Replacing the coin cell and setting the correct time in BIOS resolves it immediately.
CMOS checksum error on boot after fitting a new cell
A checksum error after installing a new coin cell usually means the CMOS SRAM was completely cleared before the swap — the stored checksum no longer matches the default values. This is not a fault with the replacement cell. Enter BIOS setup, load default settings, set the correct date and time, then save and exit. The checksum recalculates on the next boot and the error clears.