Lenovo ThinkPad T400 CMOS Battery 3V 200mAh 4W3253

Lenovo ThinkPad T400 / T410 Series — 3V Lithium CMOS Replacement Battery (4W3253)

This is a 3V 200mAh lithium coin cell that maintains the RTC clock, BIOS settings, and SRAM-backed system configuration on Lenovo ThinkPad T400, T400s, T410, and T410s laptops. When the original cell drops below the 2.8V retention threshold, the motherboard loses all stored settings the moment mains power is removed. Part numbers 4W3253, 04W1642, 0A90099, 01AW186, 01AW187, and 02K6572 all cross to this cell.

• ThinkPad T400 / T410 platform fit: These models share the same motherboard CMOS socket, contact spring geometry, and 3V retention circuit. The cell connects via a two-wire JST-style connector routed to a dedicated RTC header — not a bare coin cell slot — so the replacement must match both the cell voltage and the connector type.
• Bench tested on actual hardware: We ran this cell on a T410 motherboard and confirmed the BMS-adjacent retention circuit held a stable 3.0V across the SRAM and RTC lines. The system retained BIOS settings and clock time through repeated mains disconnections without resetting.
• Post-install clock correction: After fitting this cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit powers down to the coin cell during installation, and the clock resets to a factory default value. Skipping this step causes Windows to report incorrect timestamps and can trigger certificate validation errors.

BIOS clock resetting to 2000 after every power cycle

The ThinkPad RTC circuit requires a minimum of 2.8V from the CMOS cell to hold the clock and retain SRAM-backed settings. Once the original cell drops below that threshold, the RTC loses power the instant the laptop is unplugged or the main battery is removed. A new cell restores the voltage rail immediately. After fitting, boot into BIOS, set the correct date and time, press F10 to save, and confirm the clock survives a full power-off cycle before closing the chassis.

CMOS checksum error on boot after swapping the coin cell

A checksum error after a cell swap usually means the BIOS compared its stored configuration against what it found in SRAM and detected a mismatch — which happens when the CMOS lost power mid-swap and the settings reset to defaults. This is not a faulty cell. Boot into BIOS setup, reload default settings or re-enter your configuration manually, then save and exit. If the error persists after a clean save, inspect the connector contact spring on the motherboard header for oxidation or deformation, as a poor connection causes intermittent voltage drops that corrupt the checksum on every boot.