Dell Latitude LST C400ST CMOS Replacement Battery 7.2V 20mAh

Dell Latitude LST C400ST / L400 / Ls Series — 7.2V Ni-MH CMOS Backup Battery (PP01L)

This is a 7.2V, 20mAh Ni-MH CMOS backup battery for the Dell Latitude LST C400ST, L400, Ls, and LST series laptops. It replaces OEM part numbers PP01L, 9604T, 6P466, and B-4002. The cell powers the RTC circuit and SRAM on the motherboard, keeping BIOS settings and the system clock intact when mains power is absent.

• Latitude LST / L400 / Ls platform fit: These Latitude models share a common motherboard architecture with the same CMOS cell connector, voltage rail, and physical envelope — 43.93 x 24.00 x 3.75mm. All four OEM part numbers cross-reference to this single cell specification.
• Bench tested on actual hardware: We verified cell voltage at delivery and confirmed the RTC circuit held a stable clock reading across repeated mains-power interruptions. The cell maintained retention voltage without drop to the 2.8V cutoff threshold during testing.
• Post-installation clock reset: After fitting, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a factory default the moment the old cell is removed, and it will not self-correct — you must set it manually on first boot.

BIOS clock resetting to January 2000 after every power cycle on the Latitude L400

The Latitude L400 and LST motherboards use the CMOS cell exclusively to power the RTC oscillator and retain SRAM values when AC is disconnected. Once the cell drops below 2.8V, the RTC loses its reference and resets to the firmware default — typically 01/01/2000 on Dell Latitude hardware of this era. A new cell alone does not fix the clock; you must enter BIOS setup after replacement and write the correct date and time before saving. If the clock resets again within days of a new cell, check the connector spring on the motherboard — a weak spring contact will cause intermittent voltage dropout even with a good cell seated.

CMOS checksum error on boot after cell replacement

A checksum error means the BIOS has detected that stored CMOS values no longer match their checksum — this happens when the cell was fully depleted and all SRAM contents were lost. Fitting the new cell restores power to the circuit but does not restore the data. Enter BIOS setup on the next boot, load optimised defaults, set the correct date and time, and save. If the error persists after saving, confirm cell voltage with a multimeter — a fresh Ni-MH cell should read at or above 7.2V at the connector before installation.