Dell Latitude C800 CMOS Battery 2664E 7.2V 40mAh

Dell Latitude C800 Series — 7.2V Ni-MH CMOS Backup Battery (2664E)

This is the CMOS backup battery for the Dell Latitude C800, C810, C840, and Inspiron 3700 series notebooks. It runs at 7.2V with a 40mAh capacity and uses Ni-MH chemistry. The cell keeps BIOS settings and the real-time clock alive when mains power or the main battery is absent.

• Latitude C800, C810, C840 and Inspiron 3700 compatibility: These models share the same CMOS circuit layout, connector pinout, and voltage requirement. One cell covers all of them without modification.
• Bench tested on actual hardware: We seated the cell, powered up the board, and confirmed the CMOS circuit held voltage across repeated mains disconnects. The RTC retained the correct time with no BMS faults logged.
• Post-install clock correction: After fitting this cell, enter the BIOS immediately and set the correct date and time, then save and exit. The RTC circuit loses its reference point during the swap, and it will default to a wrong date until you write the correct value into CMOS and let the new cell hold it.

BIOS clock resetting to January 2000 after every power cycle

The RTC on the Latitude C800 draws continuous low-level current from the CMOS cell to keep the clock running between power cycles. When cell voltage drops below 2.8V, the circuit can no longer maintain the SRAM state, so the clock resets to its factory default — typically 1 January 2000. A new cell alone does not fix this immediately. You still need to enter BIOS, set the correct date and time, save, and exit so the value is written back into CMOS and held by the fresh cell.

CMOS checksum error on boot after replacing the cell

A checksum error on the Latitude C800 means the BIOS has compared stored settings against a checksum and found a mismatch — usually because the CMOS SRAM was wiped during the cell swap. This is expected after any CMOS cell replacement, not a sign the new cell is faulty. Enter BIOS setup at the F2 prompt, review all settings, and save before exiting. That write cycle regenerates a valid checksum and clears the error on the next boot.