The CMOS, or reserve battery, provides a steady trickle of power to a computer motherboard so that the computer can effectively remember the date and time. If the date and time on your Dell Latitude C840 constantly resets each time you power the computer on, you need to replace the CMOS battery. In order to access the CMOS battery in your laptop you must partially disassemble the computer, a difficult task if you are unfamiliar with computer assembly.
Disconnect the power cable from the Dell Latitude C840.
Turn the laptop over and slide the battery release switch over and remove the battery. Remove the three screws on the bottom labelled "circle K" and the screw labelled "circle K/M."
Turn the laptop over and open the display cover. Insert a Flathead screwdriver into the blank key next to the arrow keys on the keyboard and lift up to free the keyboard.
Lift the right side of the keyboard, standing the keyboard vertically. Locate the keyboard cable and disconnect it from the board beneath. Remove the keyboard.
Close the display cover and insert a Flathead screwdriver into the right hinge, behind the display cover. Pry the hinge cover to loosen it.
Open the display cover and pry the hinge cover near the "F8" key. Work your way around the hinge cover until it is free. Lift the hinge cover off.
Locate the copper-coloured flex cable beneath the hinge cover, directly over the "F7" key. Remove the relief screw securing the cable. Unplug the cable.
Close the cover and remove the two screws on each side of the back of the laptop, beneath the hinge cover, using a Phillips screwdriver.
Separate the display cover from the base of the laptop and lay it aside.
Turn the laptop upside down and remove the nine screws labelled "circle P."
Turn the computer over and pull the pull loop, located directly over the touchpad, to disconnect the palm-rest flex cable from the base of the computer.
Pry the right and left side of the palm rest away from the computer base using a Flathead screwdriver. Lift the palm rest up to remove it.
Locate the four screws securing the metal bracket on the underside of the palm rest. Remove the screws with a Phillips screwdriver and then disconnect the palm rest flex cable from the bottom of the palm rest. The CMOS battery is located between the palm rest and the palm rest bracket.
Disconnect the CMOS battery and replace it with a new battery.
Reassemble the Dell C840, following disassembly instructions in reverse.