The CMOS battery is the power source for a small computer chip on the laptop's motherboard known as the complementary metal oxide semiconductor (CMOS). This chip retains information about the computer's hardware configuration and other instructions known as the BIOS (basic input output settings). The CMOS battery supplies power to the chip to preserve this information, even when the computer is turned off. If your Compaq laptop's CMOS battery is dead, you will be unable to boot the system and initiate the operating system. Replacing a CMOS battery may be a simple affair, or be a tedious and involved process, depending on the model of your Compaq laptop. In order to replace a Compaq laptop's CMOS battery, you will have to access its motherboard.
Remove the screws to detach the plastic shells from the bottom of the laptop. Undo two screws from the hard drive cover and three screws from the memory module cover. Raise the plastic pieces from the base and remove them. Dismount the hard drive by pulling a small tab to raise and disconnect it from the motherboard.
Remove memory modules (RAM) and the wireless card. Undo clips from either end of the memory slot by pushing them outward to release the RAM modules. When a module pops up, pull its teeth out of the slot; remove all modules similarly. Dismount the wireless card by disconnecting its two antenna cables. Uninstall the card by pulling it out of its slot.
Remove the optical drive (CD-ROM or DVD-ROM). Undo a single securing screw from the bottom of the laptop and slide the drive out. With the drive out, undo all marked screws from the laptop's base. Depending on model, you may have to undo up to 12 screws.
Loosen the strip from the top of the keyboard panel, known as the switch cover. Turn the laptop over, open its lid and work with your fingers to raise the strip from the top left corner of the keyboard. Manoeuvre to loosen small latches holding the strip in place. Be careful that you don't snap a thin wire running from the strip to the motherboard.
Remove the keyboard. Gently raise the panel from the laptop and reach over to its back to access the connecting cables. Locate the connector adjacent to the touchpad, disengage its locking tab at the head of the cable and pull the keyboard away. Unplug a cable running from the switch cover to the motherboard and remove the cover.
Dismount the display assembly. Unplug the LCD cable from the motherboard and pull the wireless card antenna cables through a small opening in the motherboard. You will have to release the antenna cables because they are permanently attached to the LCD panel. Remove four screws adjacent to the hinges and completely detach the LCD panel.
Locate the CMOS battery, a small circular component resembling a watch battery. With the wireless card, RAM modules, hard drive and optical drive removed from the system, you will be able to access the CMOS battery on the motherboard. Depending on model, you may have to remove the battery from the base of the laptop, or through the keyboard opening.
Pry open the battery and install a new one. Firmly attach the replacement battery to the motherboard and reassemble the laptop. You should have no leftover pieces when you are done putting the computer back together.
In some Compaq models, you may be able to access the CMOS battery by simply removing the memory module cover. Refer to documentation for information on the exact location of the battery.