Laptop computers, like their desktop counterparts, typically contain a CMOS, or complementary metal-oxide-semiconductor, battery. This battery is mounted on the mother board, and retains the BIOS information of your machine when it is completely powered down, as well as maintains the date and time information, and backs up what is on your RAM in the case of a power interruption.
A CMOS battery typically lasts between four and seven years in a computer before it begins to die, however system trauma or a defective battery can, rarely, cause a battery to malfunction well before its time. You’ll know when your CMOS battery has gone bad when you see messages that mention something like “CMOS checksum error” or a similarly described problem. Oftentimes, you will see this error reporting infrequently on boot up, indicating that the battery is dying but not yet completely gone. This is the ideal time to replace the battery. However, if you get the error every time you boot, the battery is likely completely dead, and you may have lost your BIOS settings along with some other system information.
Before removing a CMOS battery for replacement, be sure to enter set up mode on your computer, and copy down all of the settings in your BIOS menus. This is what is stored on that battery, and when you remove that battery, that data will be erased. After replacing with a new battery however, you can go into BIOS and restore those settings from your copy. Also, some programs will back up your BIOS settings to an external disk of some sort, allowing you to reload them at any time.
In laptops, CMOS batteries are typically found beneath the keyboard. This can pose a challenge, as the keyboard on a laptop can be difficult and delicate to remove. It is also important to note that on some models, the CMOS battery is actually soldered to the mother board, in which case you will not be able to remove the battery without professional assistance, either through a repair company or the manufacturer. In laptops with removable batteries, there will typically be a tension clip holding it in place. Simply open the clip, remove the battery, and replace it with a new one. Make sure however that the new battery matches the replaced battery. Consult your computer’s hardware manual or a local or online retailer to be sure you get a direct replacement that will function with your mother board.