If the CMOS battery in your computer or laptop dies, the machine will be unable to remember its hardware settings when it is powered up. It is likely to cause problems with the day-to-day use of your system.
Background
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The CMOS (complementary metal oxide semiconductor) battery in your computer is used to store your hardware configuration settings. It also keeps a record of the date and time when your machine is switched off. The CMOS battery is inside your machine and attached to the motherboard.
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Problems
If the CMOS battery dies, settings will be lost when the computer is powered down. You will probably be asked to reset the time and date when you start the computer up. Sometimes the loss of settings will prevent the computer loading the operating system.
Replacement
It is usually possible to install a new CMOS battery and replacements are inexpensive. Be aware that this replacement does require you to open up your PC and may invalidate your warranty–you may wish to enlist the help of a qualified computer technician.