If the computer forgets, it stops operating. The CMOS information keeps track of the date, time, memory, type of hard drive and all other hardware requirements. If your battery fails for any reason, and the CMOS loses the system information, it will not be able to boot from the hard drive. You will be required to run the Setup program and re-enter the information CMOS stores. This is why it is imperative to save the CMOS information on paper, or on a floppy, so that you will know what information to enter if the CMOS must be updated.
Some batteries are rechargeable and last about two to three years.
Some look like little barrels soldered directly on the motherboard. Others are attached to the computer case away from the motherboard to lessen the danger of a leaky battery. If the original motherboard battery had previously died, and was replaced with a rechargeable battery, the system is probably good for another three years depending on the type of battery installed. If you replaced a failed battery with an alkaline pack, you will have to replace it every year, but it will last a full year provided it continues to be recharged. If you leave the computer turned off for extended periods of time the batteries can become discharged, or develop a retained memory as being discharged, causing CMOS to lose system information.