The CMOS battery governs an often-overlooked, yet essential, part of every IBM-based computer, and is the equivalent of PRAM on Macintosh computers. The actual CMOS chip is soldiered onto the motherboard, and stores system settings so that they are not erased or corrupted when the computer turns on or off. It is able to store these settings without direct power from the computer because it depends on a battery to maintain its data. This battery, which typically lasts around ten years (http://www.computerhope.com/help/cmos.htm), occasionally fails because of environmental circumstances. With that being said, there are several easy ways to determine if your CMOS battery is failing.
Since the CMOS retains data like system settings and the date and time, a sure indication of low battery is buggy, flawed, or inconsistent system information. Symptoms range from the innocuous (like the computer giving you a time that is consistently wrong) to the annoying (having to reinstall drivers for system components every time you restart the computer) to the frightening (your computer telling you it cannot start normally because system settings have changed). Although these problems can be caused by more than a dying CMOS battery, changing the battery or resetting the CMOS is always a good first step when troubleshooting. Another simple tip-off of a dying battery is low battery indicators within the BIOS start-up screen. Finally, your computer may produce error messages like "CMOS Read Error," "CMOS Checksum Error," or "CMOS Battery Failure" that provide the most obvious indication that something is awry.
Rectifying the problem of a dying CMOS battery is fairly easy. The best first step is to leave your computer running for 24 hours or so, as this usually charges the battery. If your problems persist, however, replacement is your next option. Today's batteries are usually 3v lithium models, and inexpensively available at major retailers or at websites (like http://shopping.microbattery.com/s.nl/sc.2/category.763/.f). Replacement is as simple as popping the old battery out and inserting the new one. Be sure to test your system settings to ensure that the problem is solved, because symptoms persisting after replacement are often indicative of a larger problem, typically with the motherboard.
If shown a motherboard, the average computer user would probably not be able to identify the CMOS chip and battery. Ideally, the same user should have no reason to. The CMOS is vital to the smooth operation of a PC, but it runs quietly in the background without user input. A dying CMOS battery disturbs the otherwise smooth operation of your computer, so it is in your best interest to discover problems before they disrupt your computer unnecessarily.