From the Kenosha News on 1-6-08
Dead CMOS battery can be the missing link in a computer problem
As I explained last week, part of my desktop computer's problem at home was a dead CMOS battery. If I hadn't figured this out, I would probably still be troubleshooting the problem and/or could have replaced the hard drive or other parts needlessly. If you have a desktop, you may not even be aware that your computer has a battery. If you have a laptop, then your computer actually has TWO batteries – the regular one you use when you're not plugged in and the CMOS battery.
The purpose of the "CMOS battery" is to provide power to a chip that stores your computer's time and system settings. The computer's clock and some basic information about the computer have to stay active even when your computer is disconnected from power. To do that, it uses a small, inexpensive battery. Check out www.computerhope.com/help/cmos.htm for an explanation of CMOS and pictures of various types of CMOS batteries. I've found that most computers have either the "coin cell" type – like a large watch battery about the size of a nickel – or the "1/2 AA" type (not shown on the web site) that is half the length of a AA battery. Look for it in a small plastic case directly on the motherboard of your computer. Ours was the coin cell type.
One pretty quick test to see if your CMOS battery is dead is to check your computer's date and time. You can do this in Windows by going to the Date and Time control panel. (If your computer won't start up in Windows, you can also see the date and time in Setup mode. Enter Setup by pressing a specific key – often Del, F1, or some combination such as Ctrl-Alt-Esc – while your computer is booting. Each brand can be different.)
If your desktop computer has a dead CMOS battery, it is pretty easy to change yourself. With the computer off and unplugged, locate the battery and remove it. Purchase a replacement and install it. On a laptop, the CMOS battery is harder to find and replace. It is usually the external Lithium type, looking like disks or cylinders shrink-wrapped in plastic with a wire pigtail. I recommend that you have a professional replace it for you; I believe that Batteries Plus offers this service. Once you change the battery, you will need to reset the date and any other lost setup information.
In our case, our battery likely went dead recently because we went had decided to turn off all electronics not in use, and this computer gets used much less now that my oldest son is off to college. The more a computer is off, the more it relies on its battery. Unlike a car battery, your computer's battery is not rechargeable, so it will run down eventually, and it will run down quicker if the computer stays off or unplugged for longer periods of time. This isn't a reason to leave your computer on all the time, but just be aware how it all works!
By Carol Sabbar from the Kenosha News on 1-6-08