Most people don't spend much time thinking about the CMOS battery inside their PCs. This battery protects some important data, though. Without it, your computer wouldn't know the time and date, your basic hardware settings, how to boot up or how to talk to the motherboard and hard drives. CMOS battery symptoms may be intermittent or continuous, making it harder to tell what's wrong. Fortunately, if you know what to look for, this issue is relatively simple to diagnose.
A dead or dying CMOS battery may cause your computer to take a long time to boot up. This is because your computer has to actually recreate the BIOS settings from scratch every time you reboot. Figuring out the number and location of hard drives, the available amount of memory and other information necessary for booting may cause your computer to delay significantly. This can also cause the computer to run at the wrong speed, since the BIOS settings aren't retained. A computer with a bad CMOS battery may refuse to boot entirely.
If the time and date in your operating system keep resetting themselves to a date and time in the past, there is a good chance the CMOS battery is at fault. The CMOS battery is responsible for backing up the system clock on shutdown. This is a sign of a CMOS battery that is on its way out. It's a good idea to replace it right away if you see this symptom, since it may soon die entirely.
If you suddenly find yourself unable to get important information about the amount of memory available on your computer or how large your hard drive is, a failing CMOS battery may be the culprit. Fortunately, the battery is relatively easy to replace. Other explanations for this problem may include a failing hard drive or motherboard.
A failing CMOS battery may also cause your computer to produce mysterious error messages during operation. These include Error 201, any error message containing the word "CMOS" and the System Configuration Lost error. You may also discover that your keyboard, speakers or other small peripherals suddenly stop working, or that your computer reports being unable to find the operating system.