Say "computer battery," and people are likely to think about the rechargeable brick that goes into a laptop. But computers have another battery that's just as important: the complementary metal-oxide semiconductor, or CMOS, battery. This tiny button-style battery on the motherboard provides just enough power to keep critical start-up information stored in memory when the computer is powered off and unplugged. When this battery goes bad, as Smart Computing magazine says, "unpleasant things will happen."
Lost Date and Time
The CMOS battery backs up the system clock. If your computer seems to "forget" the date and time whenever you shut down, the culprit is probably a weak CMOS battery. If the date and time reset to midnight on Jan. 1, 1990, then this is almost certainly the case. Take the time now to replace the battery; if it goes completely dead, the problems may get worse.
Missing System Information
If you can't access information about your computer's installed memory or disk space, a failing CMOS battery may be at fault. In fact, if the computer suddenly loses track of how much RAM it has or how much free space is available on the disk, a bad CMOS battery might be the best-case scenario. The worst case? Total hard drive failure.
Mysterious, seemingly random messages alerting you to errors in the CMOS RAM, the system memory or your disk configuration are also symptomatic of a bad CMOS battery.
Trouble at Startup
The system backed up by the CMOS battery stores the computer's BIOS settings. Those settings are what your computer consults when it starts up. They tell it what devices to turn on, where to find the operating system on the hard drive, and how to load the operating system into RAM. If the battery dies, or just goes bad, those settings will be lost, and the computer won't know what to do with itself. You may see error messages about lost or missing Setup or CMOS data, or saying that the system doesn't match the configuration. Or the computer might not boot up at all.
If these problems are intermittent--they prevent you from starting up sometimes but not others--that's a strong sign that the CMOS battery is on the fritz. Once again, a bad battery may well be the best-case scenario.