PC Hardware

The CMOS Battery

The CMOS chip requires a small trickle voltage from a battery to keep its memory alive. When the battery gets low or runs out of current, the computer will experience a sudden memory loss and thus lose settings. It might not be able to find the floppy disk or first hard disk drive and therefore signify an error indicating that it cannot find the system or non-system disk.

The voltage of CMOS batteries ranges from 3 to 6 volts. Check the motherboard or the motherboard documentation to determine the actual battery requirements. Batteries come as either on-board (NiCad batteries, soldered in place or in a fixture, that last from five to seven years) or external (nonrechargeable AA alkaline batteries that last from two to four years). The 3-volt lithium watch battery is becoming very popular with motherboard suppliers. Many of these are mounted in a special holder so that the battery can be easily changed; however, some manufacturers solder them in place.

The first clue that the battery is weakening is that the CMOS clock begins to slow down. Go to a C prompt and type time. If you notice the clock is slow, it's time to change the battery.

Remember that an MS-DOS machine (this includes one running Windows 95 or 98) uses the CMOS clock only to get the date upon startup. After the computer is running, MS-DOS uses the memory refresh timer on the memory controller to keep time. This works well, but because the refresh timer is not very accurate about seconds, you will lose one or two seconds per day. If you never turn off a computer, it could lose time. Do not confuse this with a bad battery. When you reboot, the computer will update itself to the correct time from the CMOS. If the CMOS battery is low, it will still show the incorrect time.

When the CMOS battery dies completely, you will get lost CMOS errors, as previously described. If you reload the CMOS data and the errors return, you must change the battery. Although the computer will hold CMOS information during the week, sometimes, over the weekend-when the computer is turned off for two days-the CMOS data will be lost. Do not let these seemingly "intermittent" problems fool you. And sometimes, before a battery dies, but after it has started to fail, it will still be able to hold CMOS settings for a short time after the computer is off. Any time a computer loses the CMOS information more than once in a week, replace the battery immediately-if only to eliminate the battery as the source of the problem. After replacing the battery, you must run the setup utility and restore any CMOS settings.

The CMOS chip contains a capacitor that allows replacement of the battery without losing data. For motherboards with soldered on-board batteries, there is usually a connection that allows you to add an external battery to replace a worn-out internal one. Be sure that the external battery has the same voltage as the on-board battery you are replacing. Some of the older PCs use a battery pack with four AA cells or a single nine-volt battery. These should be replaced with a special PC battery pack to ensure longer life.

The best source of information about replacing a CMOS battery is the documentation that comes with the motherboard.

Today's computers are becoming less reliant on battery backup for CMOS. With Windows 95 and 98 and Plug and Play technology, devices come with their own BIOS, which the system reads each time the computer is booted. This does not eliminate the need for CMOS or batteries, but it minimizes the impact of a battery failure. At the very least, you will still need to retain the date-and-time information.