CMOS batteries and Clock Backup batteries perform the same function in desktop and laptop computers: when the computer is turned off, the battery maintains the time and date, insuring their accuracy when the system is once again restarted. More importantly, the battery saves the computer's CMOS set-up configuration, which allows the system to efficiently re-boot once it is restarted. The computer knows what type of hard drive it is dealing with, etc. Not surprisingly, these batteries are known alternatively as CMOS batteries, Real Time Clock (RTC) batteries, or simply internal batteries.

The most common CMOS battery chemistries are Lithium, Nickel Cadmium (NiCad) and alkaline. They are usually somewhere in the 3 to 7.2 volt range and either solder onto the motherboard or simply plug in via a snap-in connector (depending upon the computer manufacturer's design).

In most cases, replacement of the CMOS battery is an easy task. It is simply a matter of locating the battery on the computer's motherboard, removing it, and plugging in a new one. As a rule, internal batteries should be replaced by the same type of battery which was originally used in the machine, or according to the manufacturer's specifications. The major exception to this rule are older IBM compatible computers which come with a NiCad battery soldered onto the motherboard. These computers usually have a three or four pin male plug, with two of the pins connected via a jumper (this is generally found in the same area of the motherboard as the original battery). This plug gives you the option of leaving the soldered battery in place and replacing it with a plug-in type battery. Removing the jumper tells the computer to ignore the battery soldered onto the motherboard and to look to the pins for its power source. IMPORTANT NOTE: NiCad batteries are rechargeable, whereas Lithium and alkaline batteries are NOT. Therefore, Lithium and alkaline batteries must be replaced by equivalent batteries of the same type. Attempting to replace these non-rechargeable batteries with a NiCad will result in a non-functioning battery, due to the fact the computer lacks the proper charging circuitry. If a motherboard lacks the above-discussed provision for an external battery, the NiCad battery MUST be unsoldered and replaced by a NiCad battery ONLY. Attempting to use an alkaline or lithium battery in place of NiCad on such a board could be hazardous. These batteries are not designed to be recharged, and an attempt to do so may cause the battery to "burst", or explode.

CMOS batteries generally last for two to three years, although some (especially the lithium type) have been known to last much longer. Ironically, the less you use your computer, the faster the CMOS battery will run out. This is because when your computer is turned off the battery begins to function. It is recommended to replace the CMOS battery approximately once a year, or when servicing the computer. If your computer has been idle for an extended length of time it is a good idea to change the battery. Changing the battery is a relatively easy and inexpensive task, especially (as I'm sure many of you out there know) when compared to trying to reconfigure a computer which has lost its CMOS settings.