CMOS BATTERY
When was the last time you thought about the battery in your PC? Didn't know there was a Battery? Thats the battery that protects your valuable CMOS data when your desktop isn't powered on. Most recent motherboards come with coin shaped lithium/manganese-dioxide batteries that look like large watch batteries. These batteries are advertised to have a life span of five years. That's longer than many people keep their PCs!
When should you replace your battery?
While the lifespan of the battery might be five years under the best of circumstances, why would yopu wait until failure to replace? Remember that the battery is the only thing standing between you and the loss of your CMOS settings when you power down your PC.
If you keep your PC long term without swapping out the motherboard, you should consider changing the battery every two to three years. Why change the battery sooner? Simply this. If you wait until the battery begins to die, your PC can forget its CMOS settings. And remember its only about $3.00 to replace if you do it yourself.
Often, a dying battery will first manifest itself intermittently. Sometimes, you will boot with no problem. At other times, you will see an error indicating that the system doesn't match the configuration. When the battery fails entirely, the message will either come up every time you reboot, or your BIOS may report an error number with a message stating that the battery is dead.
If you have a voltage meter, you can check whether or not the battery is healthy by measuring the voltage between the battery terminals that connect to the motherboard. The voltage should be higher than 2.5 volts. If the voltage reading is less, you should change the battery right away.
Before we change the battery lets look at several types of batteries motherboard manufactures use.
Coin Cell Batteries
The most common battery used in todays computers .3 volt Lithium coin cell battery. Replacement for CR-2032 computer battery.
4.5V Alkaline Batteries 
4 pin connector. Velcro strip provided to attach it out of the way on the power supply or case.
3.6V CMOS Batteries
3.6 volt ni-cad battery - 60mah
- 2 pin connector - solder on.
Replacement for most soldered on "barrel" type
batteries.If your motherboard has a 4 pin connection near your
old battery use B220 which you just plug on.
PS2 Computer Clock Battery 
6V Lithium battery replacement for PS2 50 and up computer battery.
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A good Internet Friend asked me about Laptop CMOS Batteries, so this is for
Goddess of the Toshiba
(who's identity I will not publish)
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LapTop CMOS Batteries
Computer clock (CMOS or
PRAM)batteries are used in virtually all laptop computer systems
that use CMOS memory to retain the date, time and configuration
information when the power is off. The types of devices used vary
greatly depending
on the manufacturer and the application. Laptops may contain a
clock/CMOS battery and a resume battery to retain RAM memory
information in the event the system battery loses power.
Some IBM compatible laptop computers contain a real time clock (RTC) battery and a resume (somtimes called "sleep") cell. The RTC battery runs the clock and provides memory back-up power for configuration (CMOS) information. The resume (or "sleep") cell provides RAM back-up power when the main powercell fails. This allows the user to save information in RAM to disk when the system is connected to power or a fully charged battery is inserted into the system. Note: the resume function usually does not work the if laptop's powercell is not in the system.
Some laptops can contain more than
one internal battery to power clock, CMOS, resume and sub-battery
functions. Type vary from Lithium coin cells to Nickel Cadmium
(NiCd) and Nickel Metal Hydride (NiMh) packs. These can be
soldered directly to the system board (or sub-card) or attached
using a wire harness and sub-minature connector.
Locations of these devices vary from model to model but, in most
cases, these devices are found underneath the keyboard. Some
manufacturers locate the cells in odd places like underneath a
mouse ball, touch pad or in the screen frame housing.
Use extreme caution when removing the keyboard on any laptop!
Keyboard assemblies are connected to the system board by flat "ribbon" cables. These cables are usually made from
Mylar™ (or similar materials). Cable assemblies can be
expensive and very difficult to find, even through an authorized
dealer.
ALERT - Many Compaq laptop owners are reporting that they cannot
find a replacement for the Rayovac BR1632 or that costs quoted
exceed $50.00. Resource 800 can provide a CR1632 with wire leads
for $16.00, plus shipping (Please order the CR1632/WL). The item
is shipped as a kit with insulating shrink tube and Velcro
fastener. Installation requires
cutting the old wire harness (with subminiature plug) off AT THE
battery and soldering the new battery leads to the old wire
harness/connector assembly. SEE: http://www.resource800.com/
Replacing a CMOS Battery on a PC
When you replace your battery, the CMOS settings will
immediately be lost. In addition, battery corrosion (more common
in older PCs) or other battery problems may cause the battery to
die more suddenly than its expected lifetime. For this reason, it
is important to keep a record of your CMOS settings. Many utility
suites, such as Norton Utilities, will allow you to create a
floppy disk backup of your settings. This can be extremely handy
for battery problems as well as recovering from other CMOS
errors. If you do not have any of these utilities then here's
what to do!
Warning
Before proceeding remember to observe proper anti-static precautions when operating inside the case of your PC. If you do not have a anti-static strap, rub your hands on the metal portion of the case to get rid of static.
If you can't see your battery right away, try removing expansion cards or unplugging cables. Most newer motherboards use lithium batteries that look like large watch batteries.
Typical motherboard with
a Coin Cell Battery 
1. Boot your PC and enter its setup mode
2. Write down all of the settings from the various menus.
3. Power off your PC.
4. Open the case and locate the battery on the motherboard.
See your user manual for specifications about the battery
and its location.
5. Obtain a replacement battery from a local or online computer parts dealer or Radio Shack.
6. Remove the old battery and replace it with the new one.
7. Document the date you replaced the battery.
8. Replace the case and power on the PC.
9. Enter your PC's setup mode.
10. Reenter the settings you have written down from the various setup menus.
Congratulations you just replaced your CMOS Battery
