Fundamental system configuration settings are stored in CMOS (Complementary Metal-Oxide Semiconductor) NVRAM (Non-Volatile Random Access Memory) and therefore require very little power to maintain. The CMOS RAM backup batteries that provide this power when the computer is switched off can last for years, and are usually lithium / manganese dioxide batteries.
Although CMOS RAM backup batteries can last for over five years, it’s good practice to replace them every two or three years. This will avoid the situation where the fundamental system configuration settings are lost at the worst possible time, when the computer is being used for very important work and starts playing up.
An early sign of CMOS RAM backup battery failure is finding that when you switch the computer on, it seems to have lost track of the date and time. Many people regard this as a minor annoyance, and simply enter the correct date and time on system start-up, or run the system with the wrong date and time, which can cause serious problems with security certificates. However, the best thing to do in these circumstances is make a note of all the fundamental system configuration settings, then change the CMOS RAM backup battery and re-enter the settings.
Some older systems would be unable to start up once they had lost all the fundamental system configuration settings, but modern systems can usually start up automatically using basic defaults. Modern systems also often have an automatic facility for finding out the best possible settings, although sometimes trying to be optimal doesn’t work and you have to drop back to the defaults temporarily. However, with a modern system you’re never going to get into a situation where changing the CMOS battery means that the system will no longer start up, so there’s no need to note down all the current settings on paper before changing the CMOS battery, just in case it becomes difficult to get back to your original state.
There are several programs you can use to save the fundamental system configuration settings into a file on the hard disk drive before changing the CMOS battery. Then, after you’ve changed the CMOS battery, you can start up the system using basic settings then restore the original, probably more advanced, settings from the file. This often saves a lot of time with more sophisticated systems, such as gaming computers for example, which often have complex settings for doing things like overclocking and tuning memory timings.
Most CMOS batteries are of the CR2032 type. When replacing an old CMOS battery with a new one, it’s a good idea to wear a pair of latex gloves to prevent contamination of the battery’s conductive surfaces with natural oils from your skin, which reduce electrical conductivity. Also, it’s a good idea to use a well-known make, such as Duracell for example, rather than some unknown make that you happen to have lying around. You might be able to buy a whole pack of miniature batteries from the market for a pound, but it may not be sensible to use any of them in computer systems because you’ll only have to repeat the replacement procedure if they quickly run out. This is especially true for laptops, where it often takes considerable effort to gain access to the CMOS battery.