What is CMOS? CMOS Battery Failure Signs: Resetting CMOS

CMOS battery failure – it doesn’t matter what you are using; laptop or desktop, if you’ve inspected your device carefully, you have obviously seen a strange-looking tablet-like battery sitting on the device motherboard. In case, if you don’t know, this battery is called CMOS battery. Don’t be confused, this battery doesn’t provide any kind of power that a computer needs to work properly. We use this battery in our system for completely different purposes.

Because of the CMOS Battery Failure we try to remove the battery from the motherboard
CMOS Battery

CMOS (Complementary Metal Oxide Semiconductor) chip is a small component continuously gets power from the battery. In this chip, all the low-level system functions like BIOS settings, real-time clock information are stored. Now you are probably thinking why it’s necessary to power the chip constantly? What happens if the CMOS battery fails? In this article we’re describing every detail you should know about the CMOS battery failure, CMOS battery failure symptoms, and CMOS battery failure effects.

What is BIOS?

As I told, CMOS chip is a very important component for the BIOS. Now a question obviously comes to our mind What is BIOS? In short, BIOS is a piece of program stored in EEPROM which is a physical chip located on the motherboard. BIOS is the first software that runs when powered on which performs hardware initialization during the booting process. Basically, after getting the power, the CPU turns on itself and loads the BIOS (Basic Input/Output System) or in modern computers UEFI (Unified Extensible Firmware Interface) from the EEPROM (Electrically Erasable Programmable Read-Only Memory).

Some motherboard has Dual BIOS Chip. if one of them gets failure the other one is used as a backup
Dual BIOS Chip

Next, BIOS or UEFI find and load the low-level configuration settings like date and time, frequency and voltages of the CPU & RAM that you have set during overclocking from the CMOS. After loading the information from CMOS, the BIOS AKA UEFI (modern BIOS) starts a process called ‘POST'(Power on Self-Test). After ‘POST’, BIOS or UEFI finds a boot device to load the operating system’s bootloader. Then the bootloader loads the full operating system – Windows, Linux, Mac OS, Ubuntu.

Read more in “Why Does My Computer Take So Long to Startup?

What is CMOS?

In the early days, CMOS was a physical chip located on the motherboard. CMOS stored all the necessary information that the BIOS needs to work with. This chip was a type of volatile memory just like our RAM which needs constant power to store all the data into it. If the power is lost for a second, all the data into it will be completely removed. Now you are thinking in the right direction, yes, the BIOS battery AKA CMOS battery provides constant power in it.

When the computer is running there is no problem for the CMOS chip. It gets the power from where the computer is running like the battery or from electricity but when the PC turns off, the BIOS/CMOS battery comes in handy. Normally we use ‘CR2032’ Li-Ion battery in our PC for the CMOS chip but some laptop manufacturers also use different kinds of battery to power the CMOS chip.

CMOS Chip in Modern Computers:

Here, I should mention that in modern days computers, one can’t find any CMOS chip into the motherboard. Manufacturers integrate the CMOS chip into the chipset of the motherboard. As you know in modern computers, there is only one chipset is located but a couple of years back, there were two chipsets into our motherboard ‘North-Bridge’ and ‘South-Bridge’. Actually, firstly this CMOS chip was integrated into the northbridge of the motherboard. Besides this location changes of this CMOS chip, everything is still the same now.

One Chipset is found on modern Motherboards. In this case Z370
CPU-Block-Diagram

As I said, CMOS is a type of memory where BIOS stores all the necessary information. If you change any settings in BIOS, it always stores in CMOS. Modern motherboard manufacturers often use nonvolatile memory to store the BIOS settings. That means it doesn’t require to constantly power that chip to store the BIOS data. This is the reason why now and then you have noticed that all the BIOS AKA UEFI settings still remain intact after removing the CMOS battery with some motherboards.

Why does the Motherboard battery still exist then?

As I said, some motherboard manufacturers nowadays don’t use a volatile memory to store the BIOS data then you may have thought why we need the battery on the motherboard. Here we should not forget that all the computers have a Real-Time Clock (in short ‘RTC’). This clock always powers on whether the computer is on or off. Have you ever thought why your computer knows always the right time whether it is connected with the internet or not?

CMOS battery failure effects

Hope you guys have sometimes noticed these error messages in your computer screen during the booting process; “CMOS Checksum Error”, “CMOS Read Error” or something similar. These are the indication of CMOS battery failure AKA there is no juice left into the battery. Normally, this battery will be dead between two to ten years, depending upon the computer usage. As much as you use your computer, the life of the battery will increase because when the computer starts to get the power from electricity or the battery, the CMOS chip receives the power from whatever the source is.

During the process of boot, because of the low battery you can see the "CMOS Checksum Error".
CMOS Checksum Error

As I said before, some messages like “CMOS Checksum Error”, “CMOS Read Error” or “CMOS battery failure” are displayed on your computer screen and the computer refuses to load the OS after this messages although you can bypass these errors and turn your computer on forcefully but that’s not the point here.

Major Problem of CMOS Battery failure:

Actually the major problem that a computer has to face after the death of the CMOS battery or you can say CMOS Battery Failure is, the “Boot Device Priority”, if your computer has multiple drives to work with then because of the lack of information, the computer becomes confused which drive should be used to boot the machine and from where it loads the bootloader.

Boot-Device-Priority settings of UEFI

In modern computers, it doesn’t create any problem anyway if the battery is dead. Only the RTC (Real Time Clock) shows random dates and times instead of the correct one. If you correct the date and time manually, it will again change into a random date and time when the computer turns off next time.

CMOS battery model is CR2032.
CR2032-CMOS-Battery

How to test CMOS battery

To check the CMOS battery whether it is dead or not is very simple and also, I don’t think that you need to check it on your own. Our computer is obviously smart enough to detect if any juice is left into the battery. Also, your computer creates some problems if the CMOS battery becomes dead. Like the date and time changes after every restart, some error messages are also displayed before booting into the OS and your machine might shout with ‘beep sound’ to indicate you the low “CMOS battery”.

Test CMOS battery with a RED LED
CR2032-Battery-Check With RED LED

If you can’t rely on your PC then, of course, you can test this battery with a RED LED or a multimeter. If the LED glows like shown in the picture below, the battery might be OK if it is not glowing or the intensity is very low then replacing the battery is a good idea. Also, as I have mentioned earlier that a one can use the multimeter to check the battery. If the meter shows the voltage above 2V then the battery is fine and it can drive the RTC or the CMOS chip but if the voltage is less than 2V then the replacement is essential.

Test CMOS battery with Multimeter
CR2032-Battery-Check-with-multimeter

How to Clear CMOS?

Because of the wrong configuration sometimes we do in the BIOS AKA UEFI the computer doesn’t turn on or sometimes gets unstable. In these situations, resetting the CMOS is a pretty good idea, you know why. To reset the CMOS most of the motherboards have a jumper. By changing its location one can easily clear the CMOS chip. Also, in the early days by removing the BIOS battery CMOS resetting was also possible. After deleting all the information from CMOS bios goes into its default state and everything works normally. Now you know how to clear CMOS or you can say how to reset CMOS.

Resetting CMOS
Resetting CMOS

Long story in short,

If I say shortly, about all the problems of CMOS battery failure then the answer is “it depends”. Depending upon the motherboard, the computer and the model, the problems may vary. Messages which display on the monitor also may vary so there is no fixed answer. I hope you guys have enjoyed this article. If so, then don’t forget to comment down below to appreciate my work and share this post with your friends if they want this information too. Thanks for visiting.

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