What Happens When CMOS Battery Dies?

When a CMOS battery dies or if its failing, it can manifest itself in myriad of ways all of which are ugly and lead to system malfunction.

In this article I will discuss in detail what happens when CMOS battery dies or if it is failing. 

Basically, when a CMOS battery dies or if its near depletion, one of the most common issues you would notice is that your PC will simply not boot. Other times, you may notice the POST screen displaying warning and errors.

An issue with CMOS battery is often confused and conflated with larger issues such as a motherboard or CPU dying. Fortunately, CMOS battery is very easy to replace and if you suspect any issues with system booting, then the first course of action should be to change the CMOS battery.

What is CMOS and CMOS Battery?

CMOS or Complementary Metal Oxide Semiconductor is basically a battery powered chip located on the motherboard that holds critical system information such as the Real Time Clock (RTC), date as well as hardware information.

This seemingly menial chip is essential to the working of your entire PC. Think of CMOS as ignition switch on a car. If the ignition switch is faulty, the car will simply not start.

Similarly, if something goes wrong with CMOS, then your PC will simply not start.

A CMOS battery is basically what powers the CMOS chip. Therefore, even when the PC is turned off or when plugged out from the wall socket, the motherboard retains the clock, date and the critical hardware information.

A CMOS battery is fairly hard to miss. It is located on the motherboard.

Motherboard CMOS Battery Labelled
CMOS Battery Labelled
CMOS Battery

The CMOS battery is a CR2032 coin-cell battery and it is universal across all motherboards.

A new battery is estimated to last for around 3 years if the PC is not plugged into the wall socket. 

So What Happens When CMOS Battery Dies?

There are a few things that can happen if your CMOS battery dies or if its failing. The following are thus signs of a failing CMOS battery:

  1. Your PC Will Simply Not Start
  2. You Will See Flashes of LED on Motherboard
  3. You Will See Error on the POST Screen on Startup
  4. Beeps if You Have a Beep Code Speaker

1. Your PC Will Simply Not Start

A failing CMOS battery can have different effects on different PCs.

On certain systems, you will notice that the PC will simply not start.

Pressing the turn on button may start the fans on the motherboard for a bit, but you will see no display at all.

With everything else considered such as the cable connections and if you are certain that the your PC was working fine and it has suddenly developed this issue, then it can indicate that the CMOS battery is dying.

2. You Will See Flashes of LED on Motherboard

In some cases, if you have a motherboard with LED for troubleshooting, then you may see flashes of LED. 

These are referred to as POST (Power On Self Test) LED lights. 

motherboard flashing leds
Motherboard may flash POST LED lights indicating something is wrong (missing CPU)

You may refer to your motherboard’s manual to decipher the exact meaning of the LED flashes, but in most cases if everything else is in order, LED flashes with no display can indicate a dying CMOS battery.

3. You Will See Errors on the POST Screen on Startup

Power On Self Test or POST is a test that all computers conduct in order to see if all the critical hardware is in order.

If not, then the POST screen indicates where the issues lies.

In some cases, the issues the POST screen shows are ambiguous such as this:

what happens when cmos battery dies
Source: Dell

This error simply points to the fact that there is an issue with the time “Time-of-day not set”. It doesn’t tell you exactly that the issue lies with CMOS battery.

However, since we know that CMOS holds the real time clock, it is thus most probably pointing to a dead CMOS battery.

Other times, the POST screen may show errors related to hard drives. One of the key words that you will notice in most issues pertaining to a dead CMOS battery is:

Invalid Configuration Information

CMOS holds the configuration information of your hardware. Hence, a failing CMOS battery can lead to hardware configuration being inaccessible. 

If the hardware information cannot be accessed, the hardware cannot be initialized and which would thus lead to a system wide error.

4. Beeps if You Have a Beep Code Speaker

Beep code speaker
Beep code speaker

Finally, if you have a beep code speaker, or the system internal speaker, then you may hear a constant buzzing sound. 

The amount of beeps a beep code speaker produces indicates where the problem lies. So 1 beep refers to memory problems, 5 beeps refers to CPU problem, 10 beeps indicates issues related to CMOS.

For the entire list, refer to this article.

Also Read: What Should I Put Under My PC?

Fixing the Problem – Replacing the Battery

CMOS related issues are perhaps the simplest to fix. Often people confuse CMOS related issues with more complex issues such as a dead CPU or a motherboard.

However, before you discard your expensive hardware as being defective, always replace the CMOS battery of your motherboard to check and see if that fixes the issues.

Cmos battery remove
Try replacing the CMOS battery of the motherboard for starters.

All CMOS batteries are replaceable. The battery sockets have a small latch on the side which when pressed with a tool like a screw driver can pop the CMOS battery out of its socket.

Once the battery is out of the socket, it is then just a matter of replacing it with a shiny new CR2032 3 volts battery.

You can also test your old battery with a multimeter to check if its voltage level is significantly lower than 3 volts. 

Also Read: What is a Bricked Motherboard?

Do Not Cheap Out on the Battery Brand

It is highly advisable that you do not go for the basic CR2032 brands. Not only do the generic batteries have a lower life, they are also prone to heat damage.

Hence, I would always advice to choose a battery from the respectable brands such as Duracell, Energizer, Panasonic etc.

Removing or Replacing CMOS Battery Resets the BIOS Settings

It should be noted that removing or replacing the CMOS battery resets the system clock as well as any previous BIOS settings.

Replacing or removing the CMOS battery will have NO effect on your operating system, however, if there were any custom settings you had performed on BIOS such as changed the boot sequence, overclocked your component etc, all of that would go back to its factory default.

Also Read: How to Tell If Motherboard is Bad or Dead?

Final Words

So what happens when CMOS battery dies? Well nothing but the worst. Your PC will simply not start if the CMOS battery dies.

If you are experiencing issues with system start up or if you have noticed that your PC sometimes starts but other times it will simply not boot up, then that can indicate an issue with a CMOS battery.

Fortunately, replacing the CMOS battery is very simply.

Photo of author


Atif Qazi
Atif Qazi is the founder of PCGuide101. He is a digital nomad who loves everything PC. He is a PC builder, tech enthusiast, engineer, and a lover of single player lore-rich RPG games.

1 thought on “What Happens When CMOS Battery Dies?”

Leave a Comment