If your CMOS battery has died or if you are simply just curious, then you may wonder what the little coin cell battery in the middle of the motherboard does.
This may beg several questions such as why have it in the first place and can computer run without CMOS battery at all.
The short answer to the question is that a CMOS battery is very important as it stores BIOS settings and the Real-Time Clock. In majority of the cases, your computer CAN run without a CMOS battery. However, in some cases you may experience issues with booting.
In the following text I will talk in detail about CMOS battery in general and what would happen without it.
CMOS Battery in Brief
Very briefly speaking, CMOS stands for Complementary Metal Oxide Semiconductor. This is basically a chip that is powered NOT by the wall-electrical supply but through the coin cell battery on the motherboard.
CMOS is an important chip as it stores the Real-Time Clock as well as the BIOS settings. BIOS settings can also include your boot device settings if you have multiple drives. The boot device settings or priority tells the PC which drive has the operating system installed.
Other settings can include your overclock settings, DRAM frequency settings etc.
The CMOS battery stores these settings in real time. Meaning even if your PC was to be disconnected from the wall socket, the settings in your BIOS as well as your PC’s clock will remain intact and up to date.
So Can Computer Run Without CMOS Battery?
The answer to the question isn’t straightforward. There are cases in which the computer will run without a CMOS battery but there are some cases where your PC will malfunction.
Computer Will RUN If There is NO Boot Order
When you remove the CMOS battery, all the settings including the Boot Device Priority RESETS!
Boot Device Priority, as mentioned earlier, is a setting that tells your PC which drive to access for the operating system.
If your PC only has a single hard drive, be it SSD or HDD, then it will typically also be the drive containing the operating system.
In this case, even if the CMOS battery is removed and your BIOS settings change to their default state, your PC will have no issue in booting. This is because even in the default state, the PC will have no option but to access the singular hard drive to boot from.
It should be noted, however, that while your PC will run and boot in this case, the clock will have to be set each and every time you start your PC.
Computer MAY NOT RUN If There are Multiple Drives
You set up a Boot Device Priority in BIOS when you have multiple drives installed but only one of them has the main operating system installed, you will thus have to select the appropriate Boot Drive to start the PC with.
Since this BIOS setting is stored in the CMOS chip as well, when the CMOS battery is removed the setting also resets to its default.
In such cases, when you start your PC, you may see “Disk Boot Failure” error. In this particular situation, this error does NOT entail that the hard drive is failing or that your motherboard has malfunctioned.
It simply means that the hard drive that has the operating system installed CANNOT be accessed in the default state of BIOS settings.
There are two ways you can circumvent this issue.
- Plugging the Boot Drive into Default Boot Slot
- Changing the Boot Drive Order
1. Plugging the Boot Drive into Default Boot Slot (For Multiple Hard Drives)
A motherboard typically has multiple M.2 or SATA slots for plugging your hard drives into. Take the following motherboard for instance. This motherboard has six SATA ports.
Typically, when CMOS battery is removed and as the BIOS settings and the Boot Device Priority turn to their default state, the first SATA port is what the PC turns to for finding the Boot Drive when you turn it.
If it cannot find the boot drive in the first, or the default, SATA port, it gives the Disk Boot Failure error.
You can generally tell which SATA port is the first or the default SATA port by its number on the motherboard. For instance in the motherboard above SATA3_0 is the default SATA port.
As such, you should always aim to install the boot drive, or your drive with the operating system, into the default SATA port. Now when the CMOS battery is removed, your PC will still be able to access the boot drive in its default settings.
Another way to look at this is that the non-boot drives should ALWAYS be installed in SATA ports that are further down the priority line in relation to the boot drive. Meaning, a non-boot drive should not be installed in the SATA_2 port while the boot drive is installed on the SATA_3 port. The boot drive in this case would have to be plugged into the SATA_0 or SATA_1 slot.
The same is true for M.2 drives and SSDs installed into the M.2 slots.
2. Changing the Boot Drive Order
Another way to circumvent the issue of Boot Drive Failure in case of multiple drives installed is to change the Boot Drive order when you turn your PC on.
When in POST, you can press a certain key such as F1, F8, or Del key (the specific key differs from PC to PC), to access the Boot Drive order.
When in the Boot Drive Order menu, you can then choose the drive that has the operating system installed to start the PC.
However, you will have to do this every time you start your PC when the CMOS battery is removed since this setting will not be stored in the CMOS chip.
POST Issues When CMOS Battery is Removed
There are some instances when your PC will simply not start without the CMOS battery even when you have taken care of the boot drive order.
In such cases you may see POST error. POST stands for Power On Self Test and this is the first test the PC goes through in order to check if all the core hardware is intact.
In case of failure you may hear beeping sounds or you may see LED lights flashing on the motherboard. You may also see errors related to CMOS itself displayed on your monitor i.e “CMOS Checksum Error”, “CMOS Checksum Bad” etc.
All motherboards are designed differently, so you can never truly put your finger on a singular issue that can be caused if the CMOS battery is removed.
Long story short, it is obviously not ideal to start your PC without a CMOS battery.
While in majority of the cases you should be able to run without CMOS battery installed. In other cases it may open a can of worms.