CMOS Battery Dead? How to Replace the CMOS Battery
Complementary Metal Oxide Semiconductor (CMOS) battery is a small, mostly button/coin cell battery that provides power to the small memory that stores information about the BIOS settings.
When the CMOS battery dies, BIOS resets its setting, including date, hard drive settings, boot order, etc., and in many situations, OS can't load properly.
When something like that happens, the only course of action is to replace the CMOS battery.
Published: January 3, 2022.
On This Page:
- What Kills a CMOS Battery?
- What Happens When CMOS Battery Dies?
- CMOS Battery vs RAID Disc Controller Battery
- Common CMOS Battery Models
- How to Replace CMOS Battery
- Few Final Words
What Kills a CMOS Battery?
On average, CMOS battery lasts 3-10 years, sometimes even more.
The drain current of the CMOS battery is very low, but over time, it discharges the CMOS battery completely - the CMOS battery dies.
Also, the temperature inside computers is usually increased, when compared with the temperature in the typical office or home - increased temperature also leads to the increased self-discharge rate.
So, constant current drain and increased temperature eventually kill the CMOS battery.
Note: when the computer is turned ON, CMOS battery drain is practically zero - PSU (Power Supply Unit) provides power for the computer, BIOS memory included. So, if You use your computer very often, CMOS battery load is decreased.
What Happens When CMOS Battery Dies?
When the CMOS battery dies, the BIOS memory is deleted leading to:
- lost time/date,
- lost boot order,
- CMOS errors of various kinds will probably be displayed on the screen (CMOS Read Error, CMOS Checksum Error, CMOS Battery Failure, etc.)
- Operating System (OS) will not load,
- some computer peripherals may not work properly,
- BIOS password reset (if it was present), etc.
CMOS Battery vs RAID Disc Controller Battery
Before continuing further, there is also one thing that we must clarify.
Many servers and even PCs with advanced disc controllers feature lithium rechargeable RAID disc controller batteries.
RAID disc controller battery provides power to the RAID disc controller memory chip in the case of power failure.
The reason is simple - in order to speed things up, when OS sends data to be written on the disc, the RAID controller stores that data in the RAID memory and signals to the OS that the data is written.
Since discs are much slower than memory, such activity speeds things up significantly.
However, if the power is lost, there is a danger of data inconsistency and corruption if the data is written into the memory, but not on the disc.
So, the RAID disc controller battery allows the RAID memory to store that data for a few days, usually 3-7 days.
And when the power is restored, data is first written from the memory to the disc, and only then the RAID disc controller gives the OS access to the data, preventing data inconsistency or corruption.
In the event of a weak RAID disc controller battery, it is possible to disable data caching on the disc controller until the new battery arrives - this slows down the disc subsystem, but that is better than data corruption.
Common CMOS Battery Models
The most common CMOS battery is a CR2032 lithium non-rechargeable 3.0V Lithium Manganese Dioxide (Li-MnO2) battery, although other similar batteries are in use, including CR2025, CR2020, and CR2016.
Note that certain motherboards may use cylindrical lithium batteries or even rechargeable batteries based on the NiCd, NiMH, or some of the lithium battery chemistries. But, this is very rare, at least in recent years.
CR2032 and similar batteries feature a wide temperature range and very low self-discharge rate, allowing them to hold the charge up to 10 years, sometimes even more.
But, if required, some devices support the use of Carbon-Monofluoride Lithium batteries, which labels start with 'B', for example, BR2032.
Carbon-Monofluoride Lithium batteries (for example, BR2032) feature a somewhat smaller capacity than Lithium Manganese Dioxide batteries (for example, CR2032), but have an even wider temperature operating range and ultra-low self-discharge rate, allowing them to easily operate 10 or more years.
How to Replace CMOS Battery
If You familiar with computers, the best course of action is to take your computer to the computer store/service and let them replace the CMOS battery for You - it will cost You more than the battery itself, but nonetheless, it is much cheaper than the new motherboard, memory modules, processor, etc.
Note: If the computer is within the warranty period, do NOT open it - take it to the computer dealership or store and let them replace the battery for You - opening the computer while under warranty, renders the warranty void!
If your computer warns You about the weak CMOS battery, backup the BIOS to the USB memory stick.
If You can't back up the BIOS, but You get warnings about the weak BIOS battery, it is (at least in theory) possible to replace the CMOS battery while the computer/server is turned ON, but this is not recommended - should You decide to do it, remember that whatever You do, it is your own responsibility.
In order to replace the CMOS battery, do the following:
- backup BIOS, if possible,
- turn Off the computer,
- unplug the power cord and press the On/Off button for 10-15 seconds,
- open the computer, without touching the internal components,
- ground yourself using the anti-static wrist strap band,
Note: If You don't have an anti-static wrist strap band, touch the computer chassis in order to remove static electricity from your hands. Actually, it is a good practice to hold the computer chassis with one hand and to the work with another hand.
- locate the battery and replace it with the new one,
- close the computer,
- plug in the computer and turn it On, go to the BIOS,
- either restore the BIOS from the backup or set it manually.
After the battery is replaced and the BIOS is set like it previously was, load the OS and check if everything is OK there.
And that would be all.
Few Final Words
As one can see, replacing the old CMOS battery with the new one is rather easy.
But, if You plan to do it on your own, note that the static charge can destroy sensitive components easily and if the computer is within the warranty period, it should be taken to the authorized service or dealership so that they replace the battery.
But, CR2032 and especially BR2032 and similar "CR" and "BR" batteries are intended to last for years - since most computers are used for 3-5 years, the possibility of having a weak or dead CMOS battery is low.
But, if that happens, the battery can be replaced relatively easily.