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Replacing a computer's CMOS battery isn't difficult, but since CMOS batteries last so long it's often not even necessary.

In all desktop PCs there is a battery - what is the purpose of this battery and how long does it last before it has to be replaced? Does the replacement of this battery affect the BIOS of the computer? I've had a PC for 6 years now and never replaced the battery.

Actually all PCs, desktop and laptop alike, have a battery like this, typically referred to as the "CMOS" battery. It serves an important function, even though it's actually possible on most machines to run without one ... which quickly becomes annoying.

The good news is that, as you've seen, they typically last for years.

"CMOS" stands for "Complementary metal-oxide-semiconductor", the original technology that was originally used in some of the circuitry that the battery was used to power. While no longer the case, the term "CMOS" or CMOS battery lives on anyway.

The purpose of the CMOS battery most often today is simply to allow your computer to remember what time it is. I think of it as a glorified watch battery. If the battery dies or is removed, then when your computer boots it will have forgotten the current date and time.

Typically the CMOS battery no longer affects whether or not BIOS settings are remembered. Most of those settings are kept in a type of dedicated flash memory, not unlike the kind that might be inside a USB flash drive. In the past, back when the battery was actually powering "CMOS RAM" to retain those settings removing the CMOS battery would cause your BIOS to lose not only the date and time, but also any customizations you might have made to the BIOS configuration.

The reason you still need a battery is very simple: no matter what happens to the computer, be it unplugged or turned off, the clock needs to keep running in order to keep track of the current time.

"I think of it as a glorified watch battery. If the battery dies or is removed, then when your computer boots it will have forgotten the current date and time."

And that's by far the most common symptom: one day you boot up your machine and it think's that it's January 1, 1980, or some other random date years in the past. That's when it's time to replace the battery.

For the record, I've never proactively replaced the battery. I just wait until it dies, and replace it then. These batteries last so long that more often than not the computer is old and no longer in service anyway by the time that happens.

On desktop PCs the battery is typically relatively easy to get at and replace. All you need do after removing power is open the case and look on the motherboard and it's likely there.

Laptop users have a harder time. Not only are laptops typically not designed to be easily opened, the components are often in hard to reach places. It's not uncommon to partially disassemble a laptop in order to reach the CMOS battery. I did have one case where I just let the battery die and didn't replace it ... it was too difficult.

I've avoided a lot of specifics because, quite frankly, the specifics vary tremendously from manufacturer to manufacturer, and even from battery to battery. Some might last a year or two, others a decade. Some BIOS will lose only the time, others will lose more. Some machines are designed to make it easy to replace the battery, others not so much.

My advice: don't worry about it until your PC starts losing the current time and date on every boot up. Then check with your computer's manufacturer for instructions on replacing the CMOS battery, and which battery to replace it with.

Article C3330 - March 24, 2008 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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24 Comments
Ken B
March 24, 2008 1:47 PM

It's been so long since a computer of mine has had the CMOS battery die, that I honestly couldn't tell you what would happen on any of the computers I currently have. But, as you said, back in "the good old days", that battery kept every BIOS setting you had, and removing it would reset everything to the manufacturer's default.

While most systems use a standard CR2032 battery, I did see one system years ago which used some customer-looking battery that was actually soldered to the motherboard. Needless to say, we didn't replace it when it died.

Houssam S.Mousa
March 24, 2008 9:25 PM

Actually, I found out that in Laptops you Really never will have to replace the battery, as I noticed one time when I had to remove the CMOS Battery that the laptop uses it's Battery Pack for thismatter also, I do not know if this is a standard for all laptops but my Acer Does uses it's own battery instead of a CMOS CR2032 battery

Mark
March 24, 2008 9:36 PM

A random thought here. I have had computer troubles fairly recently where one trick recommended was to re-seat the CMOS battery. Is there anything else that the battery can influence?

Eli Coten
March 29, 2008 2:16 PM

It should be noted that the CMOS battery may not be used at all when the machine is plugged in (or in the case of a laptop, when the main battery is present).

Therefore its only function would be to keep the time if the computer was unplugged or the laptop's main battery removed.

ryan
July 11, 2008 12:34 PM

I have an older computer and the CMOS battery powers the BIOS 24/7. It died. I would replace it if I knew were it was. Just can't find it.

Adesh
September 30, 2008 12:00 AM

I removed my CMOS battery from my Mecer Laptop for +- 10 min to flash the BIOS as i had a Worm that was corrupting my computer. I first tried a fresh re-installation. This did not work. Now that i removed my CMOS - thinking that the worm might be corrupting my BIOS, My Screen just stays off, No POST when booting up. My CPU runs and the Battery light does come on. The HDD light does flicker when the laptop is turned on. How can i get my screen back on and how can i use my Laptop now. NB: I did recal removing the RAM so i could get to the CMOS battery.

You removed the CMOS? That's the problem - PC's require the CMOS be in place and the BIOS be installed in order to do anything.
- Leo
29-Sep-2008

Donald
September 30, 2008 8:44 PM

I realize this may be a old thread, but I'm haveing a major problem with my cmos battery. It keeps dyeing, Or if the battery voltage drops any, it won't boot up all I get is a blank screen. When I put in a fresh battery, it works fine for about 2 weeks or so, Then does the same thing. I tried to flash my bios and it came up with "There was a unknown error, accessing a unknown file" I'm ready to throw this thing out the window. please help

adesh
September 30, 2008 10:59 PM

Hi, When I did boot up the laptop, the CMOS was in place. Everything was in place as it should be. NOW THAT'S ASTOUNDING. i just dont understand why the laptop screen doesn't come on. By removing the CMOS and putting it right back, Would this remove the BIOS? Any Other Suggestions.

Maurice G.
November 5, 2008 9:43 PM

I have a question, I was in my bios I left my computer for a minute,my little nephew pressed or held down the enter key and my computer locked up, I've tried using the passwords that was set, my computer dosen't recognize them anymore, can you tell me what has happen or if I have any recourse ? I was thinking I could change my cmos battery .... after reading some threads I don't think so

larry adams
March 13, 2009 3:01 PM

i was having problems with my software shutting down for no reason, i replaced the cmos battery, and now everything is working with no crashes.i wasnt sure if this would fix the problem, but i had to try the cheapest way first. it was simple to replace, and i got it in a two pack at walmart for 1.99 in the watch section. just glad it wasnt somthing major.

jennifer
April 2, 2009 12:13 PM

May be, you can help me, i have a laptop acer aspire and then suddenly its keyboard, touchpad usb port does not work. My question is what may be the cause, I ask someone that have knowledge and he said that it may be the cmos battery. But when i open my laptop i might see a battery but it is attached with the motherboard. Can you help me by answering if the cmos battery can cause this? And if that's so, what can i do?

Actually I'd be surprised if it were the CMOS battery. Sounds more like a loose cable or other hardware failure inside the laptop. I'd have a technician look at it.
- Leo
03-Apr-2009

Marius
July 12, 2009 12:58 AM

i am having problems with my Acer L320 mini pc.. the cmos battery is dead i think, so am i able to get it other than going back to acer? It uses a different battery though..CR2032CL-3.. any help?

I'm not familiar with the specific battery/model, but if you can get a compatible replacement battery and feel comfortable replacing it yourself - or perhaps have a local technician - you need not go back to the manufacturer. (Off chance you may only be able to get the battery from Acer, but unlikely. Even then you can replace it yourself if you like.)
- Leo
12-Jul-2009

faru
December 22, 2009 10:21 AM

can a low cmos battery affect the lcd display of a laptop?

Christo Present
February 19, 2010 11:45 AM

I have a mecer laptop. The problem I have is that when I press the start button to boot . I can only hear the fan run for about 10 seconds and then everything is dead again. Whan can be the problem ?

Gary Roche
March 3, 2010 2:00 AM

I have a Dell desk top that is practically new and wont boot up.I last used it about 2 years ago. I noticed there is a green LED light working on the motherboard when plugged in. If the CMOS battery is dead, would this cause the system not to boot up? Thanks.

m.sanath
May 25, 2010 1:13 AM

I have a problem with laptop.
model:lenovo 3000 Y5oo model.I don`t have "Bios" psw.Now I want get that.how it will do that.can any one explain..
& I want to chang my OS vista to sp.

I am waiting for ANS

You'll have to contact Lenovo to see if there's a way to reset that password.
Leo
26-May-2010

Dale Gupton
August 16, 2010 9:07 PM

Hi, Leo. I 1st replaced my cmos battery a couple of years ago. My pc had sat unused for several mos. during a move to another city. On my ist boot I noticed my AVG security settings were not working properly. Finally I rechecked the clock settings and noticed that the year setting had reverted back several years, probably before AVG was born. I replaced the cmos battery and as soon as the pc date, time and year got syncronized with the AVG settings, (just a few seconds), everything was back to normal. I didn't realize AVG was dependent on the pc's time. Thank you for the services you provide to us struggling "techies".

Ali
November 12, 2010 11:43 AM

Dear Sir ..

i would like to ask you about my problem .. can you help me please ..

My pc it was working fine but Suddenly Display Not Working but when i remove the battery and re plug again my pc working fine .. just i want know what is the link between battery and display ..
Please Reply to me ASAP
Warm Regards..

Ali Alsayegh .

Michael
June 30, 2011 10:20 AM

Dear Leo:Shalom Aleichem Sir! (Peace, grace and mercy to you)
I'm having major problems with a "Toshiba Tecra 8200 Windows XP 256 MB 4.0 laptop".
It is Not booting up ever since I attempted to upgrade my memory to 512 MB and also replaced
the old 3300mah main battery with a new 4400mah which was not holding a charge any longer.
I purchased only(1) memory module from memorystock.com and installed it into "Slot A."
An"Error message appeared":"Remove incompatible memory"...so I did,and reinstalled the original one.
But now it won't come on any longer! I called memorystock and one of there Rep's said:Oh,we have had this
problem in the past with other toshiba's! So I replied:"Why then would you send Me one knowing that."
I did manage to restart it a few times and receive assorted message screens...Boot options
then start windows normally,and last message was CMOS problem press F1. It is also running very hot on bottom!
The only way I restarted it was:*I pulled out both memory sticks slots A & B then I push reset slot on front a few times.
Then I held PWR button while pulling out battery to allow diodes and capacitors to drain out all current .Lastly,I reinstalled
both memory sticks,battery and held:PWR+F2...booted then hit D to boot. I have gone through this for a month now.
I repeat above steps over and over but each time I do I need to hit something else,F8,F11,but Now "Nothing is working!"
Sorry for the lengthly letter but I wanted you aware of all going on here.Todah,Thank you very much .Michka'el.

Thedog
September 2, 2011 12:53 AM

You should replace it when it says Error : User not yet born.

andrew
April 11, 2012 11:07 AM

Hi, i changed the battery of my desktop but i still have to press f1when the computer starts and adjust the time.should i change the settings after changing the battery?

Mark J
April 11, 2012 1:11 PM

@Andrew
After changing the battery, it is necessary to reset the time and date just like you would with a battery powered clock or watch.

Knoxs
November 13, 2012 7:53 PM

I have had 12 computers and 5 of them I ran with out CMOS battery's and went fine

Absolutely, a computer will RUN just fine without a CMOS battery. It's simply that if the computer is turned off (and in some cases the power also needs to be unplugged) then it will forget what time it is, and possibly other BIOS settings will revert to their default state.
Leo
16-Nov-2012

ian geer
March 7, 2013 5:22 AM

Hi have an e442 notebook, 2 years old, every time i boot, time and date resets, and sometimes have a blank screen, that clears after about 10 mins, but other than that everything ok. replaced cmos battery, but still the same, found a sort of solution, removed the the main battery, and increased percentage of cpu power from 5% to 100% and everything is fine, unless i refit the main battery, then time and date resets again, but no more blank screens.

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