Summary: 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.
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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.
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"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.
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.
Related:
Ask Leo! - Why does my clock reset on every power-on?
Ask Leo! - How do I maximize my battery life?
Article C3330 - March 24, 2008
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?
Posted by: Mark at March 24, 2008 9:36 PMIt 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.
Posted by: Eli Coten at March 29, 2008 2:16 PMI 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.
Posted by: ryan at July 11, 2008 12:34 PMI 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.
29-Sep-2008
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
Posted by: Donald at September 30, 2008 8:44 PMHi, 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.
Posted by: adesh at September 30, 2008 10:59 PMI 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
Posted by: Maurice G. at November 5, 2008 9:43 PMi 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.
Posted by: larry adams at March 13, 2009 3:01 PMMay 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?
03-Apr-2009
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?
12-Jul-2009
Posted by: Marius at July 12, 2009 12:58 AM