Helping people with computers... one answer at a time.
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.
Article C3330 - March 24, 2008
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 .
Posted by: Ali at November 12, 2010 11:43 AMDear Leo:Shalom Aleichem Sir! (Peace, grace and mercy to you)
Posted by: Michael at June 30, 2011 10:20 AMI'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.
You should replace it when it says Error : User not yet born.
Posted by: Thedog at September 2, 2011 12:53 AMHi, 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?
Posted by: andrew at April 11, 2012 11:07 AM@Andrew
Posted by: Mark J at April 11, 2012 1:11 PMAfter changing the battery, it is necessary to reset the time and date just like you would with a battery powered clock or watch.