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CMOS Battery Replacement


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Koot's Avatar
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22-Jan-2009, 10:40 PM #1
CMOS Battery Replacement
My PC is about 7-8 years old and I would like to change the CMOS battery on the motherboard before it fails...instead of after it fails. My question pertains to finding out if I can change the battery with the PC "On" so I don't lose my BIOS settings.

Can anyone tell me if the motherboard powers the CMOS without the battery in the clip as long as the PC is "On"? In other words, if I have the PC "On" and remove the CMOS battery will I still lose the BIOS settings because no voltage would be going to the CMOS chip? Or is voltage supplied to the CMOS even without the battery as long as the PC is "On"?

My motherboard is an ABIT AT7 MAX2 (VIA KT400-8235)

Another thing I thought about doing was changing the battery with the PC "Off", but feeding 3 volts to the battery clips (to keep voltage on the CMOS) as I remove the old battery and install the new battery. (This is what many auto dealers/shops do when changing a car battery so the car's radio doesn't lose its channel settings and theft code.) This would be done by clipping leads from a spare CMOS battery to the battery holder, which would supply 3 volts in parallel with the old battery prior to removal and during the few seconds before a new battery can be installed. Any thoughts on this?


Also, does anyone know which battery my motherboard takes? It looks like a CR2032, but I'm not absolutely sure. Confirmation would be helpful.



Thanks!

Last edited by Koot; 22-Jan-2009 at 10:52 PM..
Jack Hackett's Avatar
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22-Jan-2009, 10:51 PM #2
Yes you can change the BIOS battery with the pc powered on, I have removed and fitted them with the pc on in the past and have had NO ill effects whatsoever, just take the usual ESD precautions etc

CR2032
Alex Ethridge's Avatar
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22-Jan-2009, 10:55 PM #3
Power is supplied to the CMOS when the computer is on; however, replacing the bettery while it is on is risking accidentally touching something with the battery that might damage the board. Having written that, I will also write that I have done that very thing scores of times without incident.

Just be careful.

Clipping leads to the battery's contacts while changing the battery would be next to impossible because of the size and the way the battery fits into its holder.

Losing the settings isn't a big deal. You can always just choose one of the default settings and the computer will boot just fine.

It's been so many years since I ran across a computer that didn't use the CR2032, I can't remember when it was.
Koot's Avatar
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22-Jan-2009, 11:12 PM #4
Thanks guys! That's great news. I was hoping the CMOS would still get voltage from the motherboard without a battery as long as the PC was "On". I would prefer to change the battery with the PC "On" instead of losing the BIOS settings and going default. The battery is mounted vertically (as shown in above image) so I think it should be easy to get to and change without a problem...but I'll be careful not to drop the battery and short anything, and also use ESD precaution.

I'll forego feeding voltage to the battery clips in parallel with a spare battery since the motherboard will keep 3 volts on the CMOS. I wasn't sure if that would be easily done, but was just a thought if needed.

Also, thanks for the confirmation on the battery being a CR2032.

Any other thoughts would be appreciated.

Thanks!
Koot's Avatar
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23-Jan-2009, 12:29 PM #5
I have another question or two...for anyone.

When I remove the CMOS battery will the positive and negative contacts touch each other and short out? Keep in mind that I want to change the battery with the PC "On". If so, should I use an insulator (see Mylar battery insulator images below) to prevent the contacts from touching and shorting out the circuit? (Please look at the style of my battery holder and contact clips in the image directly below.)


Also, Jack and Alex said they have successfully changed the CMOS battery with the PC "On" without any problems. However my question is - when this was done (changed the battery with the PC "On") are you sure the BIOS settings were actually retained...or was the PC in a state of repair where the BIOS needed to be re-setup anyway? (In other words I'm trying to determine (for sure) if the BIOS settings were retained/saved as they originally were before changing the CMOS battery.)

Thanks!

Last edited by Koot; 23-Jan-2009 at 02:05 PM..
kiwiguy's Avatar
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23-Jan-2009, 03:28 PM #6
The + and - will not short out.

Whatever settings you must have changed in the BIOS must be complex as the normal change with power off has never been a problem for me. 99% of BIOS settings are default, they automatically pick up HDD etc on power on anyway.

Be VERY careful not to break the top clip.
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23-Jan-2009, 04:02 PM #7
you might check the documentation that came with your pc,
concerning the time you need to leave the battery out to clear the cmos / bios settings,
most usually say to remove the battery for 2 - 5 minutes,
therefore,
in that changing a battery takes a matter of only a few seconds,
personally, i'd just do it with the pc off,
Koot's Avatar
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23-Jan-2009, 04:04 PM #8
Quote:
Originally Posted by kiwiguy View Post
The + and - will not short out.

Whatever settings you must have changed in the BIOS must be complex as the normal change with power off has never been a problem for me. 99% of BIOS settings are default, they automatically pick up HDD etc on power on anyway.

Be VERY careful not to break the top clip.
Thanks! I wasn't sure if the + and - contacts would meet each other and short out or not. It's nice to know they won't.

I know the BIOS settings were changed (somewhat) by a friend that built the PC. I really like the way the PC performs. I just don't want to have to change the settings back to what they are now, which is something I would need to do if it went back to the default settings.

Yes, I will be very careful not to break the top battery clip.
Koot's Avatar
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23-Jan-2009, 04:10 PM #9
Quote:
Originally Posted by daniel_b2380 View Post
you might check the documentation that came with your pc,
concerning the time you need to leave the battery out to clear the cmos / bios settings,
most usually say to remove the battery for 2 - 5 minutes,
therefore,
in that changing a battery takes a matter of only a few seconds,
personally, i'd just do it with the pc off,
I really don't want to take a chance of losing the BIOS settings or causing a quirk in the system somehow, although I have seen it mentioned that if you change the battery quickly you won't lose the settings. I could find nothing about this in my documentation that addressed it however.
crjdriver's Avatar
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23-Jan-2009, 06:34 PM #10
It really is no big deal at all to "lose" the bios settings. It just resets to default.

I would much rather have to set a few bios settings rather than try and find a replacement board for something that old. New socket A boards are very rare now.
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