Request advice on changing CMOS battery

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I look after a network of six computers for a club.  On one computer, the
CMOS battery (CR2032) died today, and I replaced it.

However, I had to re-set several BIOS settings to get it back to operating
normally.  Did this by starting up another computer with the same
motherboard and looking at the settings known to work OK.

Now I've decided I should replace the other CMOS batteries as all the
computers are about the same age, and better to replace now than wait till
they fail.

What I would like to know is, can the CMOS batteries be removed and
replaced safely whilst a computer is running (thus preserving the BIOS
settings during the operation), or is there a risk of corrupting the BIOS
settings in the process?


John S

Re: Request advice on changing CMOS battery

John S wrote:

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The capacitor across the battery should provide you with a minute in
which you can replace the battery before the capacitor's voltage drops
too low to retain the settings saved in the modified CMOS copy of your
BIOS settings.  If the CMOS battery is completely dead, you already lost
the modified settings in the CMOS after you powered down the host.  If
the host is powered up, the battery isn't used to retain those modified

Otherwise, use the option to reset the BIOS to its standard settings
(which won't be the fastest or optimal but will work for most mobos and
then you decide what to tweak up after that).  If you lose some CMOS
settings due to loss of power then it is best not to use what remains
since it be corrupted.  Instead short the jumper to reset the CMOS after
replacing with a new battery.  This forces the load of the default
settings from the EEPROM for the BIOS and copies into the CMOS table.

If the battery is already dead, it's not likely that any tweaked changes
to the default BIOS settings are still in the CMOS copy.  You'll have to
start from scratch.  If the battery isn't dead yet (or the host hasn't
been powered down during which the battery became too weak) then leave
the host powered up when you replace the CMOS battery (obviously there
will be power so be careful).  If the CMOS battery isn't completely dead
yet (i.e., settings are still retained during power cycling) then you've
got a minute or two before the capacitor discharges and you lose any
modified settings in CMOS.  If none of the BIOS settings were modified
in the CMOS copy then you don't care because you'll be resetting back
the defaults after replacing the battery.

For your hosts that still retain their BIOS settings between power
cycles (i.e., the battery is still good enough to hold the settings in
the CMOS copy), just power down those hosts, replace the battery in
under minute, and power them back up.

Re: Request advice on changing CMOS battery

John S wrote:
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It is true, that the CMOS well is powered by two power sources.
The +5VSB rail of the power supply, is one source, while the
CMOS battery is the other. So there is some room for playing
around. As long as one of the sources is present, the CMOS
remains powered. The 1K ohm resistor, between the battery socket
and the common point, provides some isolation if you were
pulling the CR2032 out of its socket. And the diodes in the path,
even prevent a short in the socket, from pulling down +5VSB.
So the circuit is relatively well protected - part of the
reason that protection exists, is to prevent the battery from
getting charged by the motherboard. (That battery type is not
safe to charge.) The diodes only allow current flow in one direction.

      +5VSB --- (regulator) ------ diode --+
                                           +------ CMOS well on Southbridge
                                   --->    |
    CR2032_battery --- resistor -- diode --+

When you pull the battery from its socket, it can flop around
on the surface of the motherboard. Ideally, you wouldn't want
power to be present on the chassis, if that was the case. The
battery could contact something it shouldn't and cause damage.

If you power off the computer, there may be enough capacitance
on the CMOS well power rail, to maintain settings for a short
time. You can take a chance on that if you want.

Or, you can use a digital camera, to record the BIOS screens,
so you have a reference for later. You can even do that now,
and not spend a penny on batteries. If, one day, another
battery fails, replace it, and use your collection of
digital camera photos, to set up the BIOS again. So if you
take digital camera photos of all the BIOS screens now,
you'll never have to worry about this again.


Re: Request advice on changing CMOS battery

John S wrote:
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    There are several methods to avoid the loss of the settings as
mentioned, but there are also small programs that operate via DOS or
Windows that can save them.  One I have used is CMOSRAM2.  If you search
you will find others that will save your current settings if you fear
their loss during the battery replacement process.

Re: Request advice on changing CMOS battery

On Wed, 4 Nov 2009 17:57:39 +1300, John S wrote:

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Just a follow up to my own post, many thanks for the helpful advice and
information from Vanguard LH, Paul, and Ken.

I have now successfully replaced batteries on three of the computers whilst
they were powered on, but first took the precaution of recording all the
BIOS settings as suggested.  

The suggestion of photographing the BIOS settings is a neat idea, though I
found  it simpler to write the settings down, as I could get the lot on one
sheet of paper (using a bit of shorthand), which is easier for me to refer
to later.

I managed to find and download the CMOSRAM2 utility suggested by Ken, and
will use it to make an electronic record of the settings.

Thanks again for your assistance.


John S

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