Problem with the CMOS motherboard battery.

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My old XP system (from 2006)  was having booting problems, with a
halting slow and freezing startup.

I turned off the machine and removed the cmos motherboard battery
(CR2032) and found a voltage reading of 1.5V which is half the nominal

I bought a new one, and all was mended, with a rapid startup and even
a faster screen shortcuts loading.

I need to know:

1/ Do I have to make any alterations in the setup &/or cmos menus?

2/ Why is there no warning of a failing cmos battery?   And is there
such a warning for my new Windows7 system?


Re: Problem with the CMOS motherboard battery.

On 7/22/2011 9:34 PM, Peter Jason wrote:
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1. Only if you made changes from the default originally.
2. I'm not aware of any machine/motherboard/operating system
that has such. That battery is pretty much  isolated
from the rest of the motherboard parts.

Re: Problem with the CMOS motherboard battery.

Peter Jason wrote:
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1) If you needed to make custom BIOS settings at some point,
    you'd need to reload those. Usually, it would be something
    along the lines of either boot order, or disk controller
    operating mode. If you can boot the machine, chances are
    it's all right as is. Sample disk controller operating
    modes are IDE (Enhanced or Compatible), AHCI, or RAID.
    IDE works with most OSes (within reason), while the others
    require some driver work. Modern laptops and desktops come
    set to AHCI by default (since Vista/Win7 support AHCI out
    of the box).

2) None of my machines here, warn about the battery. I've
    seen SuperI/O chips with a VBAT input or the like, but
    the capabilities of SuperI/O chips, seldom seem to match
    what is displayed in the BIOS hardware monitor (or a Windows
    application like SpeedFan/MBM5/Asus Probe etc).

    I just wait for mine to fail, as it's too much trouble to be
    checking them all the time. They can be checked with a multimeter,
    while the battery is sitting in the socket. Use a low DC voltage
    readout range on the multimeter (measuring 3V on say the 20VDC range).
    Connect the multimeter black lead, to a grounded screw in the
    I/O plate area on the back of the computer. Then probe the top
    surface of the battery with the red lead. At around 2.7V or so,
    the battery may be down to the last 10% of its life. At 2.3V,
    the real time clock (RTC) and CMOS cells, as just about to
    wink out and lose settings or stop telling the time. The chipset
    is rated for 2.0V operation, but a Schottky diode in the path,
    accounts for 0.3V or so more voltage drop, raising the acceptance
    level to 2.3V or so. The distance from 2.7V to 2.3V on the battery,
    is about 10% of its operating life.


Re: Problem with the CMOS motherboard battery.

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Thanks, I guess this will need to be replaced every 2 years.  They
only cost $5


Re: Problem with the CMOS motherboard battery.

Peter Jason wrote:
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The current draw is around 10 microamps. It could vary, from one
chipset to the next, especially as things like leakage currents
vary with geometry.

Based on the 10 microamp number, a CR2032 lasts around 2.5 to 3
years, when the computer is left unplugged during that time. If
you keep the computer powered (run it in sleep mode, otherwise
known as "suspend to RAM" mode), the battery will last for as long
as its shelf life, which could be ten years. But if you unplug
the PC and store it in the garage, the battery lasts 2.5 to 3 years,
as then the CR2032 is the only source of power for the CMOS/RTC.
In something like "suspend to RAM" mode, the +5VSB on the ATX
supply is still running, and it powers the CMOS/RTC and zero
current flows from the battery. That's why the battery lasts for
ten years.


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