Bad CMOS Battery

[MrTsquare]

After buying a couple of 2032 cmos batteries last year to replace the
one in my 5-year old build. I sprung for a new build including an ASUS
Z97Pro series motherboard. Put that together in August 2014. recently,
the new build began losing time, ie the time would continue to display
the time that the system was last turned off. After resetting it a few
times and playing with the choice of Internet Time servers displayed in
Win7, I finally replaced the alledgedly 4 month old CMOS battery. Life
is good and now time marches on. ONLY 4 MONTHS ON A CMOS BATTERY??

T2

 

[Paul]

After buying a couple of 2032 cmos batteries last year to replace the
one in my 5-year old build. I sprung for a new build including an ASUS
Z97Pro series motherboard. Put that together in August 2014. recently,
the new build began losing time, ie the time would continue to display
the time that the system was last turned off. After resetting it a few
times and playing with the choice of Internet Time servers displayed in
Win7, I finally replaced the alledgedly 4 month old CMOS battery. Life
is good and now time marches on. ONLY 4 MONTHS ON A CMOS BATTERY??

T2

The important thing, is to keep track of how long the new ones last.
Long term, you may decide the motherboard has a problem.

*******

The battery should last a little less than 3 years, if the PC is
kept unplugged all the time (24 hours a day). If you run the PC 16
hours a day, and unplug each day for 8 hours, then you'd expect
9 years (or getting closer to the 10 year shelf life). If the
PC has +5VSB (motherboard green LED lit all the time, whether
sleep/hibernate/run), then there is no excuse for less than
ten years from the battery. In other words, if you don't switch
off at the back or unplug each day, the +5VSB provides the clock
current and not the battery.

The Asus motherboard box has a "Serial number" on the outside
sticker. The first two characters of the serial number are
the manufacturer date. My last purchase was "E4..........",
which would be 2014 May or so. So I know my CMOS battery,
even if the PC is left unplugged, the battery should last
another two years minimum.

These numbers are only approximate. They assume the Southbridge
CMOS well draws maybe 10 microamps or so. The CMOS battery
is only used, if the +5VSB is not running (the +5VSB runs that
green LED on the Asus motherboard surface and is proof the
supervisory circuits have power).

If you place the "Clear CMOS" jumper in the wrong position,
that could drain the battery. But you would figure that out,
based on how that messed up the settings. If the motherboard
has an electrical fault in that section of the design,
and draws more than 10 microamps, that too could
flatten the battery. There have been some defective motherboards,
where they drained one battery after another, in short order.
And it's some sort of motherboard problem.

And that's why this is a "wait and see" situation. If each and
every battery only lasts four months, then you'd suspect excessive
current draw from that battery.

You cannot draw more than 3000 microamps from the battery at
any one time. There is a 1K ohm series resistor to the battery,
which limits current flow. And that seems to be there for some
sort of safety reason, is all I can figure. So at least you know
the drain rate cannot exceed more than 300x the normal rate
The battery should last at least one day, even with the worst of
luck.

Paul

 

[MrTsquare]

The important thing, is to keep track of how long the new ones last.
Long term, you may decide the motherboard has a problem.

*******

The battery should last a little less than 3 years, if the PC is
kept unplugged all the time (24 hours a day). If you run the PC 16
hours a day, and unplug each day for 8 hours, then you'd expect
9 years (or getting closer to the 10 year shelf life). If the
PC has +5VSB (motherboard green LED lit all the time, whether
sleep/hibernate/run), then there is no excuse for less than
ten years from the battery. In other words, if you don't switch
off at the back or unplug each day, the +5VSB provides the clock
current and not the battery.

The Asus motherboard box has a "Serial number" on the outside
sticker. The first two characters of the serial number are
the manufacturer date. My last purchase was "E4..........",
which would be 2014 May or so. So I know my CMOS battery,
even if the PC is left unplugged, the battery should last
another two years minimum.

These numbers are only approximate. They assume the Southbridge
CMOS well draws maybe 10 microamps or so. The CMOS battery
is only used, if the +5VSB is not running (the +5VSB runs that
green LED on the Asus motherboard surface and is proof the
supervisory circuits have power).

If you place the "Clear CMOS" jumper in the wrong position,
that could drain the battery. But you would figure that out,
based on how that messed up the settings. If the motherboard
has an electrical fault in that section of the design,
and draws more than 10 microamps, that too could
flatten the battery. There have been some defective motherboards,
where they drained one battery after another, in short order.
And it's some sort of motherboard problem.

Interesting, Paul. Your figures on their lifetime do match my more
limited experience as I am used to them lasting 5 years on a desktop. I
don't believe I have the green led you refer to but then I have the
whole thing turned off probably 20 hours a day, but then that's just
like the like old ones that lasted 5 years. The original box for the MB
is long gone and I'm not going to open the case to check, but the MB
manual is dated April 2014. What occurs to me is that the ASUS stock of
CMOS batteries may be old. Maybe they got a good deal on them 4 or 5
years ago... ;<)

T2

 

[Paul]

Interesting, Paul. Your figures on their lifetime do match my more
limited experience as I am used to them lasting 5 years on a desktop. I
don't believe I have the green led you refer to but then I have the
whole thing turned off probably 20 hours a day, but then that's just
like the like old ones that lasted 5 years. The original box for the MB
is long gone and I'm not going to open the case to check, but the MB
manual is dated April 2014. What occurs to me is that the ASUS stock of
CMOS batteries may be old. Maybe they got a good deal on them 4 or 5
years ago... ;<)

T2

Asus motherboards usually have a green LED. It's one of the few
LEDs that they have consistently provided over the years. It's
to prevent damage while working on the PC - if the green LED
is lit, don't add or insert any RAM, PCI, PCI Express cards.

Other brands don't connect a LED to the +5VSB, so you have no
warning about when it is safe to work on the PC.

*******

My own informal records seem to get pretty close to the
3 year figure. And that's for machines that sit in storage,
unplugged.

There are datasheets for the CR2032 cells, and you can
divide the milliamp-hour rating by 10 microamps to work out the
expected "unplugged life" of the battery.

What's quite curious, is why the figure is 10 microamps.
A digital watch is rated 2 microamps, to perform the same
function. Why do all the Southbridges cluster around the
10 microamp number so closely ? You'd think some motherboards
would do significantly better than others, and there would
be a spread on battery life. My limited experience here,
suggests the results cluster quite closely, for something
that could have a random leakage component.

Paul

 

[MrTsquare]

Well at least its nice to have a "canary" like the time display to tell
you its time to replace. Concerning the green light... After reading
the right section of the book and powering down to verify, the Z97pro
has a little "Power On" red light/push button right below the CMOS
battery that performs the safety feature you discribed as well as the
ability to "kitchen table" turn on test the MB.

T2

 

[Paul]

Well at least its nice to have a "canary" like the time display to tell
you its time to replace. Concerning the green light... After reading
the right section of the book and powering down to verify, the Z97pro
has a little "Power On" red light/push button right below the CMOS
battery that performs the safety feature you discribed as well as the
ability to "kitchen table" turn on test the MB.

T2

Interesting. So the ole Green LED got the boot
It used to be a T-1 3/4 sized LED, which is a bit out of
style now. The surface mount LEDs would be more popular with
the staff on the soldering equipment (reflow machine).

I downloaded the manual, but missed the illuminated switch detail.
I was scanning for a "LED" feature.

Paul

 

[bruce56]

I have built and bought dozens of PCs over the last 20 years.
I have had at least 3 PCs with this problem, they were LGA775 or LGA1156
boards, but different makes. The battery would go flat after a few months.
So eventually it would boot up and say I have lost my settings,
hit FX to load failsafe defaults and continue or something like that.
I replaced with known new good batteries and they expired too.
Yet these same boards had no other flaws at all.
So I am puzzled what sort of fault causes this.

 

[Paul]

I have built and bought dozens of PCs over the last 20 years.
I have had at least 3 PCs with this problem, they were LGA775 or LGA1156
boards, but different makes. The battery would go flat after a few months.
So eventually it would boot up and say I have lost my settings,
hit FX to load failsafe defaults and continue or something like that.
I replaced with known new good batteries and they expired too.
Yet these same boards had no other flaws at all.
So I am puzzled what sort of fault causes this.

Stick your multimeter across the 1K ohm current limiting
resistor that comes from the battery.

If the load is drawing 10 microamps, times 1K ohms, that would be
10 millivolts (if normal). Any value higher than 10 millivolts,
you'd have to figure out where the current was going, and that
would not be easy.

It could be excessive Southbridge leakage, but just as easily,
a mistake in the motherboard design, causing the Southbridge to
leak. Big chips like that, you have to be really careful about
how the logic inputs are strapped when they're not being used.

The CMOS/RTC circuit block sits in a "well", with transmission
gates for interconnect to the rest of the chip. That scheme is
intended to reduce leakage out of the well area, into other
(unpowered) circuits. And keep the current draw under 10
microamps.

*******

I learned about I/O leakage, on my first job. We had a
setup like this.

Power Power
Supply Supply
| |
Circuit --- I/O ------- Circuit
Board signals Board

We turned off one of the supplies in the lab, expecting
the circuit board to be safe to pull out of the system.
Instead, the circuit board continued to run. Sufficient
power flowed through the I/O signals, to keep the
second circuit board running, including lighting up
up the status LEDs on the faceplate. One side was powered
by 5V. And the "phantom powered" circuit on the right,
it was running at somewhere between 3.6V and 4V or so. The
leakage doesn't charge up the other circuit to 5V or anything,
but enough power is available, that it was still performing
its logic function. This holds for "lightweight" circuits,
like stuff using CD4000 series logic. Any logic family with
a huge appetite for power, the leakage isn't enough to keep
them running.

The method Intel is supposed to use, is like this. The
CMOS/RTC would be the circuit on the left. The circuit
on the right would be the rest of the Southbridge. Killing
the PSU output, causes the power to go off on the right hand
side. The transmission gates are then set to "open circuit",
so the current can't leak across. That helps the battery
holding up the stuff on the left, to not end up powering
the rest of the Southbridge on the right hand side.

Power Power
Supply Supply
| |
Circuit --- transmission---- Circuit
Board gate Board

HTH,
Paul

 

[Flasherly]

I have built and bought dozens of PCs over the last 20 years.
I have had at least 3 PCs with this problem, they were LGA775 or LGA1156
boards, but different makes. The battery would go flat after a few months.
So eventually it would boot up and say I have lost my settings,
hit FX to load failsafe defaults and continue or something like that.
I replaced with known new good batteries and they expired too.
Yet these same boards had no other flaws at all.
So I am puzzled what sort of fault causes this.

Had, once, a subbrand newer-name MB that would simply lose it's CMOS
settings regardless the battery. Either of my Gigabytes AMD/Intel 775
are both flawless or near so after years and years solid running. Then
for a Gigabyte it would nothing short of scandalous to expect anything
worse. Before Gigabyte I ran for a long time with Asus, now no longer
a fair value consideration and among the most expensive. Before that,
MSI, which interestingly has made something of a competitive comeback
after nearly fading out.

Stick to the best and there's no really no reason to have to deal with
any of that nonsense.