"Lee Harris" said:
Now this might sound really stupid, but I'm getting this message frequently
when I boot up (Dell Dimension 8300 I think), the PC is probably a couple of
years old - but I thought the motherboard battery would recharge when you
have the power on? I do have my PC on for up to 20 hours per day most days
Is this an easy/safe thing to change - I've put together my own PC a couple
of times but believe it or not, have never had to fiddle with the
motherboard (I assume by "system battery" that's what it means) battery
It still allows me to continue booting with F1 (or setup on F2) but is this
harming my system or just running slower than normal?
Do I need a generic battery or one specifically obtained via DELL?
thanks in advance
LeeH
This is what a typical CMOS powering circuit looks like.
The lower diode prevents the battery from being charged. Current
can only flow out of the battery, not back in. The battery
is never called on to provide a large current, and that
is why they can get away with the 1K ohm resistor in
series with the battery.
|\ |
3.3V_standby -----------------| \|-------+
reg from 5vsb |/ | |
|
1K ohm |\ | | To Southbridge
Battery ----/\ /\ -----| \|-------+-----+---------->
CR2032 \/ \/ |/ | |
Lithium resistor diode --- 1uF
3.0V nom, --- filter
min about 1.4V | cap
by spec ICH5 |
plus diode drop __+__
___ GND
_
The purpose of the two diodes, it to implement an "either-or"
scheme. When you switch off the computer via the switch on
the back, or unplug the computer, the upper power source is
off. Then, the battery provides a tiny current through the
lower diode.
When the computer is plugged in, and the switch on the back is
ON, then the upper power source is engaged. If that power
source has a slightly higher output voltage, the current flow
from the battery is cut off. In other words, it is possible
no battery current is used while your computer is running for
its 20 hours a day - and neither would the battery be used
if the computer is in a sleep state. Only killing power via
the switch on the back, puts the motherboard back onto
battery power. If the upper voltage was weak for some
reason, then a tiny bit of current could be drawn from
the battery.
The CR2032 battery is common to a lot of ATX motherboards, so
should be readily available, even at places like Radio Shack.
If you have a multimeter, you can measure the battery, and
if it is ~3.0V, then leave it alone, as it is fine. The spec
sheet for the ICH5 says it has a 1.0V minimum, but you have
to add a diode drop, which at the low current involved, might
be on the order of an additional 0.4V. So, if the voltage at
the battery drops below about 1.5V, then the voltage point
after the diodes will drop below the minimum needed by the
Southbridge. The result in that case, would be the CMOS clock
could stop, or thing stored in the small CMOS RAM logic block
inside the Southbridge could be erased.
You can enter the BIOS and write down your settings before
changing the battery. A Dell might not have that much
in the way of custom settings, but you might have a look
anyway. Based on user accounts, if you are not too slow
in removing the old battery, and putting the new one in
place, the "filter cap" on the right can hold enough
energy to run the CMOS clock for a short period of time.
On a digital watcn, the current to run a 32768 Hz oscillator
is about 2 microamps, so the Southbridge shouldn't gobble
up the charge on that capacitor too fast (the ICH5 spec
sheet says the battery current is 6 microamps).
Consult your manual, to see if there is a recommended
procedure for battery replacement. If it was my machine,
it would be shut off and unplugged, before opening the
side of the case. Remember to bring your body to the same
potential as the computer case, by touching the case
before touching the motherboard. That will remove any
large electrostatic charges that could flow if you just
touch a component on the motherboard first, without
discharging yourself to the case first. I like to handle
batteries like that with a soft cloth or tissue, so no
fingerprints all over the nice shiny battery
In the above circuit, even if the battery is flipped upside-
down by accident, the diode should prevent the reverse bias
from damaging the Southbridge. Current should only flow if
the (+) of the battery is connected to the 1K ohm resistor.
So, the dual diodes can do three things:
1) Prevent battery discharge, as long as +5VSB is powered
from the ATX PSU.
2) Prevent the battery from being charged, which I understand
can have explosive consequences. No, I don't want to test
that hypothesis
http://www.duracell.com/procell/safety/pdf/2032_3c.pdf
3) Prevent damage to the motherboard, if the battery is flipped
over by accident. A reverse biased diode will not conduct.
HTH,
Paul
