Re: Motherboard batteries

PC Guy wrote:
Paul wrote:

I've had two batteries run flat while the computer was stored,
and that is where I get the 3 year number from.

Any idea what the shelf-life is of a 2032 battery?

When such a battery is installed in a motherboard, and the board is
supplying the power for the RTC even when the PC is turned off, is there
any chance that some current is feeding back into the battery to keep
it's charge up?

In other words, if I take 2 batteries from the same production batch,
and keep one in it's package and put the other in a motherboard that is
connected to main AC power indefinately, then will both batteries age or
discharge at the same rate?

They should age at roughly the same rate. The idea being, that when
+5VSB is available, no current is drawn from the CR2032.

You can see in the spec here, the maximum reverse charge is one microamp,
so the battery maker, really don't want the circuit design to be charging
the battery. The Schottky dual diode scheme, will have a small amount of leakage
back into the battery, and one would hope, not exceeding that
one microamp specification.

http://data.energizer.com/PDFs/cr2032.pdf

There is an example of the actual dual diode used to prevent backward
current flow here. You can see "K45" printed on the dual diode package,
which makes it a BAS40W-05. (I have three motherboards here, that use
K45 for the dual diode near the battery.) Figure 2 shows the leakage
current. The diode would have to be raised to 125C temperature, before
the leakage would be a problem. And normally, that diode will be living
at 35C to 40C temperatures. (Leakage in this case, means backward flow
of current from the +5VSB source, into the battery, in effect attempting
to charge the battery.)

http://www.diodes.com/datasheets/ds30114.pdf

The reason those particular diodes are used, is for the low
forward voltage drop.

Paul
.

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