From: Ed Tanton (
Date: Sun Oct 11 1998 - 12:50:42 EDT

Hi Barry... I work with PCs... I HAVE had a bad, NEW, battery (for the
CMOS) show up every once in a while over the years. Even checking the
voltage isn't really enough with some batteries-note the way Duracell tests
them under load in that builtin tester. It isn't a common occurance-but it
would be the first thing I would redo, before looking more deeply. And then
there's whether it's in correctly. Some/most such batteries appear to be
diode-isolated... so reverse voltage from the battery won't affect anything
(and any voltages higher than the battery's won't go INTO the battery,
avoiding a nasty little bang in the case of Lithium cells.)

My first step after this doesn't help would be to check that isolation
diode. Then there is probably a very-high-value "super-cap" somewhere that
may have shorted... they stuff a lot of foil and goop into quite a small
package for the usual values ranging from 1/2 to 1 farad at 5VDC. Finally,
there is the CMOS memory itself. It is much more likely to be something
else (like a leaky capacitor) but if the chips are socketed, you can simply
pull them and see if the voltage hangs in there.

I would put a high-input-impedance DVM on the CMOS voltage bus and (with
the chips and a new battery in place) turn off the power supply, and watch
it drain-noting the rate. Then, remove the CMOS (with power OFF!) Turn it
back on, note the new voltage, and, again, turn it off watching the drop
rate. If it stays the same as it was WITH the memory IC(s?) removed, it's
not the chip(s). You can follow the same procedure for that super-cap, and
other components right there around the memory and battery circuit.

Let me know how it goes, please. Good Luck.

Ed Tanton N4XY EMAIL:
189 Pioneer Trail
Marietta, GA 30068-3466 TEL: (770)579-3933 V/MBX/FAX
INTERESTS: QRP BoatAnchors Test Equipment Photography
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"Think you can, think you can't: either way you're right!" Henry Ford

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