"Greg Mayman" bravely wrote to "Mike Ross" (18 May 04  11:49:00)
 --- on the heady topic of "SILICON CHIP ON LINE"

 -=> Mike Ross said to Roy J. Tellason
 -=> about "SILICON CHIP ON LINE" on 05-11-04  23:31.....

 GM> That's right. But don't use 4 cells -- that would give 6 volts.
 GM> Use only 3 cells = 4.5 volts. Even two cells might be enough for
 GM> the job.

 GM> They use special circuits to power the CMOS memory and the clock
 GM> from whatever has the higher voltage. When the computer power is
 GM> on, they run from the +5v rail. When it is off, they run from the
 GM> battery.

 GM> So using a 6 volt battery pack, the CMOS and the clock would take
 GM> their power _permanently_ from the battery pack, until the
 GM> battery pack dropped to less than 5 volts.

 GM> And at that point, the battery _might_ not be capable of holding
 GM> up the clock and CMOS when the computer was powered down.

 GM> And _DO_ remember to remove the link that supplies charging
 GM> current to the battery when the computer is powered up.

The external battery connector usually has some sort of steering diode
but the charging voltage for the internal nicad would cancel that
circuit if the external battery jumper isn't changed. The LSI circuit
that has the cmos memory and clock has a pin that gets the backup
battery voltage. I recall reading this pin will accept from about 2 to
8 volts input but you're right, 4.5v seems to be the most common.

Note that boards that used internal lithium batteries had no charging
circuit. Those that used nicads can be altered to lithium by cutting
the lead off the charge steering diode. Those dollar coin sized
lithium batteries with the pcb mounting leads last almost forever.

Mind good quality alkalines have a shelf life of about 5 years as is
but the lithium babies can go for 10 and even 15 years. I have a
number of devices using these for backup and they've been in them for
a longggggg time. I've noticed the often seen CR2032 batteries in a
holder don't seem to last quite so long.

Anyhow, there is a barrel shaped lithium battery that fits the same
spacing as the nicad barrels but remember to cut the charging diode.

 Mike
 ****

... # <--- electrocuted tribble
--- Blue Wave/DOS v2.30