Most folks here are going way overboard (using Nicads, chargers,
telephone batteries, etc.) on this problem.
Buy a battery holder for three or four common AAA, AA, C, or D-size
batteries. If you don't have some handy nearly-dead flashlight
batteries ("torch" batteries to some), go buy the cheapest, most
common NON-alkaline, NON-rechargeable batteries you can find to fill
the holder.
(Most CMOS will maintain time and info when supplied by anything
between about 2.2 vdc and 7.2 vdc. That's why it doesn't matter if
you buy a three- or four-battery holder.)
Solder/attach the holder's leads to your CMOS battery connection,
then plug in those nearly-dead batteries. You're done.
Using four new zinc/carbon "D"-size batteries (I repeat: NOT
alkaline, nicad, nor any rechargeable), you'll throw away that
computer before replacing the battery pack... about a dozen years
from now.
If you REALLY never want to bother with the CMOS battery again,
install a small diode (soldered to a resistor in the 150-3000 ohm
range) from ANY 5 vdc source on the motherboard to the positive
battery pack connection. Don't even be concerned about Ohm's law or
any of that... you don't need much of a charge to maintain 2.2 - 5
volts inside the battery pack!
(I put four AA batteries in my old 286 sometime around 1990/91, with
no "charging" circuit at all. It's still running fine, every time I
turn it on... even though I don't turn it on much anymore.)
The only critical things in the above:
- NO expensive alkaline batteries or rechargeables.
- You can always use a lower voltage/smaller number of batteries
than the original, but don't go more than about 1.2 volts higher.
(Each AAA, AA, C, or D is 1.2-1.5 volts when new.)
- If you use a diode/resistor charger, make certain the diode points
the right way. If you don't understand this, don't try it.
- Don't even try recharging the original CMOS battery. It is made
from exotic (and expensive) metal alloys - like lithium, mercury
- and really CAN explode or go on fire. Same deal with the
alkaline high-power batteries.
This is the only part of your computer that is really amazingly
"forgiving" concerning the voltage applied. And LOW voltage is much
better/safer than any higher voltage - but you can probably go as
high as 6 volts on every computer ever made.
- John T.
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