Build A Simple Rechargeable CMOS Battery

Build A
Simple Rechargeable CMOS Battery


Occasionally we come across a
computer motherboard that seems to eat those CR2032 batteries
every 4 to 5 months. This is particularly frustrating because the
computer works perfectly once booted but all the important BIOS
settings are lost when the CR2032 battery fails. This is very
irritating if the affected computer will not boot up with the
default BIOS settings. I recently had a computer motherboard with
this same irritating phenomenon. A test revealed that the current
drain on my CMOS battery was about 30 microamps. While this does
not seem excessive, it is large enough to drain a CR2032 battery
in a few months. While the simplest solution may be to replace
the motherboard, it seems wasteful to trash a perfectly
functioning motherboard just for this reason. I had seen
rechargeable battery systems in catalogs with prices as high as
$50 dollars. In the Amateur tradition, I searched for a better
and less costly solution.

The circuit in Figure 1 is an
inexpensive rechargeable CMOS battery system that can be built
from readily available parts. The heart of the circuit is the 3.6
volt rechargeable cordless phone battery pack. With the
popularity of cordless phones, these replacement battery packs
can be found in many department stores for as little as $6. The
particular brand used is not important only that it is a 3.6 volt
(3 nicad cell) type. To get the necessary 5 volts from the
computer power supply, a Y power connector is needed. One of the
power leads has 12 volts and the other has 5 volts. Since wire
color coding schemes vary, use a voltmeter to connect to the
right lead. The remaining two black leads go to ground. A
connector will also be needed to connect the battery system to
the external battery connector on the motherboard. It may be
necessary to add or remove a jumper on the motherboard to use an
external CMOS battery. Check the computer motherboard
documentation for information on using an external CMOS battery.
Resistor R1 is a 10 ohm � watt resistor and D1 is a 1N4001

The resistor limits charging
current to about 40 to 50 milliamps and diode D1 prevents the
battery from discharging through the computer power supply when
it is turned off. When the circuit is first installed it will be
necessary to run the computer for several hours to get the
battery pack charged up. After this initial charge, the computer
can be run for as little as 2 hours a week.

I have used this circuit for
several months and I have found it to be an excellent external
CMOS battery that never needs replacing.


Build A Simple Rechargeable CMOS Battery

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