Computer Batteries

Most all computers have a battery inside. Portable Laptop and Notebook 
computers have batteries to power them. They also have a separate battery to 
power the configuration CMOS and clock. Desktop computers also have a CMOS 
battery.

CMOS batteries come in many sizes and shapes as well as voltages. You need to 
select the right one when it comes time for replacement. There are nicad, 
alkiline, and lithium batteries and they come in different voltages for 
different manufacturer's machines. They also have diffrent mounting methods 
and connections. Many clone manufacturers use a standard replacement battery 
with a 4 pin plug that plugs into the computer motherboard. Usually a jumper 
is changed to disable the onboard battery and enable the new replacement 
battery.

The old PC/XT computer had the option of having a clock and battery. Many
XT's used a flat disk Lithium type battery.  Configuration was accomplished by 
changing jumpers or switches.

The AT series computer including the 286, 386, 486, and Pentium computers all 
use a battery to hold the time in the built in clock as well as the system 
configuration. The AT configuration included how many and types of floppy 
drives, amount of memory, type display card, hard drive type or discription,
and even more changable options in the advanced CMOS. The hard drive type or 
user definable type is crutial to the proper operation of the computer. The 
exact cylinders, heads, and sectors per track must match. These numbers are 
all held in the CMOS. 

The CMOS battery is also guaranteed to die one day. These batteries usually 
last from 3 or 6 months to several years. Any battery older than 2 years is 
on borrowed time.

The worst part of all this, is the computer owner rarely knows what numbers 
should be in CMOS. These numbers should be recorded before the computer 
battery dies. Many technicians do not use standard numbers, that is they 
change the number of cylinders or use an odd translation scheme so you will 
need a technician to reset the numbers so that the hard drive can read data 
without scrambling it.

Most motherboards have chargable nicad batteries mounted on them. Technically, 
this should give years of service, provided you use the computer as the 
manufacturer intended, that is regularly almost every day. If you do not use 
it regularly, the battery gets into an odd charging cycle and doesn't retain a 
full charge. Soon, it will not retain any charge. Then it goes dead and you 
loose the numbers required for configuration.

Portable computers also have charging requirements. Nicads must be fully 
discharged and immediately recharged or they get "memory" problems of not 
knowing what a full charge is. Lead acid gell cells can be recharged at any 
place in the discharge cycle but must not be left discharged fully. You must 
know what you have and how to use it.