CMOS Backup Battery - Why Did It Die?
CMOS Backup Battery is NOT a laptop power battery!
There is a chip or IC (Integrated Circuit) on your motherboard that has two functions. It retains your customized or default hardware parameters (BIOS) and has a clock that keeps your computer time and date current.
To keep this IC powered as to not lose these important settings there is a small battery mounted on the motherboard. Usually it is a Lithium 2.5 to 3.0 Volt battery (voltage of the battery varies from one motherboard manufacture to another) that has a life of over two years. Some will last longer, some don't, depends on the manufacture of the battery and if you completely power your computer down (that is either pull the power cord out of the AC socket or have a switch that removes power from the power supply such as a surge suppressor that you turn off when the computer is not in use).
CMOS stands for Complementary Metal Oxide Semiconductor this is a programmable IC or chip. The motherboard has circuitry that will write the selected parameters to the chip.
This chip contains the information called a BIOS (Basic Input Output System). The BIOS is a program, it is NOT an IC! (Some people including the Nerds and Geeks intermingle these two terms... )
The IC is the CMOS and must have continuous power to retain the information written to it, thus it has a circuit that receives power from the power supply while the power supply is connected to an AC outlet (even if the computer is powered down) or when not connected to AC power a CMOS Backup Battery.
"Well then Sparky, why does mine only last a year or less?"
Three of these answers would mean you either keep changing the battery or replace the motherboard, time for an upgrade?
An indication that it has failed is the time and date have reverted to the default date of the BIOS, another would be that the hardware parameters have changed and on POST you get an error telling you that you need to set the time and date or hardware parameters.
Just thought you would like to know why your time and date needs to be set every time you remove the AC power from your computer... :)