What does a CMOS battery do for a mot...

The battery allows the motherboard to remember the time and settings when it power is off.
Yes, it dies.
Just like every battery, when it expends all of its power it stops working.
They can fail prematurely if there is a short circuit or power surge to the motherboard.
They usually last pretty long. (5-10 years)
It is safe to remove it, test it and replace it.
You will not lose any data on your hard drive.

I hope this helps.

cmos battery is for back up power ,yes it will boot with power cord,reason to die is old age mainly