What is expected life of the PRAM battery? Is it user replaceable?

it depends on if you are constantly using your Mac or letting it sit unplugged or in storage for long periods. the PRAM [Parameter RAM] battery supplies power to the system clock and PRAM, which in turn controls many start-up and system controls. in storage, the battery can run dead in a few years, but if the machine is plugged in and used on a regular basis, it can last for up to 7 to 10 years, or more... however, they do eventually die.

they are generally very easy to replace in PowerMacs and some iMacs. the barrel shaped 3.6v cells are simply held in place with a plastic clip and pop out like any AA or AAA battery...if you can get to them. on some iMacs, and all Mac laptops, dis-assembly of the case to get to the battery is the biggest problem. be sure to familiarize yourself with the procedure before tearing into it.

on PowerMacs, the matter is much easier. the battery sits in its own mount on the system board usually next to the ATA connectors. once you pull the battery, you will need to reset the system time and some other settings commonly controlled by PRAM. be sure to unplug and power down any computer before getting into the internals to insure not damaging anything like circuits....or shocking yourself.