I do not know much about computers & I just learned of the existence of CMOS batteries. I have an old T-4x Thinkpad & am wondering if I should have the battery replaced. Having read some threads about replacement I don't think it's something I should undertake. I have a Thinkpad with a T-41 body, a T-42 (I think) motherboard, & the hard drive from my T-43 (whose motherboard went bad). I've read on the Forum that a CMOS battery has about a 5 year life. It sounds like I'm on borrowed time. Do people replace these batteries every so often (like smoke alarm batteries) or wait until they fail? Thanks in advance for any reply.

I usually won't replace a CMOS battery until it's totally dead, even though it's a relatively trivial task to replace one. Even on a T4x (which involves a little more than some other models) where you only have to remove the keyboard and palm rest, and plug the new battery into the socket.