The CMOS battery is being recharged when your PC is turend on. However, this battery will eventually wear out, and not be able to maintain a charge. You will then have to replace this battery.

It is a very good idea to start your PC's CMOS setup program and write down all your system settings Why? When your CMOS battery dies all your system settings are lost. That means your PC will not work. Writing these settings down will save you the trouble of scrambling through your system manual to find these settings.