I have a 2002 PT cruiser with 27,000 miles and no problem with starting How do I know when to replace the battery?

You can have it tested for free at most major auto parts stores.

I have had several Chrysler products over the years. After being stranded a few times, I reached the point where I started thinking abou replacing the battery after about two years.
If that is the original battery, you are definitely living on borrowed time. Testing the battery is fine but if a cell goes bad tomorrow it will go from 12 volts to zero in a heartbeat.
The cost of a battery is trivial compared to the inconvience and cost of dealing with a surprise.
My battery problems went away when I finally weaned myself from Chrysler.