> Actually, the battery in a phone is more comparable to the entire fuel delivery and ignition system in a car, not just the battery. Even fewer people could handle that work on their own.

So you know, there was a point in time where I just replaced my phone's battery. I bought the battery, opened the back cover, and replaced it. And it worked great. :)

The reason people take their cars to a service shop is one of convenience for things like batteries, not because it's a necessity created by the design of the vehicle.