I don't know if this is a bug or intended, but the Coal Generators don't stop working when the batteries reach 100% charge. They won't be refueled, but there is some coal lost if the batteries reach maximum charge and the generators still have fuel.
Steps to Reproduce
Build a Coal Generator and hook it up to some battery. When the battery is full, if there is still coal in the generator, it will keep running and consuming the fuel.
This issue has not been confirmed by a developer yet.