I've checked twice on Continental and it was very easy. I rode my
ECV to the gate--told the agent I wanted to gate check it--told her what kind of battery I had--rode it to the entrance to the plane--showed them how to put it in manual--took my basket, key, and tools into the plane. When I got off, it was waiting for me.
I did make sure to have a copy of my manual with me just in case, but never needed it.