Ok, here is how the setup works. The actual radio is the one in the instructors hands and controls the plane. It has and uses a battery. The buddy box does not use any batttery. In fact, if the buddy box has a battery in it and you turn it on it will usually mess up the connection to the other radio. Because of that the buddy boxes I use don't have any batteries in them. I am an instructor for our field and I have a buddy box for all the different types of radios, so I just them empty with no batteries in them.

So, to answer your question. Charge the main radio and take the battery out of the buddy box.

Hope this helps.