If you take the 2007 Prius to a drive-in, how do you keep the radio on?

What is your question?

My son took our 2007 Prius to a drive-in. In order to listen to the movie, you have to have the radio on. The radio does not stay on when the key in the ignition is in the "ON" position. The radio is on when the car has been started and running. This takes the main battery down to 1 bar by the end of a 90-minute movie and of course the engine kicks in throughout the movie when the battery gets below 1 bar.

What makes this problem better or worse?

start the car, keep it on

How long has your 2007 Toyota Prius had this problem?

once