I use the TuneJuice that I got from the Apple store. It works great on both my iPod Classic as well as my iPhone. It was $39.95 from the store and runs on 4 AAA batteries. Works like a charm. It also appears that the batteries don't exert power when not plugged in to anything. I used it for the first time today since the end of March on a flight and I used the old batteries I had in there and they still had plenty of life.