Between my girlfriend and I we have had a total of 5 minis. Sent two back for battery issues, and the ones we got in return were only marginally better. The other one simply stopped working.
Here is what you need to know...
There was a lawsuit that apple settled relating to the Mini and its abysmal battery performance. It may or may not be worth your time (I decided against it), but go here if you's like to learn more:
http://www.appleipodsettlement.com/
At the end of the day, the Mini was a bad product. The Nano is a replacement device that has a battery life 4X as long and is a much better player all around...