Hello!
An independent variable in an experiment is the factor being changed by the scientist to see what it will impact. In this case, the independent variable is the brand of the battery, as that is the variable being changed.
The dependent variable in an experiment is the factor that is impacted by the independent variable. In this case, the dependent variable is the length of time the batteries last, as that is being impacted by the brand of the battery.
The control variable(s) are the variables which remain unchanged throughout the experiment, and don't impact the dependent variable. Here, the control variable is your iPod, as it remains the same throughout testing all three brands of battery.
Hope this helps!