I bought the TuneJuice to allow me to watch more than 2 hours of video on my iPod. The TuneJuice allows you to connect a 9 volt batter to an iPod to extend the playback time.
One interesting thing is that the internal iPod battery is not charged with the TuneJuice. When the TuneJuice is connected, the battery indicator on the iPod just shows that there is more charge.
Since the internal batter is not charged, I wonder if the iPod Dock connector has separate pins for external battery packs than for battery chargers.