There's a pretty bad design flaw in this product.
You're only supposed to charge your iDevices when the battery is low. By doing partial charges, you kill the battery over a shorter amount of time.
There's no way to not charge and listen.
So, to listen to your iDevice, you always have to charge it ... no matter how long the drive. (5 minutes, 10 hours, etc)
I used to think the same, turns out that info is incorrect.. read this:
Basically, you're better off keeping any product with lithium batteries fully charged all the time. Every so often let it discharge completely and charge it back up to full to maintain it.