I had a 30gig 5th gen IPod hooked up to the stereo. I had to replace it, so I bought an IPod Touch 3G. It kept the old (5th gen) IPod battery constantly charged. Now, after a long period of time (working all day or asleep all night) I find that the IPod Touch battery is almost completely drained.
I want to leave the IPod connected 24/7. I don't want to disconnect it everytime I leave the car. I left the 5th gen one hooked up this way with no issues.