I recently added an iPod interface to my Accord: http://www.amazon.com/gp/product/B001JT5G4G/ref=oh_o02_s00_i00_details
It has two connections: the regular Apple 30-pin iPod dock cable which plays back and charges and then an AUX 3.5 mm headphone connector.
I get the best sound when connected to the Apple connector, but it is also charging it at the same time. I really don't use it much outside of my commute to work ~1 hr. per day, so is it bad for the battery to be plugged in all the time? The car interface won't overcharge it; it stops when full. I just didn't know if I needed to actually use the Li-ion battery in the iPod to keep it useful when I do need to take it unplugged.
Best Answer
It'll probably be fine.
The easiest way to mitigate this I can think of is to unplug the device over the weekend, remove it from the car, and let it discharge on its own. Then, on Monday, plug it back in and go. This has the added advantage that it's not subjected to the temperature extremes the interior of a car can see over a period of extended non-use.