as far as I know, as long as the red light goes out, then the charging has stopped.. Don't know if it operated on AC power after that point though, or if it runs off the battery and the charger just tops it off..
edit - Realized I didn't actually answer the original question
. a wall/car charger should work just the same as the usb charge coming from the PC; It's the kobo itself that should be stopping the charge once the battery is full.