The phone will draw current until fully charged and then stop. Normally the phone won't be totally dead, but if it is, the total phone battery capacity is roughly 10 Wh. A car battery capacity is roughly 600 Wh, so 60 times the capacity of the a phone battery. Charging a dead phone will use less than 0.02 of the car battery's capacity. Though this is small, it does add to the other drains on the battery.