Roadster's battery pack is, as we all know by now, liquid cooled. Basic reason for this is that there is not enough exposed surface of the pack for air cooling to suffice. The pack is basically just a box, where only small amount of the cells lie directly to outer box walls, most of them have other cells for neighbors. Without circulating liquid there would be only other cells to transfer heat to. Model S' battery pack is supposed to be completely flat and under the car. They will probably still use 18650 cells, so the pack will be only about 8 cm or 3 inch thick (cells are 6,5 cm high plus some housing) and every cell will have its own exposed surface through which it could dissipate its own heat. Pack of 8000 cells in 18650 format would have at least 2,3 square meters of exposed surface under the car over which the outside air will blow. It is much harder (much more added wight) to liquid cool such a flat pack than it is for a more box-like one in the roadster. Another factor is that this pack is supposed to be easily swappable. If it is liquid cooled it would have to have its own reservoir, pump and heat exchanger or a pipe connected to a car. This pipe would then had to be easily pluggable. All this brings me to the conclusion Model S' battery will be air-cooled. Maybe it will have some embedded heating, but no internal liquid cooling. This also means there will be no water pump to run 24/7 and lower total battery weight.