Drive Train - Keeping the battery comfortable
The battery plays a significantly bigger role in the drive train of a hybrid or electric vehicle, than compared to an internal combustion engine based drive train. It is therefore that a lot more care and attention goes out to the battery; it is usually kept in a well insulated compartment and has a special control system to maintain a good temperature. The reason that this temperature needs to be controlled is to preserve battery life and keep the specifications as much as possible the same in all weather conditions. The battery degrades faster in temperatures that are too warm, but the chemical reactions slow down at too low temperatures. If the battery gets too warm this results in damaged cells, which is noticed as a loss in cell capacity, or simply put: it shortens your range. At cooler temperatures the electro-chemical reactions slow down. It can slow down to such a level that the battery can not hold up with the power demand, which you notice at first that acceleration is becoming slower. The better known effect is of an internal combustion car not being able to start in winter.
Basicly there are two types of control systems at the moment; one using air to control the battery temperature, the other uses a liquid system.
A battery pack from Tesla
With the air-based system, outside air is used to cool the battery. A benefit of this system is the low cost of the system and the low weight of its components. A downside is that air can not hold as much heat as for example water. It requires much larger volumes of air to get a similar amount of heat to be transferred out of a system, compared to a water based system.
With a liquis based system a (closed) circuit of (for example) water is used to transport the heat out of the system. The components required for such a system cost and weigh more, but you need a smaller flow to achieve the same; resulting in a more compact system.
Thermal images of batteries: some areas produce more heat than others
Another aspect that is often overlooked is the spread of the temperature inside the battery. Ideally the battery has the same temperature throughout the battery, not that one side is 10 degrees warmer than the other. The more uniformly the temperature can be kept, the better this is for the battery life. With an air based system it requires a substantial amount of air to ensure that every corner in the system is cooled at the same temperature. Compared to the liquid system, which more easily accesses every nook and cranny and thus can cool very much the same in every corner.
All in all I think from a technical point of view, the liquid based system is the better. This is probably also the reason it is used in many high-end/critical cooling systems. From a cost perspective the air-based clearly has its advantages and it remains to be seen how critical maintaining a battery temperature and an even spread really is. With the current batteries around and in particular their prices, the quality of this system is allowed to cost a bit more. However, if batteries get cheaper as the technology develops and more are being manufactured, using the cheaper air-based system might be a wiser solution.