124 Chapter 10 WĂƐƐŝǀĞůLJŝƌŽŽůĞĚͬ,ĞĂƚĞĚĂƩĞƌLJ^ĐŚĞŵĂƟĐ ĂƩĞƌLJWĂĐŬ ŵďŝĞŶƚ ;KƵƚƐŝĚĞͿ ŝƌ ĂďŝŶ ŝƌ &ĂŶ ,ĞĂƚĞĚ ŝƌ Figure 6 Passive air cooling schematic. must first be filtered and the cooling ability is limited to reducing the temperature only to what the ambient temperature is. So in this instance, if the ambient temperature is at 30 °C the thermal management system will only be able to cool down the battery to 30 °C ( Figure 6 ). Another challenge of air cooling is that it makes the system design more difficult in the event that the pack needs to be a “sealed” pack design.
As the air-cooled pack is essentially an “open system,” it becomes very difficult to achieve an IP69 sealing level on the pack, which makes it a very good solution for a pack that is mounted inside the vehicle or inside a build- ing or container, but not as good a solution if it is mounted external to the vehicle. The other common method of active thermal management is by forcing a liquid, often a 50/50 water and glycol mix similar to that used in engine cooling, through a series of plates that are mounted next to the cells. This system involves a liquid distribution system that must be inte- grated into the pack, most often done through a series of hoses and heat exchangers.
The benefit of the liquid cooling system is that it is quite an effective medium for quickly transferring heat away from the cells. It can also, with a heating element in the system, be used to provide heated liquid in order to warm the battery in the cold weather. The disadvantages are that it tends to be a heavier system (greater mass) and there is always a risk of leaks in the battery pack. As the liquid-cooled system is essentially a “closed system,” it tends to be much easier to seal to the environment making it a good solution for packs mounted external to the vehicle.
In the liquid-cooled pack, there are essentially two methods for managing the heat generation of the lithium-ion cells. In the first method, you can develop a plate that is affixed directly to the cells and flow the cooling/heating liquid directly through these plates ( Figure 7 ). The second method is to create a single plate through which the fluid flows, but rather than affix the cells directly to this plate, a series of “fins” are attached to the heat sync plate. The lithium-ion cells are then attached directly to these fins ( Figure 8 ). One other aspect to examine in actively cooling and heating lithium-ion batteries is direct and indirect cooling.
These terms are sometimes used interchangeably with active and passive. Direct cooling refers to moving the cooling/heating medium directly across the cells, whereas indirect cooling/heating involves moving the cooling medium through a heat exchanger that is