Thermal Management 123 In a liquid-cooled system, a heat pump can be added to the overall system to provide warmed liquid through the cooling loop, which will slowly heat up the batteries. Other methods may also be employed such as using a thin-film heater. In this instance, the heater may not actually be used to heat, but rather to slow the rate of cooling such that if the vehicle sits for a week- end the cells will only be down to 25 °C after several days. This means that the system would have full performance, however, would suffer from some energy loss as the thin-film heater would need to be powered by the battery itself. However, this would be only a minor and temporary capacity loss.
Active Thermal Management Systems Active thermal management involves using some medium such as air, liquid, or refrigerant that is forced through the pack and over the cells to reduce temperatures. The two most common methods are air cooling using chilled air that is directed throughout the pack and over the cells and electronics to reduce the temperature of the cells and liquid cooling. This generally requires the integration of a fan, ducting, and heat transfer plates of some sort. The benefits are that it can be relatively effective in responding to rapid changes in tem- perature and has a lower weight than a liquid-cooled system.
The other benefit is that the cooled air is directly flowing across the cells. The disadvantages are that air is not as effective a cooling medium as liquid. And depending on air-flow design, it can cause the cells at the beginning of the air flow to be cooler than the cells at the end of the air flow because as the air passes by the initial cells, it begins to pick up the heat so by the time the air passes by the last cells it is warmer than when it in initially entered the pack. This uneven cooling can cause the cells to age at different rates, thereby reducing the life of the pack ( Figure 5 ).
In addition to using chilled air, some systems will pull ambient air from the outside of the pack, or even inside the cabin, and circulate that air through the pack. In this case, the air ĐƟǀĞůLJŝƌŽŽůĞĚͬ,ĞĂƚĞĚĂƩĞƌLJ^ĐŚĞŵĂƟĐ ZĂĚŝĂƚŽƌ ŵďŝĞŶƚ ;KƵƚƐŝĚĞͿŝƌ ǀĂƉŽƌĂƚŽƌ ĂƩĞƌLJWĂĐŬ &ĂŶ ,ĞĂƚĞĚ ŝƌ Figure 5 Active air cooling schematic.