Automotive electrification is the new path for the automotive and transportation sectors to reduce CO2 emissions and impacts on global warming. These new propulsion systems, such as hybrid (HEV), plug-in hybrid (PHEV), and electric (BEV), are sold by new and traditional OEMs in all continents.
Lithium batteries of different cell chemistries are used in these vehicles due to their long lifespan, high energy density (Wh/L), and high specific energy (Wh/kg). As the Lithium-ion battery is the most expensive component in electric vehicles, roughly 50% of the vehicle’s overall cost, the automotive industry has focused on increasing their longevity and performance. An effective thermal management is essential to reach this goal, avoid premature failure, and reach greater performance, durability, safety, and operational stability.
It is known that a relationship between lithium-ion cell temperature, battery capacity degradation, and battery life can be calculated by the Ahrrenius equation. This relationship indicates that the degradation of batteries increases exponentially with temperature. And its degradation intensity also depends on cell chemistry, design, geometry, and form.
According to the most accepted industry standards, this degradation takes the battery to the end of its first useful life when cell initial capacity (Ah) decreases by 20% or internal resistance (Ohms) increases by 100%. In simple terms, it means the battery can store less energy, has reduced performance, needs more charging events during the day, and generates more heat, which degrades battery life even further.
Although the BMS (battery management system) software can limit delivered current and power for safety reasons when cells are exposed to extreme temperatures and minimize the risk of a thermal run-away event, the battery temperature control function must be exercised first by an efficient thermal management system.
This thermal management should act to avoid exposition to extreme high and low temperatures. When exposed to high temperatures (above 45°C), chemical reactions within the cells take place at a faster rate and degradation increases exponentially. And when exposed to low temperatures (below 5°C) the risk of dendrites formation when charging or overheating when discharging can permanently harm the battery lifespan. Thus, the presence of an efficient thermal management system is essential, and several companies in the sector have adopted different strategies.
There is a variety of thermal management systems in the market, and they can be classified according to their purpose, source, and medium. As for the purpose, they can be just for cooling or cooling/heating. However, dual-purpose cooling and heating systems are more commonly used globally because countries in Europe, China and North America can easily face temperatures above 40°C and below -30°C.
The source can be passive, although its efficiency is low, where the air from inside the vehicle and from the external environment is used to cool-off the battery bank. Or active source, where a cooling/heating system is specially dedicated to the battery bank, and it provides a more efficient temperature control. The active thermal management channels are in direct contact with the lithium-ion cells. This method is preferred by the new and traditional OEMs, including Tesla, General Motors, Honda, Nissan (new generation Leaf), Ford, BMW, among others.
The medium can be air itself or coolants, as mentioned above. Using air for cooling, generally, leads to a low efficiency due to the limited specific heat of air (J/(kg.K)). While the use of liquid refrigerants is more efficient, it requires a dedicated and active cooling system with greater thermodynamic efficiency, but with a higher cost due to the need to install compressors, chillers, pumps, among other items.
Furthermore, to increase the heat exchange between the cells and the external environment, several technical solutions are incorporated into the battery pack design in addition to the management systems mentioned above. One of them is to direct the heat to the external faces of the battery pack through gels, extra thin thermal membranes, or other thermoplastic membranes with heat dissipation properties.
Finally, batteries thermal management must remove or add heat from cells quickly and efficiently throughout the entire battery bank, avoiding delta of temperatures between cells. A high temperature delta is an indication of poor and inefficient thermal management. Therefore, keeping all cells (hundreds, thousands of them) in the battery pack at the same temperature is the biggest technical challenge.
An efficient thermal management system sized correctly for each battery pack type is the only way to guarantee high performance and safe operation, minimize degradation and uneven cell aging, and ensure a long lifespan for electric vehicle batteries and low risk of an early replacement.