Batteries can only operate within a certain temperature range. If they are at too hot or too cold, their safety, performance, and lifespan will be affected.
Battery thermal management is essential in electric vehicles and energy storage systems to regulate the temperature of batteries. It uses cooling and heating systems to maintain temperature within an optimal range, minimize cell-to-cell temperature variations, enable supercharging, prevent malfunctions and thermal runaways, and maximize the battery’s life.
In this article, you will find everything you need to understand battery thermal management.
- Understanding Battery Thermal Management
- Battery Cooling Methods
- Thermal Interface Materials
Battery thermal management is a complex subject. Before delving into the types of thermal management systems, it’s important to understand key concepts such as where heat comes from in batteries, why heat affects batteries, and what are the optimal operating temperatures.
The electric current going through batteries encounters resistance in various materials. As a result, some of the electrical energy is loss as heat. Resistance occurs during charging and discharging when the current passes through electrodes, electrolyte, current collectors, busbars, and various interconnections. The higher the current, the more heat is generated.
Batteries are affected by temperature changes because temperature impacts the kinetic energy of the molecules found in battery materials (such as the electrolyte and the electrodes). This makes these materials more or less conductive based on their temperature.
Temperature’s Impact on Kinetic Energy
In colder temperatures, molecules have less kinetic energy, moving slower and colliding less frequently. In higher temperatures, molecules move faster, causing more collisions and hence more chemical reactions.
From a presentation on Thermal Energy
Example with Li-Ion Batteries
During the discharge of li-ion batteries, lithium ions move from the negative electrode to the positive electrode, passing through the electrolyte and the electrodes.
In colder temperatures, electrons move slower through these materials because these materials become more resistant to the flow of ions, resulting in lower conductivity. During cold winter days for example, EV batteries need heat themselves using heating loops to ensure good conductivity and performance, drawing more power from the battery.
In higher temperatures, electrons move faster, causing faster charging performance but also faster degradation of the battery’s components. Bringing batteries at higher temperatures can enable supercharging to meet consumer needs, but at the cost of a shorter battery life.
Different types of battery chemistries are affected differently by temperature.
Lithium-ion batteries, which are used in most electric vehicles, can operate between −20°C and 60°C. Their optimal operating temperature, however, is between 15°C and 35°C, the range where they perform the best.
To maximize the performance and longevity of the battery pack, it is essential to maintain a uniform temperature distribution across all battery cells. Ideally, the maximum surface temperature variation is no more than 5°C.
From the article Battery thermal management systems: Recent progress and challenges
Each type of battery chemistry has unique characteristics that affect its behavior at different temperatures. Consequently, the type of battery has a big impact on battery thermal management.
From the article Charging at High and Low Temperatures
One of the main functions of a battery thermal management system is to extract heat from the battery to prevent the degradation of its components as well as thermal runaways. Here are the different cooling methods and how they affect the battery’s design and efficiency.
Battery cooling methods fall under two general categories: passive cooling and active cooling.
Passive cooling methods use natural heat dissipation like radiation and conduction to extract heat from the battery. This can include materials with high thermal conductivity. It can also include design decisions like battery casings or structures that facilitate airflow between cells to dissipate heat.
Passively cooled batteries can be found all around us. Examples include cellphones, laptops, Bluetooth speakers, and most battery-based consumer electronic devices.
Passive cooling methods are simpler, require lower maintenance, and are less expensive. However, they are often less efficient at extracting heat, and their efficiency can vary based on environmental conditions.
Active cooling methods use external devices to actively regulate and dissipate heat from the battery. They make use of components like fans, pumps, or compressors to move air or liquid through the battery system. Active cooling systems also use sensors and other tools to monitor temperatures and adjust cooling. These components need to be powered and hence add to the energy consumption of the battery.
Active cooling methods offer increased precision and control, making it easier to maintain the battery within a specific range. They also extract heat more efficiently. However, they are more complex, consume more energy, and require additional maintenance.
Air cooling and liquid cooling are the most common cooling methods currently used in electric vehicles. Both of them are active cooling methods.
Air cooling systems uses fans or blowers to generate an airflow that extracts heat from the battery’s components. Air cooling is simple, relatively low cost, and does not require a lot of energy. However, air is not an efficient heat conductor compared to alternatives. For that reason, many EV battery manufacturers have been moving away from air cooling for higher-end electric vehicles.
Liquid cooling systems uses pumps or other mechanical components to circulate a liquid coolant through channels that are in direct contact with the battery cells or modules to absorb heat. The liquid then travels to components like heat exchangers, radiators, or fans to expel the heat. Examples of liquid coolants include water, glycol, oil, acetone, and refrigerant.
There are various approaches to liquid cooling. Some manufacturers add cooling circuits between the cells (wave plates). Others add cooling plates below the cells (bottom plates). These plates are responsible for carrying the heat away from the cells.
Why Is Liquid Cooling More Efficient than Air Cooling?
Liquid cooling is more complex and expansive than air cooling, but it is more efficient, making it a better choice for demanding thermal management applications. This is because with their higher density, liquids have a better heat capacity and conductivity than air.
Because of their higher density, liquids have a bigger mass than air. More mass means more molecules, and more molecules means more opportunities for energy to be stored. As a result, liquids can absorb more heat for the same volume.
Water can absorb about 4.18 joules of energy per gram before gaining 1°C, whereas air can absorb about 1.005 joules of energy per gram before gaining 1°C (source).
Because of their higher density, liquids have more molecules that are in contact with the hot surface, allowing them to absorb more heat at once. This means that heat is absorbed faster with liquids than it is with air.
In terms of volume, 1g of water equals 1 ml of water, whereas 1g of air represents approximately 1 l of air. This shows how much water has a better contact with the surface and hence a better heat conductivity.
Air cooling and liquid cooling are the most common cooling methods currently used in electric vehicles, but new technologies are being developed. While they are not ready for the market yet, they have immense potential to improve battery thermal management.
Phase-change materials (PCMs) are materials that manage heat by changing phase: they go from solid to liquid or from liquid to vapor when absorbing heat, and back to their original state when expelling that heat. As the battery reaches a critical temperature, these materials undergo a phase transition, expelling heat very efficiently.
The Phase-Change Composites (PCCs) solution proposed by AllCell. PCCs are PCMs mixed in a solid structure.
When choosing a phase-change material, the melting and solidification temperature is one of the most important considerations. These materials are also chosen based their heat conductivity, heat capacity, latent heat (i.e., the amount of heat absorbed/released during phase changes), their ability to undergo repeated phase changes without degradation, and their compatibility with the battery pack’s components. Materials like expanded graphite and metal foam have great potential to improve heat dissipation in batteries.
Phase-change materials are used for passive cooling. They are an integral part of the battery’s design and do not require additional components like fans or pumps that draw power.
Dielectric immersion cooling is a method where battery cells and modules are immersed in a non-conductive liquid (dielectric liquid) to dissipate heat. Dielectric cooling is very efficient and offers a more uniform temperature distribution across the battery cells, which helps improve the battery’s performance and life. It also prevents humidity in the battery—a cause of failure in current EV batteries.
There are several challenges that battery manufacturers need to address before implementing dielectric cooling. This method requires designing a battery that’s 100% liquid proof. Dielectric liquids also add weight to the battery, an important consideration when trying to optimize the battery’s range.
Another challenge is choosing the right liquid. According to research on dielectric liquids, the choice of a liquid depends on its heat conductivity, heat capacity, chemical stability, viscosity, and compatibility with the battery materials they are in contact with. Examples of dielectric liquids that are being tested for battery thermal management include mineral oils, ester oils, and transformer oils.
Thermal interface materials (TIMs) are materials inserted between two surfaces to enhance the heat transfer between them. They play a critical role in efficiently transferring heat between battery cells and cooling elements like heat sinks, plates, or cooling fluids. Examples of TIMs include pastes, adhesives, gap fillers, pads, and phase-change materials.
Before applying a thermal interface material between two surfaces, the surface often needs to be prepared to ensure a good bonding quality. Laser surface preparation is a technology used to remove contaminants, generate the right surface roughness, and modify the chemical composition of the surface. It creates the optimal conditions to ensure a strong bond of the thermal interface material with the battery components.
Future Batteries Need Better Thermal Management
Among the current developments in electric vehicle batteries, there is a need to manage heat more efficiently. Supercharging generates more heat, and the increasing energy density of batteries means that they are packing more heat into a smaller space. This just goes to show how developments in thermal management are central to the future of electric vehicles.