Advanced Mobility Platform for Electric Vehicles

Which Cooling Method Is Right For Your Battery Pack?

Introduction

The importance of the battery thermal management and choosing a proper cooling method for a lithium-ion (Li-ion) battery pack for battery electric vehicles (BEVs) and making an optimal cooling control strategy to keep the temperature at an optimal range of 15 °C to 40 °C is essential to increasing safety, extending the pack service life, best performance and reducing costs. In this article you will understand that choosing a right cooling method for your application and developing strategies, trade-offs need to be made among many facets such as costs, complexity, weight, volume, cooling effects, temperature uniformity, and parasitic power.

Popular battery pack cooling methods are:

  • Air Cooling
  • Fin Cooling
  • PCM Cooling
  • Tab Cooling
  • Liquid Cooling (Direct  & Indirect)

This will give you a brief idea through an overview of how to cool lithium-ion battery packs and evaluates which battery cooling system is the most effective for your application in the market.

The Importance of BEV Thermal Management System

To get the best out of your Li-ion battery, it is extremely necessary that the cells and the battery pack remain in the optimal temperature conditions while charging or discharging. Often these sources of heat are a function of state-of-charge (SOC), temperature and current, with these relationships changing with cell age. Charging and discharging Li-ion batteries rapidly and at a higher C Rate generates more heat. Higher currents lead to higher rates of non-linear degradation and the heterogeneous current flow puts significant extra strain on the cells that take a higher than average current which results in increased internal resistance and a reduction in usable capacity. While advancements have been made in electric vehicle batteries that allow them to deliver more power and require less frequent charges, one of the biggest challenges that remain for battery safety is the ability to design an effective cooling system. Batteries work based on the voltage differential principle, and at higher temperatures electrons inside becomes unstable and reduces the voltage between two terminals of the pack. To get the optimum and consistent performance from the battery pack the operating temperature range must be in the range of 15 °C to 40 °C, furthermore the temperature difference amongst the cells within the battery pack should be minimum (not more than 3 °C to 5 °C depending on pack size).

If there is a large difference then it can lead to fast degradation of the cell health and performance, imbalance in the cell capacity and voltage, capacity degradation, thermal runaway and in many cases fire and explosion. Hence to keep the uniform battery pack temperature within the manufacturer’s recommended range, innovation & improvement in the battery cooling system is happening continuously.

Let’s Find Out Which Cooling System Works Best For Your Battery Pack

Choosing cooling system depends on various factors like type of cells, chemistry, position or location of the battery pack in the vehicle, battery pack enclosure, energy density of the pack, weight and volume constraints for the given application, maintenance of the cooling system, cooling required for other high voltage and low voltage components etc. We will discuss these aspects based on various cooling methods as below:

Air Cooling

Air cooling system is the most easiest and simple method of cooling. It uses the convection method of heat transfer from the battery pack. Air cooling method is not very efficient compared with other cooling methods but it is useful for small capacity of up to 0.5 to 1.5 kWh of the battery packs for two wheelers and three wheelers. The forced air cooling method by providing fan is also possible to increase the efficiency of cooling system but this requires additional parasitic energy from fans, with additional weight and volume associated with fan ducts and manifolds. However, the battery pack position and location in the vehicle matters for the efficient air cooling. Improvements to the effectiveness of air cooling systems have been made in recent years. This centers around three main aspects: cell arrangement, air flow path, and flow rate. The root cause is that air has lower heat capacity as well as lower thermal conductivity compared with other mediums (e.g. liquids and phase change materials); reducing cooling capabilities and causing poor temperature uniformity in the battery pack. So not recommended for higher energy density battery packs considering the safety risks in hot climates.

Fin Cooling

Fin cooling is more efficient than simple air cooling as it increases the surface area to increase the heat transfer. Heat is transferred from the battery pack to the fin through conduction and from the fin to air through convection. The advantage with fin cooling is having high thermal conductivity and hence better cooling efficiency. But it costs additional weight to the battery pack. Unlike the electronics and ICE vehicles where fin cooling has found lot of success, it is not suitable for the Lithium battery packs with higher capacity and higher energy density. Battery packs with fins are ideally suitable for two wheelers where battery pack capacity is from 1 to 3 kWh. Fins with the added fan could be a better option for two and three wheeler applications up to 5 kWh capacity of the pack. In addition to that, the battery pack enclosure can be made out of Aluminium to keep it light weight and the position in the vehicle should be such that the air flows over the entire battery pack while vehicle is running.

Phase Change Material (PCM)

One of the cooling methods is a passive cooling system using a phase change material (PCM). PCM can accommodate a large amount of heat through small dimensions. PCM absorbs heat energy by changing state from solid to liquid. While changing phase, the material can absorb large amounts of heat with little change in temperature. It is easy to apply and requires no power in the cooling system. The latent heat associated with the material, as it undergoes a phase transition during heating, affords it with favorable specific heat capacity characteristics. This can come in different forms with the main classifications including: solid-liquid and liquid-gas PCMs. When selecting a PCM for a Battery Thermal Management System (BTMS), the melting point, specific heat capacity and thermal conductivity are often key figures of merit. One of the main organic PCMs used is paraffin, with the length and formation of carbon within the chains giving the paraffin different physical and chemical properties. The challenges with PCM cooling method is direct cooling of the cell due to the thermally conductive additives reduce the dielectric properties of the material and solid-phase heat transfer being limited to conduction. Also, the volume change that occurs during a phase change restricts its application. As phase change material can only absorb heat generated, not transfer it away, which means that it won’t be able to reduce overall temperature of the systems. Hence it is not encouraging for use in passenger vehicles or any mobile application. But PCM can be useful for improving thermal performance in static applications by reducing internal temperature fluctuations and reducing peak cooling loads.

TAB Cooling

Battery tabs are the positive and negative connectors (terminals) that carry the cells’ electrical current. Each tab is connected to a module’s bus-bar (or a collector plate) that redistributes the positive and negative current to new terminals. Several modules are connected in the same way to form the battery pack. Battery tabs provide the connection between multiple layers of current collector plates and the external target source. The tab is welded to the current collectors (foil-to-tab) and then exits the cell, enabling the transfer of power to an external source. Battery tabs play an important role in lithium-ion cell manufacturing. Typical large format lithium-ion cells use copper for the anode foil (current collector) and aluminium for the cathode foil. A ‘foil-to-tab’ weld is needed to gather all the current collector plates (foils) inside the cell and join them to a tab. In tab cooling, instated of providing cooling at the cell surface, cell tabs are cooled. It results in even cooling of each cell layer. Recent progress on pouch cell tab cooling has been proven as a promising way to achieve high temperature uniformity. However, cell tab cooling is not always better than surface cooling. In the case of a large form factor and higher capacity pouch cell where the tabs are on the same side, pure tab cooling does not remove sufficient heat, resulting in a higher rate of degradation compared to surface cooling. Tab cooling may be promising due to its ability to impose favorable temperature gradients, and potential to extend cell lifespan. Nevertheless, two bottlenecks still needed to be taken into account before tab cooling can be upscaled. Firstly, tab cooling may not be as effective in controlling the maximum cell temperature, especially for harsh utilization conditions. Secondly, the practicalities of tab cooling systems designs are still being debated, with the ease of system manufacturability a key issue. The tab should be electrically isolating but most cooling tabs are aluminium. How to accelerate the heat transfer rate while maintaining the tabs in an insulation environment is still, therefore, under debate.

Liquid Cooling

Liquid cooling system is the most effective and widely used cooling system for Lithium batteries. Liquid cooling deliver the best performance for maintaining a battery pack in the correct temperature range and uniformity. More already proven the liquid coolants have higher heat conductivity and capacity plus better heat dissipation rate than air, so cooling performs is very efficient. The studies have proven that indirect liquid cooling is more practical than direct liquid cooling. Where the other cooling methods fail at energy efficiency and compactness, liquid cooling does not consume too much parasitic power and delivers the desired performance easily for the larger size of the battery packs. Most of the EV passenger car manufacturer uses some or other form of liquid cooling system.

Since the EV and lithium battery technologies are still evolving there are certain challenges to maintain temperature range and uniformity in extreme conditions. And as industry gains the experience developing this systems, the issues should be resolved.

Let’s understand between direct and indirect cooling methods:

  • Direct Liquid Cooling

The cells are submerged into a non-conductive dielectric fluid in direct cooling system. Some of the dielectric fluids being used are: hydrocarbon oils, silicone oils and fluorinated hydrocarbons. This unique way of cooling brings several advantages. Firstly, immersion cooling has the potential to provide the best pack and cell temperature uniformity among all the cooling methods. This is because all battery surfaces are in the fluid, providing a homogeneous, high heat capacity thermal transport path for heat rejection. This direct contact with the cell surfaces further reduces the thermal contact resistances experienced in indirect cooling systems. Furthermore, the suppression of thermal runway is usually observed for immersion cooling systems as some of the dielectric fluids are also flame-retardants, enhancing the safety of the Lithium battery pack. Immersion cooling with dielectric fluids is one of the most promising methods due to direct fluid contact with all cell surfaces and high specific heat capacity, which can be increased even more if the latent heat of vaporization is used in 2- phase operation. Such high heat rejection capability can also improve safety by suppressing thermal runaway under some circumstances.

Direct liquid cooling or immersion cooling method is very complex and under the research and development stage, with no cars on the market using this system. Also, the cell form factor, manufacturing difficulties considering IP ratings of the pack makes it unsuitable for the passenger vehicle application. Nevertheless the direct cooling system can be very efficient and effective for very high voltage and high energy density packs and static application.

  • Indirect Liquid Cooling

Indirect cooling system is similar to conventional IC engine vehicle cooling system. It circulates the liquid coolant through metal pipes such that the maximum cell surface remain in to the contact. The construction of the cooling system will be much different in EVs than in ICE. To achieve the maximum temperature uniformity the shape and the structure of the battery pack will be different for each EV manufacturer. When circulating a liquid coolant throughout metal piping, it is important to protect against corrosion to protect vehicle safety and performance.  What is currently known about corrosion prevention in internal combustion engine cooling systems can be easily applied to the indirect liquid cooling system in electric vehicles. The known coolant types are glycol or polyglycol blended with some additives having properties like antifreeze, anti-rust and protection against scale and corrosion.

Conclusion

Temperature affects batteries in five major ways:

  • Operation of electrochemical system
  • Efficiency and charge acceptance
  • Power and energy efficiency
  • Safety and reliability and
  • Life and life-cycle costs

Li-ion batteries are extremely sensitive to low and high temperatures. For battery packs it is important to regulate the pack to remain in the desired temperature range for optimum performance and life, and also to reduce uneven distribution of temperature throughout a pack which would lead to reduced performance. Importantly, the attainment of even temperature distributions through the battery pack eliminates potential hazards related to uncontrolled temperature build-up or thermal runaway. Therefor it becomes very essential to choose a right battery thermal management system for the application considering performance, manufacturing, longevity and maintenance aspects.

References

AmplEV

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