Examining EV Battery Thermal Management

By nature, batteries involve multiple interacting physics phenomena, making them multi-physics systems. Optimizing battery systems requires a deep knowledge of their underlying processes and principles, as well as how they affect each other. Multiphysics simulation software provides an environment where battery designs can be modeled and understood in order to give the insights required for optimizing battery systems. In this article, we will discuss how modeling and simulation can be used to facilitate the design of thermal management systems for EV batteries – from the cell scale up.

Thermal Management and Modern EV Batteries

The battery packs within EVs consist of hundreds and even thousands of individual cells grouped within dozens of modules. The design approach depends on the goals of the system. For example, manufacturers of sporty EVs prioritize high power output to support rapid acceleration and high top speeds, while those designing long-range electric vehicles focus on using higher energy-density cells to optimize range at the expense of peak power output.

Building multiphysics models of batteries across scales provides insight into how batteries perform under various conditions as well as their thermal characteristics, which helps engineers develop the optimal EV battery designs. (Image: COMSOL)

Regardless of whether the EV is designed for speed, range, or a balance of both, thermal management is crucial to the vehicle’s success. Poor thermal monitoring and management — evident in uneven wear across modules and dangerous localized temperatures — can damage cells, packs, and even the entire vehicle. In worst-case scenarios, EVs (and their connected chargers) can catch on fire.

Thermal management poses a significant challenge in the design of EV batteries because the batteries are expected to cope with diverse situations: a wide range of outside environments and temperatures, varying road conditions, and the possibility of being involved in one of the millions of accidents that occur annually. Multiphysics simulation can help ensure that the batteries that power EVs are capable of meeting this challenge.

Optimizing Thermal Management with COMSOL Multiphysics^®

Designing an efficient thermal management system, testing different cooling mechanisms, or studying the effect of temperature on cell performance and durability requires building an electrothermal model that captures the underlying electrochemical, thermal, and fluid flow phenomena and couples them together.

Specialized Battery Modeling Functionality

Battery pack voltage and temperature over time. (Image: COMSOL)

When using the COMSOL Multiphysics simulation platform together with the Battery Design Module add-on product, simulation engineers get access to a wide range of specialized functionality for modeling batteries and electrochemistry. This functionality can be used to model various chemistries, including current lithium-ion technologies and older technologies that predate them, as well as novel chemistries currently under research, such as solid-state, lithium–sulfur (Li–S), and sodium-ion batteries. Users can study cells across all scales, simulate diverse conditions and load profiles, and monitor key performance indicators such as voltage and capacity.

Regardless of cell chemistry and type, as the cell operates, internal losses — such as ohmic losses in the electrolyte and electrodes, activation losses from electrochemical reactions, entropy changes, and losses induced by local gradients in composition — manifest as heat. These heat sources are defined in the software within dedicated electrochemistry and battery user interfaces, laying the foundational step toward building an electrothermal model. Such a model is then completed by adding physics like heat transfer and fluid flow, which the software integrates cohesively.

Heat Transfer and Fluid Flow

A dedicated heat transfer user interface specifies how the cell or pack interacts thermally with the environment, and it also includes the losses in the cell that contribute to the generation of heat (see below). By customizing this interface, it is possible to define thermal properties for different solid or fluid domains and set relevant boundary conditions that simulate operational scenarios. To design a thermal management system, the physics involved go beyond heat transfer and battery considerations to include fluid flow modeling as a crucial pillar. Fluid flow modeling is essential for predicting how coolant or air circulates through the battery pack and other components, as well as how heat dissipates from these components.

Multiphysics Couplings

The COMSOL® software couples all physics seamlessly, enabling users to capture their interconnected effects and optimize thermal management strategies accurately. For example, integration between the battery and heat transfer interfaces is automatically managed by the dedicated Electrochemical Heating multiphysics coupling. This coupling aggregates heat generation from all mentioned sources and integrates the battery or electrochemistry and heat transfer user interfaces.

This ensures that the total heat generation from the battery is accurately incorporated into the energy balance equation solved by the heat transfer interface. The heat transfer interface can also be linked to a fluid flow interface via the Nonisothermal Flow multiphysics coupling, enabling a comprehensive analysis of how fluid dynamics affect thermal management. This workflow remains consistent regardless of the cell, pack, or chemistry being modeled.

Thermal model of a 200-cell battery pack for identifying hot spots. (Image: COMSOL)

Modeling at Different Scales: Cell Scale

Even if the end goal is to design the thermal management of a battery pack, it is essential to understand how each cell behaves thermally under different conditions. This foundational knowledge allows for a holistic study and monitoring of the behavior of cells when assembled in various configurations. The better these dynamics are understood, the more effective and efficient the pack-level design will be.

Consider cells using electrode or electrolyte materials that become thermally unstable at higher temperatures, for example, or the varying dissipation requirements of different cell types such as pouch, prismatic, and cylindrical cells or different tab designs. Studying these factors to establish a solid understanding of them before proceeding with pack implementation, rather than iterating back and forth, ensures a more robust design process and avoids unnecessary loops.

From a performance standpoint, temperature significantly impacts cell operation and the occurrence of parasitic reactions. Extreme temperatures, whether too high or too low, can increase internal resistance and diminish capacity. For instance, in lithium-ion cells, temperatures below freezing can induce lithium plating during charging, whereas high temperatures can accelerate solid electrolyte interphase (SEI) growth. This highlights the critical importance of operating the cell within its designed temperature range to maintain optimal performance and longevity.

Pack Scale

At the pack scale, temperature gradients can cause cells to degrade at different rates, leading to imbalance and reduced overall performance. Combining electrochemical and thermal management aspects enables monitoring of the temperature of each cell and performance indicators such as voltage and state of health (SOH). This monitoring helps users achieve uniformity and design a cooling mechanism tailored to ensure all cells operate within their optimal range.

Additionally, utilizing the COMSOL Multiphysics software’s extensive heat and fluid flow features allows for implementing, altering, and testing various thermal management strategies, such as air, liquid, or hybrid cooling, or novel approaches like phase change material (PCM) cooling. Such experimentation helps users gain perspective on these methods and ensure their consistency with vehicle design constraints.

Avoiding Disaster with Multiphysics Modeling

In addition to effective operation, safety is also a critical concern when it comes to batteries. Batteries can experience thermal runaway when pushed beyond their normal operating range, subjected to damage, or affected by a short circuit. Thermal runaway starting in one cell can propagate across the pack, increasing the risk of fires or explosions. To get insight into how this type of failure could develop and progress in prospective designs, battery designers can turn to modeling and simulation to test their designs without damaging any materials — or themselves, for that matter — in the process. Through simulation, engineers can see inside the battery pack in a way that is impossible in a lab setting, and multiphysics simulation, specifically, ensures that the models reflect the real-world context in which the battery pack will eventually operate.

EVs and Simulation

The EV market remains one of the more promising alternatives to traditionally powered vehicles. While EV batteries are complex, their design and operation can be well understood and made more efficient through multiphysics modeling and simulation. With COMSOL Multiphysics, users have the ability to visualize and evaluate their designs from the cell level to the pack level, all within the same software environment.

This article was written by Niloofar Kamyab, COMSOL, Inc. (Burlington, MA). For more information, visit here .

COMSOL and COMSOL Multiphysics are registered trademarks of COMSOL AB.