Several possible definitions of “system” exist for electric vehicle (EV) battery modeling. They range from the battery’s interactions with the complete EV drivetrain to thermal modeling of the battery pack, battery cell development and integration into battery packs, and the battery management system (BMS), which monitors the function and health of the battery pack. Different types of software tools are used for various modeling scenarios.
A circuit-based battery model has been developed to analyze the performance of a battery pack in the operation of the overall EV drive train. The model was implemented using a series of mathematical functions to represent the nonlinear relationships between the battery’s state of charge and other parameters. The battery equations modeled a battery pack with series and parallel combinations of cells.
Elements of the model included (Figure 1):
- Battery capacity and open circuit voltage
- Battery state of charge (SOC)
- Short and long period response for charge depletion and recovery
Figure 1. A circuit-based model has been developed to simulate the operation of an EV battery pack as an element in the overall drivetrain. (Image: MapleSoft)
A power controller model was used to define the interaction between the battery and the motor. Finally, terrain and drive-cycle models were included to model vehicle operation. The completed modeling system was used to simulate various driving conditions, including different acceleration rates and driving through a hilly environment.
Benefits of lumped modeling
Two standard techniques for thermal modeling of batteries are high-fidelity and lumped modeling. High-fidelity modeling models individual cells and packs and provides detailed insights into thermal performance. It looks at current and voltage distribution within individual cells and the concentrations and transport of lithium ions inside the cell. It can be used to model capacity fading and failure mechanisms. Battery designers require various implementations of high-fidelity modeling, but most are too resource-intensive to be used by EV makers when analyzing battery pack performance.
Lumped models are much less computationally intense, require fewer input parameters, and can provide a sufficient level of accuracy to be used by EV makers when modeling the performance of battery packs with hundreds of cells (Figure 2).
Lumped models use parameters that are easily obtained by battery pack designers like:
- Cell capacity
- Initial state of charge
- Open-circuit voltage versus the state of charge
- Basic parameters related to voltage or capacity losses
Figure 2. Lumped models can be used to effectively simulate battery pack thermal performance. (Image: COMSOL)
Advanced system development
While lumped modeling is useful in many cases, it’s insufficient for every development effort. This is particularly true if a new battery chemistry, cell packaging, or other complexities are introduced into the battery system development process. In this case, extensive development efforts would be required involving multiple disciplines, from electrochemists to cell designers, packaging experts, and more.
Those design efforts can benefit from an integrated development environment that begins with initial architecture definitions and battery system sizing and extends through vehicle integration and testing (Figure 3). Development teams can benefit from a software environment that integrates multi-physics system simulation, 3D computer-aided engineering (CAE), computational fluid dynamics (CFD), electromagnetics, physical testing, data analytics, and the use of digital twins and digital threads.
Figure 3. Advanced battery system development requires a highly integrated software environment. (Image: Siemens)
Battery management system
BMSs are critical for optimizing battery system operation and safety. Designing a BMS is a complex task that includes hazard and operability analysis [HAZOP], fault tree analysis [FTA], failure mode and effects analysis [FMEA], failure mode effect and diagnostic analysis [FMEDA], and confirmation that the BMS software meets the requirements of the ISO 26262 functional safety standard for road vehicles.
Typical elements of a BMS include:
- The battery pack includes thermal management and connectivity to the EV powertrain
- An electronic control unit (ECU) includes software to monitor the voltage, current, and temperature of individual cells and modules in the pack — during driving and while charging (Figure 4)
- A switch box to disconnect the pack if needed
Figure 4. While an EV battery is charging, the BMS software constantly monitors the charging voltage, state of charge (SOC), temperature and cell voltages. (Image: Ansys)
Summary
Various software tools are available for EV battery modeling at the system level. These tools range from math modeling to comprehensive multi-physics-based environments. They have been optimized for detailed cell modeling and basic battery pack modeling. Dedicated tools are also available for modeling battery management systems.
References
- Battery Design Module, COMSOL
- Battery Innovation: Modeling and Simulation of Electric Vehicles, Maplesoft
- Battery Management Systems for Safer Electric Vehicles, Ansys
- Battery pack design for optimal performance, Siemens
- Design and simulate battery and energy storage systems, MathWorks
- Electric and Hybrid Vehicle Testing, Pickering
Images
- Figure 1 , Maplesoft
- Figure 2, COMSOL
- Figure 3, Siemens, Figure 10
- Figure 4, Ansys, screen shot at 10:20 into the webinar
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Filed Under: Batteries, FAQs, Software