ProductsANSYS Simplorer 10.0, ANSYS Fluent 14.0

KeywordsBattery, battery circuit model, electrochemistry model, CFD, model-order reduction, lithium-ion, HEV, EV, VHDL-AMS

DescriptionThe lithium-ion battery is a preferred candidate as a source of power for both hybrid electric vehicles (HEVs) and electric vehicles (EVs) because of its high energy density, high voltage, low self-discharge rate and good stability. HEV and EV applications require very large lithium-ion batteries. During high power extraction required to drive a vehicle, these large batteries may experience a significant temperature increase which can lead to safety concerns. A properly designed thermal management system is crucial to prevent overheating and uneven heating across a large battery pack, which can lead to degradation, mis-match in cell capacity and thermal runaway. Design of the thermal management system requires knowledge of the amount of heat that will be generated within cells within the battery pack as well as the cooling system.

Electrical circuit and electrochemical battery models are the two most popular models for studying battery heat generation.

The electrical circuit battery model expresses heat generation by open-circuit potential along with the potential difference between positive and negative electrodes. This model provides accurate information about temperature, but it requires experimental testing data as input. The electrical circuit battery model is implemented in ANSYS Simplorer; Figure 1 shows the battery discharge curves at different discharge currents.

The electrochemistry battery model is physics based and solves for both electro-chemistry and energy equations from geometry parameters and temperature-dependent properties. Since the model is physics based, it does not require battery testing data as inputs; it can provide battery performance data that would otherwise need experimental testing to obtain.

The most well-known physics-based model is the Newman pseudo-2-D model, which is implemented in Simplorer using VHDL-AMS. Figure 2 shows the concen-tration profiles calculated in the Newman model at the same time and discharge rate but for different temperatures.

Application Brief

Designing Batteries for Electrical Vehicles

An electrical battery consists of one or more electrochemical cells that convert stored chemical energy into electrical energy. This battery is one of the critical components in a modern hybrid and electric vehicle.

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Figure 1. Voltage discharge curves at different discharge currents using electrical circuit battery model in ANSYS Simplorer

Figure 2. Concentration profiles calculated in the Newman pseudo-2-D model at the same time and discharge rate (but for different temperatures) in Simplorer

Designing Batteries for Electric Vehicles

The CFD solver ANSYS Fluent can be used for thermal management design in desired temperature ranges, reducing pressure drop and maintaining tempera-ture uniformity. Fluid dynamics has been widely used to predict flow and heat transfer in many industries, and it applies well to battery thermal management. A complete battery thermalfluid dynamics analysis, including optimization, can be done entirely within ANSYS Workbench. Figure 3 shows the temperature distribution of a battery pack calculated in Fluent. While fluid dynamics simulation can give detailed thermal information about battery thermal management systems, it is time consuming to perform many transient simulations under different drive cycles. Model-order-reduction tech-niques exist for extracting a model from CFD results, and the extracted model can give the same solution as that from the full CFD model but it runs much faster compared with CFD. The model-order-reduction process is automated in Simplorer using CFD results as inputs. Electrical circuit and electrochemical models can cosimulate with the reduced-order thermal model in Simplorer, which is suitable for systems-level simulation. Figure 4 shows an electrical circuit model cosimulated with a reduced-order thermal model in Simplorer.

SummaryThe knowledge of the batterys heat generation and its temperature is critical for lithium-ion battery design in HEV/EV applications. ANSYS provides a com-plete solution for battery design. Electrical circuit and electrochemical models are available in Simplorer to study heat generation. Temperature distribution of the battery cell/pack can be calculated in Fluent. A reduced-order thermal model based on CFD results is also available in Simplorer for systems-level simulation. Finally, electrical and electrochemical models can cosimulate with the reduced-order thermal model in Simplorer, which provides a complete systems-level battery model.

Authors Eric Lin, eric.lin@ansys.comXiao Hu, xiao.hu@ansys.com

ReferencesHu, X.; Lin, S.; Stanton,S.; Lian, W. A Foster Network Thermal Model for HEV/EV Battery Modeling. IEEE Trans. on Industry Appl., 2011, vol. 47, no. 4, pp. 1692-1699.

Hu, X.; Lin, S.; Stanton, S.; Lian, W. A State-Space Thermal Model for HEV/EV Battery Modeling. SAE paper 2011-01-1364.

Hu, X.; Stanton, S. Simulating Rechargeable Lithium-Ion Battery Using VHDL-AMS. SAE paper 2012-01-0665.

Figure 4. Electrical circuit model cosimulated with reduced-order thermal model in ANSYS Simplorer and battery cell temperatures as a function of time in discharge process

Figure 3. Temperature distribution of battery pack calculated in ANSYS Fluent

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