Read time: 10 minutes
Target audience: EV Researchers/ Automobile Engineers
Written by: Dr Tabish Wahidi

Background:

In battery simulation, selection of accurate equivalent model is always crucial. The appropriate equivalent circuit model for battery simulation depends on many factors such as: Objective of the simulation (steady state or drive cycle analysis), Level of detail required (Simpler/less detail or more detail approach), Complexity of model, Input data available (less detailed input data or comprehensive input data), Thermal management scenario, Computational time and resource constraints etc. This blog gives an overview of the types of equivalent models, criteria of selection of appropriate model and advantages of each model for the battery thermal management.

Introduction:

In this blog we will discuss on the two different commonly used Equivalent circuit methods available for defining battery.  The first method utilizes the Newman, Tiedemann, and Gu (NTG) Equivalent parametric model of battery behaviour, while the second method employs a parametric model developed by the National Renewable Energy Laboratory (NREL). The NTG and NREL styles are similar in that the behaviour is captured in a zero-dimensional lumped parameter model. These models rely on empirical data and do not attempt to explain the physics of current transfer from first principles. Each model has its advantages and disadvantages, and the choice of model depends on the analysis objectives.

NTG-Equivalent model: The NTG equivalent circuit model (shown in Fig. 1) describes battery behaviour in terms of a voltage source and an internal resistance. In this model, the current density and voltage distribution are assumed to be uniform across the electrodes.

The internal heating of a cell is treated as a uniform heat flux over the electrodes, meaning there is no detailed cell-level analysis. This model focuses on steady-state behaviour, effectively capturing the overall performance of the battery under typical operating conditions. Based on empirical data, the NTG model does not delve into the fundamental physics of current transfer but instead uses observed data to define circuit parameters. It is particularly useful for applications requiring an accurate depiction of battery performance without the need for detailed transient response analysis. The NTG model is relatively simpler to implement compared to more complex models and requires less detailed input data. Its ease of use and accuracy in representing steady-state behaviour make it a popular choice for general battery analysis and design.

NREL-Equivalent model: The NREL equivalent circuit model (shown in Fig. 2) is like the NTG model, but there are two additional circuit features, the first is the addition of “filter circuits” in series with the voltage source and the internal resistance. The second is the inclusion of an optional resistance used to model a built-in current limiting element found in some cell designs.

The filter circuits consist of a series of parallel resistor/capacitor (RC) circuits. These simulate the response a battery exhibits to changes in imposed current that occur, for example, during a drive cycle with transient power demands on the battery pack of a vehicle. In the NREL model, some cells are equipped with resistive elements that are highly sensitive to temperature. In the event of a fault condition causing a high current, these elements heat up, causing the resistance to increase significantly and thereby limiting the current flow. This resistance depends solely on temperature. NREL is well-suited for drive cycle studies where peak cell voltages are important. However, its increased complexity means that the input data required is more detailed and developing the input parameters from charge/discharge data is more complex. Additionally, the NREL model can simulate the impact of an internal, temperature-dependent series resistance in the battery, which is designed to limit fault currents during a short circuit.

Advantages of NTG model:

1. The NTG model is simpler to implement and requires less detailed input data.
2. It effectively captures the steady-state behaviour of batteries.
3. Requires less computational power and time
4. Relies on empirical data to define circuit parameters, ensuring that the model is grounded in observed performance characteristics of batteries.
5. Its straightforward approach makes it a versatile tool for various applications, including basic design checks and performance monitoring in different battery technologies.

Advantages of NREL model:

1. Offers a high level of detail and accuracy
2. Can simulate the effects of temperature-dependent resistive elements, providing insights into thermal behaviour and fault current limitations, which is crucial for safety and reliability assessments.
3. Well suited for drive cycle studies

Conclusion:

The NTG model is well-suited for applications where steady-state performance and general battery behaviour are of primary interest, as it provides a simplified yet accurate representation using empirical data without extensive detail on transient responses. This makes it ideal for routine battery performance evaluations and design optimizations that do not require intricate transient analysis.

The NREL model is preferred for studies involving dynamic conditions, such as drive cycle analyses where understanding peak voltages and the detailed transient response of the battery is crucial. Its capability to incorporate temperature-dependent resistive elements also makes it valuable for fault current analysis and advanced thermal management. While the NREL model’s complexity requires more detailed input data and a more involved parameter development process, it offers a comprehensive toolset for scenarios demanding high-fidelity simulations of battery behaviour under varying operational conditions.