Battery System Architecture

This article looks at system architecture using lithium-ion battery systems as an example. The system architecture has a major influence on the robust design of a battery system, because it directly defines and fulfils the required product functions and characteristics.

Why systems engineering matters for battery systems

Beyond electromechanical, software, electrochemical and materials expertise, battery system design requires a thorough understanding of Systems Engineering (SE). Many aspects of battery system design and development fall within the scope of SE. The main reason to apply SE throughout the product development lifecycle is the high complexity of the battery system, which combines multiple functions. Without a solid SE foundation, system complexity and interdisciplinary work quickly create a very challenging development environment. In the automotive industry, management increasingly recognises the need for an SE-centric approach, mainly because of the high complexity of the target product.

The V-model as development framework

The state-of-the-art development process for a battery system or its components, such as the Battery Management System (BMS), follows the so-called V-model in the automotive industry. Standards such as ISO 26262 and the Automotive SPICE process assessment and reference model, which are based on the V-model, are applied in addition.

Figure 1 shows the product development process schematically, with a focus on SE and on the software engineering of BMS development according to the V-model. The approach is customer-centric: product development is based on customer requirements. The phases highlighted in blue are those in which the SE discipline plays a pivotal role and carries the main activities and responsibilities. Based on the derived system architecture elements and their functions, the sub-disciplines – hardware (HW), software (SW) and mechanical engineering (ME) – design and implement their respective component requirements. In simple terms, the left side of the V-model covers the battery system design, while the right side covers its integration and testing. Functional safety is not considered here for simplicity.

Fig. 1: Product development process according to the V-model

From product functions to the physical architecture

The general approach is straightforward: the required product functions are broken down into sub-functions derived from the overall system function. The functional structure should remain solution-independent, as it often serves as the basis for later solution principles. In the same way, the physical product architecture requires defining and analysing the physical interdependencies between the sub-components. By transforming sub-functions into sub-solutions, the product structure of a battery system can be derived. Figure 2 shows a battery system product architecture. It is a simplified version; the interdependencies between components are not shown.

Fig. 2: A simplified generic battery system product architecture

Defining the functional structure

Based on the physical product architecture in Figure 2, the functional structure can be defined. For each main system component, the high-level functions it has to fulfil are outlined (Figure 3). This list is not exhaustive and should be seen as a basic reference.

Fig. 3: A simplified overview of the function structure of a battery system architecture

Each main function is then broken down into design-relevant architecture requirements. Hardware and software functions in particular are divided into three subcategories:

1. Measurement

2. Processing

3. Monitoring

When defining these design architecture requirements, it must be ensured that the main function on the level above is fully covered (Figure 4).

Fig. 4: Interdependency of the sub-functions required to fulfil the main function

The electrical and electronic (E/E) architecture

A battery system must operate efficiently and safely over the lifetime of the battery and its application, and it must be able to deliver and accept a defined amount of power over a defined period. This is the task of the electrical and electronic (E/E) architecture. Figure 5 shows a simplified, technology-independent battery system E/E architecture with the following main components:

1. BMS slave – cell controller board

2. BMS master

3. Low-voltage and high-voltage interfaces

4. Fuses, contactors and related components

Fig. 5: Schematic illustration of a battery system E/E architecture

Summary

A robust system architecture is the foundation of a reliable battery system. Although the automotive battery system served as the reference here, the process of defining the system architecture and its main functions is largely the same across applications, with only minor differences. The full scope of such a work product cannot be covered in a single article, but the steps above provide a practical introduction.

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