Identifying Building Blocks for Second-Life-Enabled Battery Management Systems

Identifying Building Blocks for Second-Life-Enabled Battery Management Systems Julian Bl¨ umke C-ECOS Technische Hochschule Ingolstadt Ingolstadt, Germany [email protected] Claudius Laves C-ECOS Technische Hochschule Ingolstadt Ingolstadt, Germany claudius.la[email protected] Hans-Joachim Hof C-ECOS Technische Hochschule Ingolstadt Ingolstadt, Germany [email protected] Abstract—The growing adoption of electric vehicles has led to a significant increase in the demand for lithium-ion batteries. Although these batteries can no longer meet the energy and power requirements of electric vehicles once their capacity drops below a certain amount, they retain sufficient performance for secondlife applications such as renewable energy storage and backup power systems. The applications align with circular economy principles by reducing reliance on newly mined materials and promoting battery reuse. However, integrating also the original battery management system into second-life applications often involves compatibility and control systems challenges. Through a comprehensive analysis of existing literature and methodology, this study proposes a novel approach to battery management system design to enhance the sustainability and efficiency of battery repurposing. Building blocks to enable the installation of updated firmware and to access historical data were identified to ease the reuse of first-life battery management systems. The findings provide valuable insights into advancing circular economy practices and improving resource utilization in the context of sustainable energy systems. Index Terms—second-life battery, SLB, battery management system, BMS, circular economy, sustainability I. INTRODUCTION Due to the rise of electric vehicles (EVs), the demand for lithium-ion batteries (LIBs) is also increasing similarly [1]. LIBs are usually used in an EV until 80% of the original battery capacity is left, since energy and power demands cannot be satisfied anymore [2]. However, these batteries are often good enough for second-life applications, e.g., to store power generated by renewable energy sources (RES) or as a backup system during power outages [3]. The characteristics of these second-life applications can differ, so different types of repurposing an EV battery are possible: depending on the application, the cells, the modules, or the whole pack are used [4]. Usually, they are installed with a newly developed battery management system (BMS) instead of reusing the original component from the first life. A BMS monitors and controls parameters of a LIB and is mandatory to ensure a safe battery operation [5]. Second-life batteries (SLBs), in general, already support the circular economy approach, reducing the demand for virgin materials and the production of new LIBs [6]. Further encouraging the continued utilization of used battery compoThis paper has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101138031 (REBORN). nents, this paper identifies building blocks for a reusable and second-life-enabled EV-BMS. The objective of this paper is expressed through three research questions: RQ1 What are the architectural and technological differences between the first and second life of battery systems? RQ2 What benefits and challenges are discussed in the literature concerning reusing the original BMS in second-life applications? RQ3 What mechanisms are necessary to ease the transition from first to second life while enabling reuse of the original BMS? The remainder of the paper is structured as follows. Section II details the methodological approach, including data and literature selection. Section III presents the main findings, which are further explored through an application scenario in Section IV. Finally, Section V summarizes the insights and outlines directions for future research. II. METHODOLOGY Literature review on second-life applications Classification into categories Finding reference architectures Comparing first-and second-life architectures Literature search Title & Abstract screening Paper skimming Cat. 1 Cat. 2 Cat. 3 Cat. 4 Cat. 5 Fig. 1. Methodology to find architectural and technological differences between the first and second life of battery systems In general, the analyses are based on a review of the academic literature. To answer RQ1, a literature review was conducted investigating actual applications of SLBs. These were then classified, summarizing similar applications on a high level. A reference architecture was identified for each category. These reference architectures were compared to a reference architecture of an EV battery system. This comparison then identifies architectural and technological differences between the first and the second life of battery systems. The process is summarized in Figure 1. RQ2 was addressed through a literature review focusing on the pros and cons of utilizing the original BMS in second-life applications. Finally, RQ3 was approached by leveraging the results of RQ1 and RQ2, formulating necessary mechanisms to ease the transition from first to second life.979-8-3315-9515-9/ 25/$31.00 ©2025 IEEE TABLE I CATEGORIES OF SECOND-LIFE APPLICATIONS Category Examples Source Behind the meter (BTM) peak shaving, max. use of locally generated power, UPS, feed-in-tariffs/time-of-use bill management [7]–[18] Front of the meter (FTM) peak shaving, grid stability, energy arbitrage, frequency regulation, black start, ancillary services, spin/no-spin reserves, mitigation of intermittent effects of RES [7], [9]–[12], [14], [16]–[20] Stationary peak shaving, (fast) EV charging, standalone/isolated grid, stabilize output power, residential ESS, commercial ESS, industrial ESS, street lighting, wind power storage system [3], [4], [12], [16], [20]–[28] Semi-stationary energy supply for construction sites, portable charging devices, container storage system [3], [21], [26] Mobile short-range vehicles (forklifts, recreational vehicles, ...), fast charging stations, low-speed E-bikes/motor cars, consumer electronics, mobile telecom. towers, rail transport [3], [12], [21]–[23], [26] III. RESULTS A. RQ1: What are the architectural and technological differences between the first and second life of battery systems? 1) Categorization of SLB applications: The academic literature, summarized in Table I, identifies two primary views for classifying applications on SLB. The first approach considers these applications from the standpoint of the power grid, thereby categorizing them into those situated in front of an electricity meter (Front of the Meter (FOTM)) and those located behind a meter (Behind the Meter (BTM)). FOTM use cases include matters that concern grid stability, like frequency regulation or ancillary services, but also activities related to large-scale energy storage within RES to mitigate their intermittent effects. Peak shaving, i.e., the strategic reduction of the electrical load at times of peak demand, can reduce the need for increased energy generation from conventional power plants at these times. The latter is not only FOTM, but also a BTM application, often applied to residential or industrial applications. Maximizing the use of locally generated power, e.g., by storing energy produced by a photovoltaic (PV) system, is one example for SLBs in private households. This can be combined with time-of-use bill management, where energy is bought and temporarily stored when it is cheap and used or sold when it is expensive. The second category group examines the mobility of second-life applications, distinguishing between stationary, semi-stationary, and mobile applications. The examples mentioned for FOTM and BTM can mainly be classified as stationary applications. Additionally, use cases for standalone and isolated grids not having a conventional meter are also mentioned here. The group of semi-stationary applications is only identified by a small margin of research works since these are a mixture of stationary applications, i.e., during operation, and mobile applications, i.e., transportable. An example is an energy supply for construction sites. Mobile applications include vehicles where range is not a dominant requirement, e.g., forklifts or recreational vehicles such as golf carts. SLBs can also be used as an energy supply for mobile telecommunication towers, e.g., during emergencies. 2) Architectural and technological comparison: In Figure 2, reference architectures for each high-level category are shown. For simplicity, a mid-layer dividing the battery system into modules is omitted. It is also assumed that the internal structure of the battery system remains unchanged when being transferred to a second-life application, so only external connections are considered. The architectures shown in Figure 2 do not indicate energy flow, thus, the visible connections are only shown where data, e.g., to monitor and control, is exchanged between components. FOTM and BTM differ only in the placement of the actual load. FOTM typically provide grid services, with an unspecified load not managed by the energy system. In contrast, BTM enables load control via an energy management system (EMS), which balances storage and consumption. The architectures of BTM and stationary applications can be summarized as identical. In semi-stationary and mobile applications, the grid integration is omitted. The architecture for mobile second-life applications is different from the others as an EMS does not exist there. The load in this case is an electrical motor controlled by a vehicle control unit (VCU). Connectivity across these components is established through a central gateway. The architectures all have a connection to a cloud in common. This is required for regulations such as the new Battery Passport [29], but also offers modern opportunities for remote monitoring and control of the system as well as for the use of high-performance computing, enhancing status calculations [30]. It can be observed that the relevant architectural artifacts can be reduced to a single connection from the BMS to a higher-level component that fulfills the role of managing and controlling the entire second-life system. In consideration of the broader research target of identifying building blocks for the BMS, the technologies used in this connection are therefore of particular interest. Protocols used for the communication between a BMS and an EMS across the industry for the physical and data link layers are commonly CAN-BUS, Ethernet, or WiFi [7], [31]– [33]. The Modbus-protocol is regularly employed within the application layer, facilitating data communication as a client/server architecture [31], [34]. Other possible protocols in the application layer are HTTP or MQTT, although these are not particularly represented in the literature. Comparing this with the BMS architecture in an EV, as shown in Figure 3, some similarities can be identified. For example, the BMS is also only connected to a higher-level component, i.e. the gateway, via a single connection. In addition, there is an interface from this to an electric vehicle supply equipment (EVSE), i.e., the charging station and to an in-vehicle infotainment (IVI). While there is a range of protocols for connecting to EMS, the CAN bus protocol is widespread in EVs. This results in a technological difference from these. BMS BMS BMS BMS BMS a. FOTM b. BTM c. Stationary d. Semi-Stationary e. Mobile EMS EMS Cloud UIEMS EMSInv.Inv.Inv.Inv. Grid Load Load Load VCUGateway GridLoadCloud Cloud Cloud Grid Cloud EVSE Load Fig. 2. Comparison of architectures of second-life applications. BMS GatewayIVI EVSE VCU Cloud Load Fig. 3. EV-BMS architecture B. RQ2: What benefits and challenges are discussed in the literature concerning reusing the original BMS in second-life applications? TABLE II BENEFITS AND CHALLENGES CONCERNING REUSING THE ORIGINAL BMS IN SECOND-LIFE APPLICATIONS Benefits Source Cost Efficiency [4], [35]–[39] Data-Driven Health Assessment [4], [22], [38], [40] System Integration Simplicity [39], [41] Resource Protection & Circularity [40] Challenges Communication Interface Compatibility [4], [35], [41] Limited OEM Access & Proprietary Constraints [4], [24], [38], [40], [41] Technological Obsolescence [4], [22], [24], [36], [37], [42]–[45] Regulatory and Insurance Compliance [40], [43] The second research question analyses the benefits and challenges regarding the reuse of first-life BMS in second-life applications to later use these for the identification of BMS building blocks. The research works found for each cause are summarized in Table II. 1) Benefits: Cost Efficiency: The process of disassembling, evaluating, and subsequently reconstructing a battery pack into a new system involves significant labor and expenses, whereas utilizing a battery pack proves to be a more cost-effective alternative. Data-Driven Health Assessment: The assessment concerning remaining useful life (RUL) and state of health (SoH) at the transition towards a second-life application significantly relies on the operating conditions and historical data of the first life. Access to the data stored on the BMS enables easier assessment during the transition, but also a continuous ongoing evaluation in the second life. System Integration Simplicity: The development of a new BMS is a complex and expensive task. Reusing the existing first-life BMS is considerably easier when integrating the SLB into a new application. Resource Protection & Circularity: By reusing the BMS, the approach of circular economy is further strengthened. In contrast, the use of a new BMS ties up new resources and raw materials. 2) Challenges: Communication Interface Compatibility: The integration of a SLB into a new application requires setting up a communication link to the BMS to control and monitor the battery. If this connection is unknown, the communication interface cannot be established, or complex reverse engineering to find out command and data structures must be carried out. However, integrating a new BMS allows full interface control. Limited OEM Access & Proprietary Constraints: The first-life OEM often keeps design control over their EV batteries due to intellectual property (IP) control, thus not handing over information regarding BMS design, control, and monitoring strategies of the battery, or battery parameters in general. Since this information is mandatory for developing and integrating the battery system, it is hardly possible to reuse the BMS. Then, the only option to continue using SLB is to develop a new BMS. Technological Obsolescence: New use cases, specifications, and environments often require new software and hardware components. Since the original design of the EV battery system can be 15-20 years old, technology like protocol stacks, hardware components, etc., can be obsolete. A new BMS can be optimized for a new environment, facilitating up-to-date technologies. Regulatory and Insurance Compliance: As the first-life environment can significantly differ from the environment of the second-life application, certifications and insurance cannot be covered anymore. It is noticeable that some of the reasons given contradict each other. While some research papers emphasize the simpler integration, some see this as a greater challenge. It should be noted here that different assumptions were made. Namely, on the one hand, the implementation of interfaces is known, which simplifies integration, and on the other hand, this is made more difficult by a lack of knowledge about these interfaces. C. RQ3: What mechanisms are necessary to ease the transition from first to second life while enabling reuse of the original BMS? The results of RQ1 showed that only a single communication link from the BMS to a higher-level component is relevant in different second-life applications. Across this link, however, a range of communication protocols may be used. Compared with EV-BMS, the results also showed that a change of protocols is necessary in most cases. RQ2 identified a major challenge in reusing an original BMS: the limited availability or complete absence of documentation regarding proprietary protocol implementations. These two aspects, installing either new protocols or receiving information about existing ones, are even more challenging when the aspect of different stakeholders is taken into account: direct contracts, including an exchange of design and documentation, between a first-life and second-life OEM can solve these issues. However, with the increasing availability of retired batteries in the upcoming years, a market for trading SLB is expected to grow significantly. In this case, the trading partners may not get to know each other and therefore do not exchange information. Another crucial factor resulting from RQ2 is the access to historical data to enhance the battery’s status assessment. Finding the best possible second-life application heavily relies on the quality, i.e., on the SoH and the RUL of the cells, which increases the importance of being able to read out operational data of the first life. Taking this into account, two major BMS building blocks would ease the transition from a first to a second life. 1) Enabling the installation of updated firmware: Installing new software includes several security measures, such as digital signatures, secure boot, encryption, and trust anchors. These mechanisms ensure confidentiality, integrity, and authenticity of the binary firmware and authenticity of communication endpoints. (Root) certificates, hardware keys, and public/private key-pairs must be highly protected and must not leave the controlled ecosystem of the respective OEM. However, to be able to install a new firmware, the second-life OEM needs to retrieve control over these security measures. Therefore, a method must be implemented that enables a secure transfer of security measures while ensuring no exchange of critical cryptographic material. 2) Access to historical data: Access to BMS data is currently restricted due to IP concerns raised by OEMs. As the battery represents a critical and market-differentiating component of an EV, its operational data may reveal proprietary control strategies that OEMs want to protect from competitors. However, as previously discussed, such data is essential to accurately assess the battery’s SoH and estimate its RUL. To reconcile these conflicting interests, it is necessary to establish a mechanism through which the BMS can securely and reliably communicate the battery’s current SoH. This information must be made accessible to second-life OEMs in a way that guarantees data integrity and authenticity, while simultaneously preserving the confidentiality of sensitive internal parameters and control logic. One promising approach to meet these requirements involves the integration of blockchain technology, which can provide controlled data access and ensure the verifiability and immutability of the shared information. IV. DISCUSSION The current Horizon Europe project REBORN [46] has, among other aspects, a use case in which batteries from an EV are to be transferred to a second-life system for storing renewable energy. This is taken as an example to show how the two identified building blocks simplify the reuse of the battery system. While there are non-disclosure agreements (NDAs) regarding IP within the project, this is not always the case in a real-world scenario. The EV-OEM, therefore, does not want secret data to be leaked, both on an operational level and from the perspective of security mechanisms. The two building blocks help to protect this data and still allow the BMS to be reused. It is essential for the second-life OEM to know what status the batteries to be used still have. A simple but confidential transfer of this status without having to access all the data from the first life makes it easier to make such an assessment. In the given example, the CAN protocol is used in the EV to communicate with the BMS. The new EMS, on the other hand, can only communicate via Modbus over Ethernet. This means that a new protocol stack must be programmed. Without a correct digital signature, however, the BMS will not accept any firmware. The secure exchange of cryptographic material from the first OEM to the material from the second OEM, therefore, enables the existing update options to be utilized and new firmware to be imported. V. CONCLUSION AND FUTURE WORK This paper aimed to improve the reuse of first-life BMS in second-life applications. To this end, the differences between first and second life were analyzed and identified through a literature review. 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