The emergence of electric mobility has pushed the interest in batteries to the forefront. Batteries are considered as the powerhouse of an electric vehicle (EV). Imperatives to decarbonise the transport sector has led to an increase in the uptake of battery electric vehicles (BEVs) in the automotive industry. However, with the escalating growth of EVs, the end-of-life management of the used batteries becomes a matter of crucial importance. As mentioned in the last article, battery recycling will play a crucial role in the efficient use and re-extraction of the resources ensuring a stable supply chain and contributing to the circular economy. However, in the waste management hierarchy, reuse is preferred before recycling. An approach to maximise the utilisation of the batteries once retired from its first use in vehicular application can be achieved through reuse of these batteries for other application such as stationary storage applications. Post-vehicular applications, the batteries retain almost 80 per cent of their capacity and sufficient performance to meet the requirement of other energy storage applications. This article focuses on technical strategies adopted on the batteries to estimate its health after the first life and the retrofitting required on the batteries before being redeployed for a secondary application.
Battery life cycle with second use
Batteries when used in an EV application is usually subjected to extreme temperature conditions, hundreds of partial cycles and have different charging discharging rates. The batteries degrade approximately within a decade of useful life in most of the cases. Three options can be adopted once the battery retires from vehicle applications that is disposal, recycling or reuse for other applications. In regions with tight regulations the direct disposal of damaged batteries is prohibited. Moreover, the direct disposal/ landfill of these batteries is not encouraged considering its environmental impact. Recycling of batteries helps in efficient re-extraction of resources from the spent batteries and benefits in reducing the need for new mineral extraction, thereby lowering the environmental footprint. Most of the batteries retain 80 per cent of capacity and possess resting self-discharge rate of only about 5 per cent over a 24-hour period which qualifies them to serve less-demanding applications, such as stationary energy-storage services. With the extensive growth of EVs, it is expected that the second life-battery supply for stationary applications could exceed 200 GWhper year by 2030 (McKinsey & Company ).The figure below summarizes the possible life cycle of a spent EV battery.
Figure 1: EV battery life cycle
Source: (McKinsey & Company )
Testing and evaluation procedures for reuse of batteries
Even though the batteries hold huge potential in post-vehicular applications, some technical challenges hinder the reuse of end-of-life lithium-ion batteries. The following technical procedures must be followed for efficient reuse of EV batteries (J. Neubauer, K. Smith, E. Wood, and A. Pesaran):
- Safety test: Tests such as over discharging, short circuit, heating, over charging, puncturing, squeezing, etc. are typically carried out before ascertaining fitment for reuse. To further guarantee the safety of the batteries, several assessment techniques such as state of safety (SOS) and failure analysis are conducted. SOS refers to probability that a battery works safely in the given time. Failure analysis methods includes Fault Tree Analysis (FTA) and Failure Mode and Effects Analysis (FMEA).
- Evaluation: The estimation of state of health (SoH) and Remaining Useful Life (RUL) aids in obtaining the performance characteristics of spent batteries and help in determining if the spent batteries are worthy of reuse. The table below concludes the approaches to determine both the critical entities.
Table 1: Battery health evaluation approaches
|SoH estimation||RUL estimation|
|Direct assessment: Measures the capacity or internal resistance directly and obtaining the SOH value. Recent methods to directly measure SOH include Incremental capacity analysis (ICA), differential voltage analysis (DVA), and other methods like acoustic – ultrasonic guided waves.||Empirical estimation: This approach uses a simple structure and involves low computational complexity. It includes several fixed parameters which may result in large errors while studying the nonlinear ageing process.|
|Empirical estimation: This approach describes the relationship between various ageing stress factors and SOH by simplified mathematical expressions.||Data driven approach: This approach can use historical data to extract few hidden information and use it to predict the RUL without going through the degrading mechanism as in model driven methods. This model has good flexibility but low generalization ability.|
|Model driven estimation: Model driven methods can estimate the SoH of spent batteries by identifying health sensitive model parameters based on electrochemical models (EM) or equivalent circuit models (ECM). ECM methods can establish a relation between the external characteristics exhibited by the battery and the internal state of the battery.||Model driven estimation: This approach estimates RUL by using mathematical models which describe the degradation process. Following are various model driven estimation models:
• ECM Method: It can establish a relation between the external characteristics exhibited by the battery and the internal state of the battery to estimate the RUL. It has a low computational cost and good scalability. The challenge lies in calibrating the model parameters to suit RUL estimation
• EM based method: This method reviews the electrochemical behaviour of the battery to estimate the RUL of the battery. This approach has high accuracy and high generalisation ability but involves high computational cost and knowledge about mechanisms
|Data driven approach: Data driven approaches do not depend on the electrochemistry or physical aspects of the battery. These approaches use methods like regression based on feature selection to estimate the SoH of batteries.|
|Hybrid methods: Hybrid methods combine the advantage of different approaches such as Entropy weight method (EWM) and grey relation analysis (GRA) to assess SOH. GRA methods first selects certain health indices based on the spent battery. EWM method is then used to evaluate the weight of the health indices selected using the GRA method.|
Source: ( Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ))
- Screening and regrouping: To guarantee performance safety, improvement in system life and homogeneity in battery pack, screening and regrouping processes are adopted for lithium-ion batteries. Spent batteries are visually screened and then critical screening indexes such as voltage, capacity, and internal resistance are used to select batteries. Several approaches can be used such as hybrid pulse power characterization (HPPC) and Piecewise Linear Fitting (PLF) methods. The cells and modules which meet the standards during screening are further regrouped into clusters based on their performance.
- Management: After spent batteries are screened and reassembled, installation of Battery Management System, Thermal Management System and Equalization Management System is carried out. If the second life application of the second-life battery is like its first life, the BMS might need minor modifications or programming. If the second-life application is different as compared to its first life, then a new BMS, TMS and EMS should be added based on the requirement of the application.
Battery reuse methods/ processes
After the evaluation procedures, the spent EV batteries cannot be directly used in the secondary applications. Based on the level of usefulness remaining inside the batteries, appropriate retrofitting must be performed before its deployment. The following methods can be adopted before reuse:
- Reconditioning: Battery reconditioning or regeneration refers to restoring the standard level of electrolytes in a battery pack and a battery packs’ full capacity to charge. The battery module is discharged repeatedly, and the cells which are unable to hold charge, are identified and are reconditioned to increase their life
- Refurbishing: Refurbishing involves opening the battery, replacing the degraded parts, and re-designing the BMS depending on the second life application
- Repurposing: This involves replacement of some cells or packs for use in totally different set of application
- Reusing: This involves splitting of the battery down to the cell or pack level post which the individual cells or packs are reused directly in a wide variety of applications
Post diagnosis, method of reuse to be adopted depends on the Remaining Useful Life (RUL), State of Health (SoH), specific reparation costs and safety conditions. A decision tree analysis is done based on the custom classification and grading process employed by different companies to decide between reconditioning/refurbishing/repurposing/reuse.
Figure 2: Flowchart illustrating the diagnosis and classification of end-of-life batteries
Source: ( Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ))
International battery standards for reuse
The refurbishment of retired lithium-ion batteries for reuse is highly complex and costly which is exacerbated by a lack of lithium-ion battery pack standardisation. Battery packs vary in design, size, and format (cylindrical, prismatic, pouch), which makes LIB repurposing for reuse quite complex. The table below captures the available international standards applicable to battery reuse technology.
Table 2: International standards for battery reuse
|Standard description and coverage|
|IEC 63330||This standard lists down the requirements for repurposing battery systems, battery packs, modules, and secondary cells manufactured for use in applications such as mobility. It also specifies the procedure to evaluate performance and safety parameters of batteries for repurposing. It does not cover redox flow batteries|
|IEC 63338||This standard gives a general guidance regarding the reuse of batteries and secondary cells|
|UL 1974||This standard specifies how to evaluate batteries for repurposing based on BMS measurements. This standard also covers the processes for sorting and grading of battery packs, modules, cells and electrochemical capacitors which were originally configured for EV propulsion|
Source: (IEC63330) ( IEC-Standrds for reuse of EV batteries)
Detailed information on reuse technology of EV batteries can be obtained from Chapter 8 (Page 277) of the report- Battery Ecosystem: A Global Overview, Gap Analysis in Indian context, and Way Forward for Ecosystem Development.
Efficient battery second use strategies paves way for extracting additional services and revenue from the battery in a post-vehicle application. The total lifetime value of the battery can be increased, and the cost of the battery can be reduced to both the primary and secondary users. Availability of onboard diagnostics data and accurate assessments methods for automotive and second use battery degradation becomes crucial. However, accuracy of battery health monitoring techniques, market adoption, lack of efficient repurposing methods, lack of battery monitoring standards and policy gaps act as major barriers for developing an efficient reuse ecosystem. Identification of suitable method of reuse largely depends on the application to which the battery is redeployed. The next article in this series would focus on major second-life applications of EV batteries and a brief overview of key companies and the application they cater to. Further a possible way forward for developing an efficient reuse ecosystem would be briefed.
This article is eighth in the series that elaborates the technical approaches required for an efficient second life use of an EV battery. The next article would focus on major second life applications and associated challenges.
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ). “Battery Ecosystem: A Global Overview, Gap Analysis in Indian context, and Way Forward for Ecosystem Development .” 2022.
IEC-Standrds for reuse of EV batteries. “Standards for reuse.” March 2020. https://www.iec.ch/ords/f?p=103:30:310723570881079::::FSP_ORG_ID,FSP_LANG_ID:1290,25.
IEC63330. “General guidance for reuse of secondary cells and batteries.” 09 March 2020. https://www.iec.ch/ords/f?p=103:38:310723570881079::::FSP_ORG_ID,FSP_APEX_PAGE,FSP_PROJECT_ID:1410,23,104109.
- Neubauer, K. Smith, E. Wood, and A. Pesaran. “Identifying and Overcoming Critical Barriers to Widespread Second Use of PEV Batteries.” 2015.
McKinsey & Company . Second-life EV batteries: The newest value pool in energy storage. 30 April 2019.