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■ Date: Jan 11, 2022
Sparking a Second Life of Power Battery
PART 3. Technical Aspects of Battery Reuse in China
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The reuse of power batteries is one link in the new energy vehicle industry chain that of high importance yet meanwhile with the highest environmental risk and the most urgent need for technological progress. Technological issues are one of the important factors affecting the promotion of battery reuse. Upon retiree, power batteries need to go through the process from battery collection, battery disassembly, screening & testing, regrouping, system integration to the final application to suitable scenario. This article will discuss the pain points, main technologies, and technical details throughout the whole process.
1. Technological issues during battery echelon utilization
Figure 1. Main stages for processing retired power batteries
There are typically 4 stages for the echelon utilization of retired power battery after collection:
1) Battery disassembly: battery pack disassembled into smaller pieces for further inspection and testing.
2) Screening & testing: Evaluate the state of the disassembled battery to facilitate its next-step application to suitable scenarios, mainly including battery residual capacity, state of health, etc.
3) Regrouping：Pair and regroup batteries with similar performance parameters to ensure maximum battery consistency so as to improve the performance, lifetime and safety performance.
4) System integration: Integrate additional system design such as thermal management, control function, online communication module, etc. applicable for final target application.
However, pain points exist throughout the whole process, which pose technical barriers for the development of power battery reutilization:
1) Battery disassembly
a) Difficult to disassemble owing to complexity and safety concern. Battery pack designs, module connection method and processing technologies differ from modules and battery types, which means that it is impossible to use one set of disassembling production line to fit all retired power battery packs and modules. These complexities make it extremely inconvenient and costly to recycle or dismantle decommissioned power batteries. Being high-energy carrier, if improperly operated, power battery would cause serious safety issues such as short circuit, liquid leakage, or even fire or explosion, resulting in personal injury and property loss. Therefore, ensuring safe operation during battery disassembly is also an important point in the echelon utilization of retired power batteries.
b) Low degree of automation brought by the difficulty of dissembling. At present, only a very small number of domestic companies such as Brunp湖南邦普, GEM格林美 have independently developed mechanical automatic dissembling equipment, which is not enough to support the echelon utilization market.
2) Screening & testing
a) Difficult to predict battery electrical performance. The electrical performance of power battery will de XXX during its life cycle. How battery was charged/discharge will greatly influence the performance in the future, which makes the prediction extremely difficult. At present, the field of testing and evaluating the residual value of batteries is still in its beginning stage. How to better predict battery SOH (state-of-health), cycle life, etc. by linking some parameters (internal resistance, temperature, SOC (state-of-charge), etc.) have always been the focus. Furthermore, due to the requirements of battery consistency, the evaluation of power battery cells or modules cannot be applied directly to the evaluation of the entire power battery system, and there are currently no relevant test methods and evaluation standards.
b) Difficult to predict battery safety performance. Some safety defects cannot be found only by visual inspection, such as slight swelling, liquid leakage, insulation failure, pole corrosion, etc. If these security flaws have not been checked out and reused in the new product, there will be more serious safety issues
c) High testing cost in large scale.
3) Regrouping
a) Diversified regrouping methods. Considering that power battery system is multi-level structured: pack-module-cell, the first problem faced when realizing its echelon utilization is to what level of power batteries should be regrouped. Different application scenario will adopt different regrouping methodologies. For example, for large-scale energy storage, battery would be regrouped in packs while for electric bicycles and other space-sensitive applications, batteries are regrouped in modules or even battery cell level. Regrouping method largely affects the technical difficulty and cost of reutilization process.
4) System integration
a) Need to balance many factors from various aspects. When designing system integration, it is necessary to calculate relevant matching coefficients according to the aging characteristic parameters of different battery modules and determine overall plan according to different application scenario. In addition to this, there are many factors that need to be considered: how to design the structure with BMS; how to consider the reliability, flexibility and feasibility of fast loading and unloading of the fixing method; how to improve the intelligence of the system, so that it can be upgraded remotely and provide online service in subsequent use.
As has been mentioned in our previous blog [ PART 2: Political Aspects of Battery Reuse], China's laws and regulations for echelon utilization are getting gradually improved, so do related standards, including battery dismantling specifications and residual capacity testing, etc.
Figure 2. Standards related to power battery reuse
However, more tasks remained unsolved. Technologies tackling with the pain points in each step are under close development, such as intelligent disassembly technology, SOC evaluation, SOH evaluation, remaining useful life prediction, etc.
Take remaining useful life (RUL) prediction as example.
Although lithium-ion (Li-ion) batteries are widely regarded as promising candidates among various energy storage solutions for various applications owing to their advantages of high energy density and low self-discharge, the life span of Li-ion batteries is not unlimited. The performance of Li-ion batteries will decrease with time (calendar aging) and use (cycle aging).
The aging of the battery will increase operating costs, reduce the service life of the equipment, and affect the safe operation of the equipment. Moreover, during battery echelon utilization, the prediction of service life of the collected retired power batteries will surely affect the application afterwards.
Figure 3. Capacity degradation of power batteries
RUL is defined as the time at which equipment perforamcne first or first arrival time drops to the failure threshold. The difficulty of its prediction is mainly reflected in two aspects:
1) Current point: internal status cannot be directly detected.
At certain time point, only external parameters can be directly measured, such as battery terminal voltage (V), charge and discharge current (I) and surface temperature (T). Internal status (SOC, SOH, etc.) could only be estimated by using external parameters.
2) From current point to future point: difficult to predict.
Battery aging presents nonlinear characteristics, which is influenced by complicated working conditions, e.g. charging rate, operational voltage window, temperature, DoD, etc.
There are mainly two types of prediction method that have been widely studied:
1) Prediction based on different models: Describe the aging behavior of the battery by establishing certain models and extrapolate these models to make RUL prediction.
Figure 4. Comparison classic models for RUL prediction
Advantages: Less requirement for historical data; good robustness and stability.
Disadvantages: Relying on expert knowledge to build physical models; insufficient flexibility, not applicable to all scenario.
Typical models have been proposed in the literature, such as electrochemical model, equivalent circuit model, empirical model, etc. These models try to find a mathematical representation to describe the battery’s internal degrading behavior from their own respective theory. After model selection, different filtering algorithms can help update and correct model parameters to reduce errors using historical data, such as Kalman Filter, Particle Filter, Improved Particle Filter, etc. Using updated model parameter, RUL prediction is made by extrapolating from current moment. However, this detailed mathematical representation means an increase in complexity and computational cost.
2) Data-driven Prediction: Based on statistical theory and machine learning, directly use battery historical data to make RUL prediction without relying on specific physical models.
Advantages: No need to understand the aging mechanism. No need to establish a specific physical model, avoiding complicated mathematical modeling process and expert knowledge. More flexible and easier to apply to different occasions.
Disadvantages: Insufficient or biased training data will lead to reduced prediction accuracy or complete error.
For the complex electrochemical dynamics system in lithium-ion battery, model-based methods are usually complicated to implement, but the data-driven method does not consider the electrochemical reactions and failure mechanisms inside LIB, but directly build a rough model from collected data. With a large amount of historical data, statistical models or machine learning models are continuously modified and corrected so as to improve the fit between model and real case.
For both prediction methods, the precision can be enhanced by optimizing models, filtering algorithm and machine learning algorithms. However, for data-driven prediction, the accuracy can be further improved by providing huge amount of data, which makes it more promising in the future.
Therefore, car companies and power battery companies that have better access to battery operating data have advantages in subsequent offline or even online predictive diagnosis.
Another topic to discuss is the choice of echelon utilization route. A typical electric vehicle power battery a battery pack, which can be break down into battery module and cell.
Figure 5. Breakdown of a typical electric vehicle power battery
The echelon utilization of power battery can be realized via different routes, each has its respective advantages and disadvantages:
| Disassemble route | directly via whole battery pack without further disassembly | packs dismantled into battery module or even into battery cell and reused after regrouping into battery packs |
| Advantages |
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| Bottleneck |
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Different disassembly and regrouping strategies depend on the needs of applications, technical feasibility, and cost. To date, no consensus has been reached across the industry on the route selection.
There have been cases for each technical route. For example, BAK power 比克电池 cooperated with South Grid and successfully put into operation a power battery echelon utilization project, which was the first in China that used battery package directly for reutilization. Huayou Cobalt 华友钴业 launched various echelon utilization projects, one using ternary battery packages with China Tower and another using LFP battery modules in Zhejiang.
However, with the emergence of new technologies, such as CTP (Cell-To-Pack) technology in the CATL and “blade battery” in BYD, battery manufacturers are pursuing for higher system energy density. Therefore, battery packaging method will be redesigned, and battery cells are arrayed in compact making it difficult to disassemble. As a result, when retired, reuse via battery pack without pack disassembly would be more practical.
2. Technological details for second-use application
Batteries for electric vehicle and that for electric bicycle or for energy storge differ in many ways: battery dimension, battery arrangement, nominal voltage, electrical performance (charging and discharging performance, cycle life, etc.) as well as system design. To better apply refurbished lithium-ion battery in the second-use application scenario, more technological details other than echelon utilization-related should be taken into consideration.
So far, there is no specific national standard that has clearly provided technical requirement of echelon battery in different application scenarios. Therefore, specifications for new lithium-ion battery products for such applications, grid energy storage, electric bicycle, etc. can be referred as indirect indication.
These specifications are included in the "Comprehensive Standardization Technical System for Lithium-ion Battery”(锂离子电池综合标准化技术体系), which were elaborated in five categories of topics: 1) basics, 2) battery materials and components, 3) battery design, 4) manufacturing and testing equipment, and 5) battery products for different applications.
Among the standards, two parts are related to power battery echelon utilization:
Figure 6. Comprehensive standardization technical system for lithium-ion batteries
Here we summarized and compared different application scenario in various aspects.
Figure 7. Requirement for batteries of different possible scenario of echelon power batteries
Low-speed 4-wheel EV, from the perspective of residual capacity, has the highest requirement, followed by three others.
It is worth noted that there exists gap between GB standard and association standard regarding cycle life. If we use association standards as reference, cycle life requirement seems more realistic for both LFP and ternary battery to reach. In contrast, under GB standard, LFP battery will gain more interest due to its longer lifetime.
Conclusion
1) There are many technical steps for echelon utilization, and the key technologies in these steps are still under development, which set high technical thresholds that hinders the development of echelon utilization industry.
- Government is supporting basic research with relevant R&D projects and improved technical standard system. It is expected that more policies will be released support the development of echelon utilization. Similar to NEV development in China, government supported academic R&D research with national and provincial key projects, and when technologies and know-hows accumulated, the focus gradually shifted to industrial side such as capacity expansion, intelligent production line manufacturing, etc.
- Future trend: higher dismantling efficiency, more accurate prediction of electrical performance, higher degree of automation, and higher intelligence.
2) Either disassembled via pack, module or via cell, battery reuse route can be flexibly selected according to different application scenario.
- Safety and economy are generally the main indicators during decision making.
- Battery design has influence on the choice, as discussed above in CATL and BYD case. Battery pack design is strongly related to the feasibility of its breaking down into smaller modules or cells as well as processing costs.
- In addition, the situation of retired batteries will also affect the choice. For example, since technology was not yet mature, power battery before 2017 are usually considered unsuitable for echelon utilization. As technology matured, battery performance after 2018 has been improved, and it can be reused via module or via whole package according to the specific requirements of the application scenario.
3)Ternary battery is not current mainstream battery type for echelon utilization due to various disadvantages. The reuse of ternary battery will have more improvement once it sees considerable market size and accumulation of acedemic knowledge.
- Poor RUL performance: Ternary battery does not have advantage over RUL compared with LFP battery, which can generally reach 2000-6000 times. Yet for ternary batteries, the degradation is more rapid and can reach only 800-2000 times.
- Safety concern due to the low thermal runaway temperature (180 ℃), ternary battery is more prone to cause fire and explosion.
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