1. Introduction
The manufacturing techniques of lithium-ion batteries are rapidly developing globally nowadays, since global corporations are actively positioning themselves in battery technology sector and scholars are attracted to engage in research for the improvement of battery technology. Meanwhile, various green energy policies and reducing greenhouse gas emissions plans are introduced by governments around the world for boosting the development of battery and battery relative industries [
1]. As a result, the battery industry can be foreseen to become even more prosperous in the near future. However, there is a significant shortcoming of lithium-ion batteries is that the performance declines gradually over time with usage [
2]. This degradation problem is affected by many different factors because of the complexity of lithium-ion batteries systems. Therefore, there is a long way to go for tackling this problem. On another hand, the battery operational data, such as voltage, current, temperature, capacity, and energy, are available during its lifespan. Further, the relation between battery performance features and degradation to a certain extent has been proven by scholars [
3]. There is feasibility of predicting the battery remaining useful life influenced by the historical performance features. This estimation is called as remaining useful life (RUL) estimation.
Accurate RUL estimation is crucial aspects of prognostics and health management since the estimation result can provide reference for improving maintenance strategies, optimizing life-cycle costs, and mitigating operational risks or issues [
4]. Model-based methods, data-driven approaches and hybrid approaches are the mainstream methods for RUL estimation at present [
5]. The model-based method can be further divided into mechanism-based method and empirical-based method. These methods are highly depending on the equipment and requires for long-time experimental exercisers, which is difficult to apply to real-world applications [
6]. With the development of data minding technology, data-driven approach is able to constructing aging prediction models by historical operational data. But this approach requires large amount of data for modeling. Its adaptability and reliability are uncertain as well [
7]. Due to the rapid evolution of machine learning technology, the hybrid approaches combining optimization and machine learning techniques have gained popularity [
8]. Having the advantages of both methodologies, a powerful framework with the ability to resolve complicated problems is built. That leads to the more accurate and efficient batteries RUL estimation modules while applying this kind of approaches.
There are remaining numerous technical and practical obstacles for predicting the state of charge (SOC), state of health (SOH), and remaining useful life (RUL) of batteries, machine learning methods have great advantage in improving the accuracy and efficiency [
9]. K-Nearest Neighbor regression model is exploited for remaining useful life estimation by incorporating data from all the cells in a battery pack [
10]. Gradient Boosting machine learning approach is suitable for real world applications and handling nonlinear input features [
11,
12]. An optimized Random Forest Regression model is developed to enhance the learning and generalization ability. This model can achieve high accuracy in a short time using a small number of samples [
13].
Recently, in order to lessen the error rate, gain better robustness and achieve better prediction performance, the ensemble learning method is frequently used for battery RUL estimation. A novel ensemble learning method [
14] consists of 5 basic learners, including Relevance Vector Machine (RVM), Random Forest (RF), Elastic Net (EN), Auto Regressive (AR), and Long Short-Term Memory (LSTM), is developed. The experiment validation shows that this approach has improvement in robustness and generalization effect. Another ensemble model [
15] based on CEEMDAN and CNN-BiGRU is designed to achieve higher precision and reliability.
Remaining useful life (RUL), which is typically random and unknown, is the number of charge-discharge cycles left on a battery at a particular time of operation until its maximum usable capacity decays to a predefined failure threshold. A hybrid deep learning model [
16] for early prediction of battery RUL combining handcrafted features with domain knowledge and latent features learned by deep networks is proposed. This model enhances model generalization ability without increasing any additional training cost and has great contribution to the improvement of model prediction accuracy and generalization ability.The main objective of this proposed approach is to predict the batteries RUL from features, such as voltage, current, internal resistance, and capacity. This proposed approach jointly combines Gradient Boosting, Random Forest and K-Nearest Neighbor into voting ensembles, so as to integrate the advantages of ensemble learning.
The battery capacity degradation processes can be divided into two period. Initially, the battery is health and the capacity is stable with a steady decline. When the capacity degrades to a certain rate and then drops to a certain point, the capacity goes through an accelerated degradation. That certain point is normally called knee point [
17]. Currently, most of previous works are focusing on predicting the SOC, SOH or RUL for battery health determination. But for knee point, there is seldom researches discussed on it. Although the existing works have enhanced the model to be low error rate, high accurate and efficient, these models are normally evaluated within a single battery dataset, where they are divided into training and evaluation measurement segments. The experiments of applying the model to multiple batteries has not been considered. Therefore, the experiments taken for verification are not sufficient enough.
In this paper, to overcome the above existing issues, an innovated hybrid ensembles approach is proposed for battery RUL prediction and knee point estimation, as shown in
Figure 1. Regression methods of Gradient Boosting, Random Forest and K-Nearest Neighbors are integrated into voting ensembles is designed to enhance the generalization performance and improve the accuracy. After generating the prediction result, the binomial fitting algorithm is employed to predict the RUL knee point. With extensive experiments, the proposed approach has demonstrated significant advantages and potential in predicting battery RUL and knee point, contributing to improving the development and application of battery RUL prediction technology.
5. Conclusion
In this paper, a methodology for predicting the battery RUL based on a hybrid ensembles approach integrated with Gradient Boosting, Random Forest and K-Nearest Neighbor is presented. The battery features of voltage, current, internal resistance are extracted from dataset of CALCE are applied to this proposed approach for training and prediction. With small size of training and testing data, this approach also performs satisfactory in terms of accuracy, robustness, and adaptability. Particularly, this approach utilizes the binomial fitting for capacity faded trend prediction. The predicted RUL and knee point for batteries are precise and fits the real data very well. Compared with existing RUL methods, this proposed approach achieves better performance as indicated by lower RE, MAE, and RMSE scores and higher scores. This approach can also be promoted to wider usage for other real world application scenario.