Elsevier

Journal of Power Sources

Volume 231, 1 June 2013, Pages 50-59
Journal of Power Sources

Switching algorithms for extending battery life in Electric Vehicles

https://doi.org/10.1016/j.jpowsour.2012.12.075Get rights and content

Abstract

The battery is a key component in any Electric Vehicle (EV) and its method of operation may have a tremendous effect on its life. In this paper we focus on improving the battery's life. Each battery is a pack of cells designed to be discharged and charged with specific optimal currents, whereby other currents, i.e. higher or lower than the optimal currents, may have negative effects on its life. We model these negative effects as penalties that are aggregate over time and propose a discharge method to minimize them. The common discharge method is very simple but far from optimal since the current demand is supplied using all the battery's cells where the current from each is the same. The method we propose is advanced switching algorithms that select a subset of the battery's cells for each current demand and control the discharge current from each, based on the electrochemical properties of the individual cells. We evaluate our proposed algorithms using simulations on world-wide driving cycles. The results reveal that compared to the common discharge method almost all penalties can be eliminated and the battery's life can be significantly extended.

Highlights

► A battery's life is greatly affected by the method of use. ► Discharging in non-optimal currents has negative effects on the battery's life. ► We propose advanced switching algorithms to minimize these effects. ► The battery's life can be significantly extended by the proposed algorithms.

Introduction

Electric Vehicles (EVs) are the next generation of cars in the world of automobiles. The propulsion solutions for EVs are based on hybrid or fully battery powered electric vehicles [1]. The critical part of EVs, that determine their performance, is the battery [2]. In this work we focus on one of its problems, namely the battery's life.

The life of operating batteries is usually measured as the number of cycles, that can be obtained, until the capacity per cycle declines beyond a pre-determined threshold [3]. The energy extracted from the battery during full discharge is the integration of voltage as a function of capacity throughout the discharge process, until its cut off point (measured in Watt-hours). However, an alternative definition, which we use throughout this paper, can be the total accumulated energy extracted from a battery during its life.

Batteries are very expensive [4] and hence it is critically important to prolong their life as much as possible. The method of operating the EV battery may have a dominant effect on its life. According to results presented in Ref. [5] operating it incorrectly might reduce the battery's life to even a third of its expected duration. Thus, it is logical that the Battery Management System (BMS) in the EV includes a component that will control the battery's operation in order to extend its life.

An EV battery is actually a pack of strings of cells connected in a series, in order to reach the required high voltage. These strings of cells, i.e. cell-series, are connected in parallel, in order to provide the required current. For safety reasons, e.g. issues related to heat dissipation during operation, it is important to construct such large batteries from small individual cells. Thus, the focus in this paper is on the level of the individual cell-series in the battery, which determine the total voltage of the battery.

Developments in the Lithium-ion (Li-ion) battery technology in the last decade have made Li-ion batteries the standard choice of power sources for EVs [6], [7]. Thus, in this paper we focus on Li-ion batteries. Based on the chemistry and engineering of Li-ion batteries, it is clear that the values of the current upon discharge of a single cell and cell-series, i.e. the rate of operation, may have a significant impact on the battery's life. The use of discharge currents that are too low or too high, may have a detrimental effect on the battery's life, as shall be discussed later in Section 3.2. These effects justify the optimization efforts described in this paper. Based on these insights we propose a penalty function, which for each discharge current, defines a penalty in terms of the detrimental effect on the battery's life.

The motivation for our research emerges from the possibility of extending the life of EV batteries via their smart operation. The discharge method that is commonly used for EV batteries, presented in Section 4.1, is very simple as the current demand is supplied using all cells in the battery simultaneously, hence the load is equally divided among them. The rationale behind this method is simplicity of implementation and the assumption that the lower the current drawn from each cell in the battery the better. However, as described in Section 3.2, the behavior and performance of a real battery is more complex, and this assumption is not always true.

In this paper we focus on optimizing the process of smart distribution of the load of each demand over the cells in the EV battery. Our main theory is that not all the cell-series in the battery should be discharged together but rather, each time only part of them should be discharged. We propose advanced switching algorithms that select the cell-series to be discharged for each current demand and control the discharge current drawn from each.

The rest of this paper is organized as follows. In Section 2 we provide an overview of related work on switching methods. We formally describe the problem in Section 3. Our proposed switching algorithms are presented in Section 4. A description of the simulation data we used to evaluate our algorithms is provided in Section 5. The simulation results are presented in Section 6 and discussed in Section 7. Finally we present our conclusions and direction for future work in Section 8.

Section snippets

Related work

The subject of battery management for multiple battery systems has been widely studied. Most of the studies were intended to maximize the battery lifetime, i.e. the time until most of the cells in the battery have lost an essential part of their capacity in the course of prolonged cycling. Basically there are two kinds of discharge algorithms: sequential and parallel. In the sequential algorithms only one battery supplies the workload each time, while in the parallel algorithms a subset of

Problem description

The problem is defined as follows. The battery pack consists of m identical cell-series, s1,s2,…,sm, all having the same initial capacity C. There are n current demands, d1,d2,…,dn, where di is the required current in amperes (A) for the i-th second. The sequence of the current demands, and in particular its length n, is not known in advance. However, the total capacity of the cell-series is guaranteed to satisfy all demands, i.e. i=1ndimC. A summary of the notations is presented in Table 1.

Switching algorithms

We present three discharge algorithms: naive in Section 4.1 and two heuristic algorithms in Sections 4.2 PreferOPT algorithm, 4.3 EqualLoad algorithm. The inputs of the algorithms are the cell-series statuses, cji values, i.e. their remaining capacities, and the current demand, di. The output of the algorithms is the current values allocated to the demand by the cell-series, Iji values. For simplicity, as our algorithms refer to a single demand at a given time, di, we drop the index i whenever

Simulation data

We evaluated our proposed switching algorithm using computer simulations. The current demands were simulated using the main world-wide driving cycles used in the United States, Europe and Japan [26], [27], [28]. Below is a list of the driving cycles we used with a short description of their purpose (sources: [26], [29]).

United States driving cycles:

  • The Urban Dynamometer Driving Schedule (UDDS), also called FTP-72 (Federal Test Procedure) or LA-4 cycle or FUDS, was developed by the United States

Simulation results

The presentation of the results is divided into two. First we present the penalty of the proposed heuristic algorithms compared to the naive algorithm which is used as a benchmark. Later we describe the differences between the performances of the proposed algorithms.

In general, apparently when using the proposed algorithms the penalty can be significantly reduced and almost totally avoided as the number of cell-series increases. In Fig. 4 the penalties of the EqualLoad algorithm compared to the

Discussion

In general, the results presented in Fig. 4 imply that by using the proposed heuristics a non-linear penalty reduction is possible for almost all driving cycles. In addition, that as the number of cell-series increases the improvement is better, i.e. the total penalty is lower. This behavior is reasonable and can be explained easily. As the number of cell-series increases there are more allocation options and more penalties can be avoided.

A statistical analysis of the results and the current

Conclusions and future work

A battery's life is greatly affected by the method of use and in this paper we focus on the discharge current. We defined a penalty function to measure the negative effects of discharge in non-optimal discharge currents and presented a switching algorithm that minimizes this function. We proposed switching algorithms that select a subset of the battery's cells for each current demand and control the discharge current from each.

The algorithms were evaluated by simulations on world-wide driving

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