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Technical Paper

Impedance Modeling and Aging Research of the Lithium-Ion Batteries Using the EIS Technique

2019-04-02
2019-01-0596
As the core component of electric vehicles (EVs), batteries attach increasingly general attention along with the rapid expansion of electric vehicle market. Battery performance effect directly the safety and reliability of the EVs, so its managing technologies are more and more crucial. Among them, the methods of estimating the state of health (SoH) and predicting remaining useful life become the focuses, which are essential to ensure their dependability and optimum performance over time. This paper mainly focuses on impedance modeling and aging research (aging diagnosis and life prediction) of lithium-ion batteries. Electrochemical impedance spectroscopy (EIS) technique is used to obtain impedance characteristic of batteries. On the one hand, equivalent circuit modeling (ECM) can be motivated by EIS, with the goal to fit measured impedance data using circuit elements.
Journal Article

Influence Mechanism of Electromechanical Parameters on Transient Vibration of Electric Wheel System

2019-04-02
2019-01-0462
Electric wheel systems of in-wheel motor driven vehicles consist of the motor controller, in-wheel motor and tire-suspension assembly. The coupling between the electromagnetic excitation and elastic structure gives rise to electromechanical dynamic issues. As for the structural layout of the electric wheel system, the driving motor is directly connected to the wheel without torsion dampers or transmission in the driveline, thus making the electric wheel structure a weak damping system. Moreover, the driving torque of electric wheel can change rapidly in various conditions of vehicle. As a result, the transient vibration problem becomes one of the key electromechanical dynamic issues in the electric wheel system. To investigate this problem, the electromechanical coupling model of the electric wheel system is established first. Then the transient responses of the electric wheel under abrupt changes of the driving torque are simulated.
Technical Paper

Distributed Drive Electric Vehicle Longitudinal Velocity Estimation with Adaptive Kalman Filter: Theory and Experiment

2019-04-02
2019-01-0439
Velocity is one of the most important inputs of active safety systems such as ABS, TCS, ESC, ACC, AEB et al. In a distributed drive electric vehicle equipped with four in-wheel motors, velocity is hard to obtain due to all-wheel drive, especially in wheel slipping conditions. This paper focus on longitudinal velocity estimation of the distributed drive electric vehicle. Firstly, a basic longitudinal velocity estimation method is built based on a typical Kalman filter, where four wheel speeds obtained by wheel speed sensors constitute an observation variable and the longitudinal acceleration measured by an inertia moment unit is chosen as input variable. In simulations, the typical Kalman filter show good results when no wheel slips; when one or more wheels slip, the typical Kalman filter with constant covariance matrices does not work well. Therefore, a gain matrix adjusting Kalman filter which can detect the wheel slip and cope with that is proposed.
Technical Paper

Genetic Algorithm-Based Parameter Optimization of Energy Management Strategy and Its Analysis for Fuel Cell Hybrid Electric Vehicles

2019-04-02
2019-01-0358
Fuel cell hybrid electric vehicles (FCHEVs) composed of fuel cells and batteries can improve the dynamic response and durability of vehicle propulsion. In addition, braking energy can be recovered by batteries. The energy management strategy (EMS) for distributing the requested power through different types of energy sources plays an important role in FCHEVs. Reasonable power split not only improves vehicle performance but also enhances fuel economy. In this paper, considering the power tracking control strategy which is widely adopted in Advanced Vehicle Simulator (ADVISOR), a constrained nonlinear programming parameter optimization model is established for minimizing fuel consumption. The principal parameters of power tracking control strategy are set as the optimized variables, with the dynamic performance index of FCHEVs being defined as the constraint condition. Then, the genetic algorithm (GA) is applied in the control strategy design for solving the optimization problem.
Technical Paper

Parameter Identification of Battery Pack Considering Cell Inconsistency

2017-03-28
2017-01-1214
Lithium-ion batteries have been applied in the new energy vehicles more and more widely. The inconsistency of battery cells imposes a lot of difficulties in parameter and state estimations. This paper proposes a new algorithm which can online identify the parameters of each individual battery cell accurately with limited increase of computational cost. An equivalent circuit battery model is founded and based on the RLS (recursive least squares) algorithm, an optimization algorithm with the construction of weight vectors is proposed which can identify the parameters of lithium battery pack considering inconsistency of single battery cell. Firstly, the average value of the parameters of the battery pack is identified with the traditional RLS algorithm. Then the ratios between the parameters of each battery cell can be deduced from the mathematical model of battery. These ratios are used to determine the weight vector of each parameter of individual battery cells.
Technical Paper

Experimental Investigation of AC Pulse Heating Method for NMC Lithium-Ion Battery at Subzero Temperatures

2017-03-28
2017-01-1217
An alternating current (AC) heating method for a NMC lithium-ion battery with 8Ah capacity is proposed. The effects of excitation frequency, current amplitudes, and voltage limit condition on the temperature evolution are investigated experimentally. Current amplitudes are set to 24A(3C), 40(5C), and 64A(8C), and excitation frequencies are set to 300Hz, 100Hz, 30Hz, 10Hz, 5Hz, and 1Hz respectively. The voltage limitations are necessary to protect cells from overcharge and over-discharge. Therefore the voltage limit condition (4.2V/2.75V, 4.3V/2.65V, and 4.4V/2.55V) are also considered in depth to verify the feasibility of the AC heating method. The temperature rises prominently as the current increases, and the decrement of frequencies also lead to the obvious growth of battery temperature. The battery obtain the maximum temperature rise at 64A and 1Hz, which takes 1800s to heat up the battery from -25°C to 18°C.
Technical Paper

A Study of Parameter Inconsistency Evolution Pattern in Parallel-Connected Battery Modules

2017-03-28
2017-01-1194
Parallel-connected modules have been widely used in battery packs for electric vehicles nowadays. Unlike series-connected modules, the direct state inconsistency caused by parameter inconsistency in parallel modules is current and temperature non-uniformity, thus resulting in the inconsistency in the speed of aging among cells. Consequently, the evolution pattern of parameter inconsistency is different from that of series-connected modules. Since it’s practically impossible to monitor each cell’s current and temperature information in battery packs, considering cost and energy efficiency, it’s necessary to study how the parameter inconsistency evolves in parallel modules considering the initial parameter distribution, topology design and working condition. In this study, we assigned cells of 18650 format into several groups regarding the degree of capacity and resistance inconsistency. Then all groups are cycled under different environmental temperature and current profile.
Technical Paper

Modeling of Open Circuit Voltage Hysteresis for LiFePO4 Batteries

2015-04-14
2015-01-1180
This paper aims at accurately modeling the nonlinear hysteretic relationship between open circuit voltage (OCV) and state of charge (SOC) for LiFePO4 batteries. The OCV-SOC hysteresis model is based on the discrete Preisach approach which divides the Preisach triangle into finite squares. To determine the weight of each square, a linear function system is constructed including a series of linear equations formulated at every sample time. This function system can be solved by computer offline. When applying this approach online, the calculated square weight vector is pre-stored in advance. Then through multiple operations with hysteresis state vector of squares updated online at every sampling time, the SOC considering the influence of OCV-SOC hysteresis is predicted.
Technical Paper

A Lithium-Ion Battery Optimized Equivalent Circuit Model based on Electrochemical Impedance Spectroscopy

2015-04-14
2015-01-1191
An electrochemical impedance spectroscopy battery model based on the porous electrode theory is used in the paper, which can comprehensively depict the internal state of the battery. The effect of battery key parameters (the radius of particle, electrochemical reaction rate constant, solid/electrolyte diffusion coefficient, conductivity) to the simulated impedance spectroscopy are discussed. Based on the EIS analysis, a lithium-ion battery optimized equivalent circuit model is built. The parameters in the equivalent circuit model have more clear physical meaning. The reliability of the optimized equivalent circuit model is verified by compared the model and experiments. The relationship between the external condition and internal resistance could be studied according to the optimized equivalent circuit model. Thus the internal process of the power battery is better understood.
Technical Paper

Research on Charging Strategy of Lithium-ion Battery

2015-04-14
2015-01-1192
Lithium-ion battery charging strategy affects charging time of electric vehicles, energy efficiency of entire vehicle, service life and safety. This paper focuses on the lithium iron phosphate (LiFePO4) battery, based on the battery internal mechanism and the working conditions, taking charging time, effective full-charge capacity and charge energy efficiency as the evaluation indexes. Firstly, through a series of comparative experiments of the constant-current constant-voltage and the constant current charging strategy, the evaluation indexes variations in different temperatures and charging currents have been studied in the paper. By analyzing the respective characteristics of constant current charging phase and constant voltage charging phase in the whole charging process and their own contributions, we have found out the superiority of the constant current charging strategy.
Journal Article

A Novel ZSB-PAM Power Regulation Method Applied in Wireless Charging System for Vehicular Power Batteries

2015-04-14
2015-01-1194
Wireless charging system for vehicular power batteries is becoming more and more popular. As one of important issues, charging power regulation is indispensable for online control, especially when the distance or angle between chassis and ground changes. This paper proposes a novel power regulation method named Z-Source-Based Pulse-Amplitude-Modulation (ZSB-PAM), which has not been mentioned in the literatures yet. The ZSB-PAM employs a unique impedance network (two pairs of inductors and capacitors connected in X shape) to couple the cascaded H Bridge to the power source. By controlling the shoot-through state of H bridge, the input voltage to H bridge can be boosted, thus the transmitter current can be adjusted, and hence, charging current and power for batteries. A LCL-LCL resonant topology is adopted as the main transfer energy carrier, for it can work with a unity power factor and have the current source characteristic which is suitable for battery charging.
Technical Paper

Optimal Regenerative Braking Control for 4WD Electric Vehicles with Decoupled Electro-Hydraulic Brake System

2015-04-14
2015-01-1117
Regenerative braking control for a four-wheel-drive (4WD) electric vehicle (EV) equipped with a decoupled electro-hydraulic brake system was studied. The energy flow of the 4WD electric vehicle was analyzed during braking, and the brake force distribution strategy between the front-rear axles, regenerative braking and hydraulic braking was studied. Considering ECE R13 regulations, motors and battery pack characteristic constraints, the optimal regenerative braking control strategy using Genetic Algorithm (GA) was proposed. A Hardware-in-loop (HIL) test was built to verify the proposed regenerative braking control strategy. The results show that the optimal regenerative braking control strategy for the 4WD electric vehicle was advantageous over the comparison program, and regenerative energy efficiency reaches 78.87% under the Shanghai Urban Driving Cycle (SUDC).
Journal Article

Differential Drive Assisted Steering Control for an In-wheel Motor Electric Vehicle

2015-04-14
2015-01-1599
For an electric vehicle driven by four in-wheel motors, the torque of each wheel can be controlled precisely and independently. A closed-loop control method of differential drive assisted steering (DDAS) has been proposed to improve vehicle steering properties based on those advantages. With consideration of acceleration requirement, a three dimensional characteristic curve that indicates the relation between torque and angle of the steering wheel at different vehicle speeds was designed as a basis of the control system. In order to deal with the saturation of motor's output torque under certain conditions, an anti-windup PI control algorithm was designed. Simulations and vehicle tests, including pivot steering test, lemniscate test and central steering test were carried out to verify the performance of the DDAS in steering portability and road feeling.
Technical Paper

A Control Allocation Strategy for Electric Vehicles with In-wheel Motors and Hydraulic Brake System

2015-04-14
2015-01-1600
Distributed drive electric vehicle (EV) is driven by four independent hub motors mounted directly in wheels and retains traditional hydraulic brake system. So it can quickly produce driving/braking motor torque and large stable hydraulic braking force. In this paper a new control allocation strategy for distributed drive electric vehicle is proposed to improve vehicle's lateral stability performance. It exploits the quick response of motor torque and controllable hydraulic pressure of the hydraulic brake system. The allocation strategy consists of two sections. The first section uses an optimal allocation controller to calculate the total longitudinal force of each wheel. In the controller, a dynamic efficiency matrix is designed via local linearization to improve lateral stability control performance, as it considers the influence of tire coupling characteristics over yaw moment control in extreme situations.
Technical Paper

Vehicle Stability Criterion Research Based on Phase Plane Method

2017-03-28
2017-01-1560
In this paper, a novel method is proposed to establish the vehicle yaw stability criterion based on the sideslip angle-yaw rate (β-r) phase plane method. First, nonlinear two degrees of freedom vehicle analysis model is established by adopting the Magic Formula of nonlinear tire model. Then, according to the model in the Matlab/Simulink environment, the β-r phase plane is gained. Emphatically, the effects of different driving conditions (front wheels steering angle, road adhesion coefficient and speed) on the stability boundaries of the phase plane are analyzed. Through a large number of simulation analysis, results show that there are two types of phase plane: curve stability region and diamond stability region, and the judgment method of the vehicle stability domain type under different driving conditions is solved.
Technical Paper

Composite Steering Strategy for 4WS-4WD EV Based on Low-Speed Steering Maneuverability

2019-11-04
2019-01-5052
A composite steering control strategy, which combines four-wheel steering (4WS) and differential steering, is proposed in this paper, to optimize steering maneuverability in the conditions where the vehicle speed is below 15 Km/h, mainly for U-turning and parking conditions. A dynamic model is developed for the steering system and the tire system. Taking different steering wheel inputs into consideration, a 4WS control strategy proportional to the front wheel steering angle is quoted to improve the steering maneuverability in the low speed conditions and guarantee the manipulability by controlling the side slip of the vehicle. Based on the 4WS system, this paper explores the possibility of further improving the low-speed maneuverability of the vehicle through differential steering. And the differential steering control strategy is developed, including four hub-motor output modes. A composite steering controller is designed based on the 4WS-4WD electric vehicle platform.
Technical Paper

SOC Estimation of Battery Pack Considering Cell Inconsistency

2019-04-02
2019-01-1309
Range anxiety problem has always been one of the biggest concern of consumers for pure electric vehicles. Accurate driving range prediction is based on accurate lithium-ion battery pack SOC (State of Charge) estimation. In this article, a complete SOC estimation algorithm is proposed from cell level to battery pack level. To begin with, the equivalent circuit model (ECM) is applied as the model of battery cell. ECM parameters are identified every 10% SOC interval through genetic algorithm. The dual extended Kalman filtering (DEKF) algorithm is adopted for cell-level SOC and ohmic resistance R0 estimation. The estimation accuracy of cell SOC and R0 is verified under NEDC dynamic working condition. The cell-level SOC estimation error is below 1%. However, cell inconsistency can always result in inaccurate cell SOC estimation inside the battery pack. The impact of initial SOC inconsistency and internal resistance inconsistency between cells on battery pack SOC is specifically analyzed.
Technical Paper

On-line Lithium-Ion Battery State-of-Power Prediction by Twice Recursive Method Based on Dynamic Model

2019-04-02
2019-01-1311
State-of-Power (SoP) prediction of Li-ion battery is necessary in battery management system for electric vehicles in order to deal with limited conditions, prevent overcharge and over discharge situations, increase the life of the battery and provide effective battery operation. This article suggests a method to on-line predict the 10-s charge and discharge peak power of Li-ion battery by twice recursions. First with the dynamic battery model we use the first recursion based on a least square method to get parameters which are influenced by the state of charge of Li-ion battery and temperature, etc. The dynamic model is an equivalent circuit model. Current and voltage are input online into the battery model. By recursive least square method the parameters are updated in real time. Moreover, when we use a recursive method to get real-time parameters, we add an extra proper factor to abandon old datum, which increases the real-time capability of state-of-power prediction.
Technical Paper

A Comparative Study of Different Wheel Rotating Simulation Methods in Automotive Aerodynamics

2018-04-03
2018-01-0728
Wheel Aerodynamics is an important part of vehicle aerodynamics. The wheels can notably influence the total aerodynamic drag, lift and ventilation drag of vehicles. In order to simulate the real on-road condition of driving cars, the moving ground and wheel rotation is of major importance in CFD. However, the wheel rotation condition is difficult to be represented exactly, so this is still a critical topic which needs to be worked on. In this paper, a study, which focuses on two types of cars: a fastback sedan and a notchback DrivAer, is conducted. Comparing three different wheel rotating simulation methods: steady Moving wall, MRF and unsteady Sliding Mesh, the effects of different methods for the numerical simulation of vehicle aerodynamics are revealed. Discrepancies of aerodynamic forces between the methods are discussed as well as the flow field, and the simulation results are also compared with published experimental data for validation.
Technical Paper

Optimal Torque Allocation for Distributed Drive Electric Skid-Steered Vehicles Based on Energy Efficiency

2018-04-03
2018-01-0579
Steering of skid-steered vehicles without steering mechanism is realized by differential drive/brake torque generated from in-wheel motors at left and right sides. Compared to traditional Ackerman-steered vehicles, skid-steered vehicles consume much more energy while steering due to greater steering resistance. Torque allocation is critical to the distributed drive skid-steered vehicles, since it influences not only steering performance, but also energy efficiency. In this paper, the dynamic characteristics of six-wheeled skid-steered vehicles were analyzed, and a 2-DOF vehicle model was established, which is important for both motion tracking control and torque allocation. Furthermore, a hierarchical controller was proposed. Considering tire force characteristics and tire slip, the upper layer calculates the generalized force and desired yaw moment based on anti-windup PI (proportion-integral) control method.
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