Jinlei Sun

A high efficiency equalizer based on forward converter for series connected battery string

In the series-connected lithium-ion battery string application for electric vehicles, equalizer is needed to enhance battery cycle life and maintain the stable performance. The equalizer is taken as an important part of battery management system (BMS), which detects the voltage of each cell to determine the control strategy for equalizer. How to improve the efficiency and reduce loss is always the problem to be solved. This paper proposes a high efficiency nondissipative equalizer for series-connected battery string, which uses forward converter to transfer energy from the highest voltage battery to the lowest voltage one. Current feedback is taken to maintain the stability of output current by adjusting the duty of Pulse Width Modulation (PWM) signal. The proposed equalizer has characteristics such as low loss, high efficiency and short equalization time. The operation principle of the proposed equalizer and design analysis is presented in this paper. Experimental results verify that the proposed equalization method has good cell balancing performance.

Research on Overcharge and Overdischarge Effect on Lithium-Ion Batteries

Commercial lithium-ion phosphate batteries were tested to investigate their responses to overcharge and overdischarge conditions. During overcharge tests, cells were charged at 1C successively until certain visual symptom such as gas release or vent burst occurred. While during overdischarge tests, cells were discharged and kept at 0V or even negative polarity. Parameters such as internal temperature, cell voltage and ac impedance were monitored and recorded for analysis of the abusive process. Tested cells presented failures in different levels. Along with the post-disassembly examination and electrochemical analysis, this paper connects the symptoms, effects and consequences of overcharge and overdischarge phenomena and tries to establish the early precaution approach for battery failures to guarantee the safety of battery applications.

A New Method of State of Peak Power Capability Prediction for Li-Ion Battery

This paper proposed a new method of state of peak power capability prediction for Li-ion battery. Electrochemical impedance spectroscopy based equivalent circuit model is used as the reference model of battery dynamic characterization simulation, state of peak power capability is predicted based on the model and parameters estimation by fractional joint Kalman filter. By decomposing the voltage response of peak state to zero state and zero input responses, the peak terminal voltage can be predicted. Then, the state of battery peak power capability can be forecast quickly under the constant load current assumption.

Development of an Optimized Algorithm for Bidirectional Equalization in Lithium-Ion Batteries

Many equalization circuits have been proposed to improve pack performance and reduce imbalance. Although bidirectional equalization topologies are promising in these methods, pre-equalization global equalization strategy is lacking. This study proposes a novel state-of-charge (SoC) equalization algorithm for bidirectional equalizer based on particle swarm optimization (PSO), which is employed to find optimal equalization time and steps. The working principle of bidirectional equalization topologies is analyzed, and the reason behind the application of SoC as a balancing criterion is explained. To verify the performance of the proposed algorithm, a pack with 12 LiFePO4 batteries is applied in the experiment. Results show that the maximum SoC gap is within 2% after equalization, and the available pack capacity is enhanced by 13.2%. Furthermore, a comparison between previously used methods and the proposed PSO equalization algorithm is presented. Experimental tests are performed, and results show that the proposed PSO equalization algorithm requires fewer steps and is superior to traditional methods in terms of equalization time, energy loss, and balancing performance.

Low Current Rate Discharge with External Heating at Low Temperature

The battery performance deterioration at low temperature attracts more attention. In order to improve the performance at low temperature, a discharge thermal model with external heating is proposed. Discharging tests with and without heating are taken respectively at -27°C and -17°C. The pack energy enhancement ratio is proposed for the first time to evaluate the discharge energy promotion effect with external heating. Experimental results show that discharging with heating at low temperature is efficient to improve the battery temperature and performance.

A Synthesized Diagnosis Approach for Lithium-ion Battery in Hybrid Electric Vehicle

A synthesized diagnosis approach for the lithium-ion battery is proposed with fully understanding of the internal failure mechanism, which is suitable for dynamic conditions such as hybrid electric vehicles. In order to detect and distinguish different fault modes and bridge external electrical parameters with internal chemical mechanisms, a serial of abusive experiments, including overcharge, over-discharge, and low-temperature operation, which commonly occur during battery applications, are arranged. Fault symptoms in the form of electrical parameter variation are extracted with reference performance tests, which consist of hybrid pulse power characteristic test, federal urban driving schedule test, and incremental capacity analysis. The proposed diagnosis approach not only indicates the occurrence of fault but provides a detailed description of symptoms and physical meaning for internal mechanisms as well.

Temperature Performance Comparative Analysis of Different Power Batteries

As the mostly used battery in electric vehicle, Li- ion batteries have different electrode materials. To compare the performance difference of Li-ion batteries with different materials at low temperature, LifePO4 battery, ternary polymer Lithium battery and titanate Lithium battery are selected as the research objects. The capacity, open circuit voltage, ohmic resistance and polarization resistance values of the batteries are obtained at different temperature points. The comparison results show that, the performance of all batteries are decreased with temperature decreasing. The discharge process at low temperature is analyzed by battery equivalent circuit model, the analysis result shows that, the increase of the internal resistance is an essential reason for the performance decrease at low temperature.

Diagnosis of Performance Degradation for Lithium-Ion Battery Module in Electric Vehicle

In recent years much attention has been focused upon performance degradation for lithium-ion batteries due to safety issues. Although SOH diagnosis and remaining life predication for individual cell is relatively sophisticated, module diagnosis especially for electric vehicle(EV) has rarely been reported and applied due to complexity of system. This paper focused on the performance degradation of EV power module during practical conditions, and proposed a diagnosis approach based on wavelet transformation and statistical analysis. This approach provides not only the performance degradation information of entire module, but also the individual cell information at different frequency range, with which ageing process of module, potential failure of individual cell, location of worst cell and fault mechanism can be obtained to guarantee the safety of battery system and reduce maintenance costs.

LiFePO4 Optimal Operation Temperature Range Analysis for EV/HEV

The LiFePO4 batteries are widely used in Electric Vehicle(EV)/Hybrid Electric Vehicle(HEV) because of the high energy and power density. However, high environment temperature could accelerate the aging of batteries, while low temperature could reduce output power capability. Therefore, optimal working temperature for batteries should be determined to maintain good performance in all kinds of tough conditions. In this paper, the optimal working temperature range for batteries is analyzed. The capacity loss model is applied to determine the upper limit. The lower limit is calculated taking available capacity and output power loss into consideration. Simulation and experimental results show that the working temperature range between 10℃ and 40℃ could ensure the performance and available capacity.