Rengui Lu

Super-Capacitor Stacks Management System With Dynamic Equalization Techniques

Super capacitor has the advantage of quick charge, large power density, and long cycle life. The shortage is the lower energy density compared with electrochemistry batteries. These features make it suitable for a short-distance electric bus used in the city. Because of the capacitance difference between the capacitor cells, after a number of deep discharging/charging cycles, the voltage difference between cells will be enlarged. This will accelerate the aging of the weak super capacitors and affect the output power. So, a management system with an equalization function is essential. In this paper, a practical super-capacitor stacks management system with dynamic equalization techniques is proposed. The function of the management system includes: monitoring the current, voltage, and temperature of the stacks, control of charge, and discharge with equalization online. A switched-capacitor equalization approach is adopted showing a low-cost way to meet the accuracy requirements. The dynamic equalized charging and discharging circuit is described. The algorithm to increase the operation speed and the precision is analyzed. By dynamically redistributing the current, the equalization procedure can be quicker and more efficient. This approach has been verified by experiments

Research on the model of magnetic-resonance based wireless energy transfer system

The key of magnetic-resonance based wireless energy transfer technology (MRBWET) is the magnetic coupling system which composed of driving and receiving coils, of which the mathematical model is important to the research of the transmission principle and the optimization of mechanism structure and parameters. Based on calculating the radiation level of typical coil, a magnetic-coupled-coil model was built based on analyzing the magnetic dipole used in the experimental system. According to the model, the quantitative relationship between the power contained in the receiving coil and parameters of resonances was discussed. The optimization method of parameters matching for maximal power transfer is proposed. Simulation and experimental results validate the correction and feasibility of the theoretical analysis.

Improved Study of Temperature Dependence Equivalent Circuit Model for Supercapacitors

The supercapacitor, because of its advantages of high specific power, quick charging or discharging with high current rates, and long cycle life, is an interesting choice for energy storage for applications where high power is needed for only a few seconds, such as pulse-power supply systems. Based on the use of supercapacitors all around the world, more and more attention is focused on the effects of temperature on their performance metrics such as capacitance, internal resistance, and efficiency. To predict the terminal voltage of supercapacitors at different temperatures, a three-branch RC equivalent circuit model is improved in this paper, and it can predict the terminal voltage up to 5 to 10 min after completion of charging or discharging. Initially, a typical three-branch RC equivalent circuit model without temperature parameter is used to simulate the pulse charging and discharging behavior of a specific commercial supercapacitor. Next, with the pulse charging and discharging experiment results under -40°C, -20°C, 0°C, and 20°C, all parameters in the circuit model are reset to a function of temperature, and the temperature dependence of parameters is discussed in detail and determined numerically. Finally, for the purpose of validating the correctness and the accuracy of the improved model, compared study of simulation results and experiment curves is done. Comparison results show that the fitting accuracy of the improved equivalent circuit model is satisfied in pulse charging and discharging at different temperature.

Energetic Macroscopic Representation based modeling and control for battery/ultra-capacitor hybrid energy strorage system in HEV

Electrical energy storage system plays an important role in hybrid electric vehicles. In this paper a simulation model for battery/ultra-capacitor hybrid energy storage system is presented by using Energetic Macroscopic Representation (EMR). Based on the model a control strategy for the hybrid energy storage system is developed with a goal of improving battery life and increasing the performance of HEV drive train. The simulation results under different urban driving cycles show the validity of the model.

A high efficiency isolated bidirectional equalizer for Lithium-ion battery string

Equalization for battery pack to enhance its usage life and performance is important for xEVs. In this paper, an isolated bidirectional equalizer based on full-bridge for Lithium-ion battery string is proposed. If a battery charge is excessive, the equalizer transfer charge from this cell to battery string; if a battery charge is insufficient, the equalizer will charge this cell from battery string. Closed-loop control keeps the equalization current constant, allowing precise control of the transferred charge. On the basis of considering OCV hysteresis, we proposed an equalization strategy based on SOC estimation for the equalizer. The equalizer features small size, high efficiency, and high accuracy. Experimental results confirm the applicability of the equalization scheme.

The Development of an Electric Bus with Super-Capacitors as Unique Energy Storage

In this paper the development of an electric bus with super-capacitors as unique energy storage is proposed. Super-capacitor has the advantage of quick charge, large power density and long cycle life. The super-capacitor bus is suitable for using in city and the drive distance between two terminals is within 20 km. Its advantage is the capability of frequently start/stop, accelerate and decelerate. Especially, in low temperature the characteristic of super-capacitor is better than that of the chemistry battery. This feature makes it the best choice in the cold area. This project is supported by the government of Heilongjiang province, China. The power of the super-capacitor stacks is 14 kWh, and the voltage range is 160 V-320 V. The maximal speed is 50 km/h. A permanent magnetic motor and a two quadrant IGBT chopper controller are developed. The rating power of the motor is 60 kW and the transient power is 90 kW. Because of the short duration of charging the super-capacitor stacks (about 15 minutes) this kind of electric bus can be used in 24 hours. The performance and the principle details are described in the paper

Analysis and design of a wireless closed-loop ICPT system working at ZVS mode

Based on the T-type equivalent circuit model of loosely coupled transformer, the theory of series-series (SS) resonant inductively coupled power transfer (ICPT) system was analyzed. By researching the principle of ICPT system working at zero voltage switch (ZVS) mode, the condition for realizing ZVS mode was got that the load resistance should be bigger than its critical value. According to the characteristic of open-loop ICPT system, a wireless voltage-feedback control for ICPT system working at ZVS mode was established. Based on the analysis above, An ICPT system prototype was built and tested, the maximum open-loop output power was 3.4KW, overall efficiency was 90%; the closed-loop output voltage was stable at 260V, and the maximum output power is 1.9KW.

A new topology of switched capacitor circuit for the balance system of ultra-capacitor stacks

The quick equalization technology of ultra-capacitor stacks in series can protect ultra-capacitors and make full use of limited capacitance. Ultra-capacitorpsilas high specific power and charging current require a higher equalization speed. Therefore a method related with buck-boost conversion for switched capacitorspsila equalization was developed. The design method for circuit parameters and corresponding equalization control algorithm were introduced. The simulation and experiment show that this method with high precision can realize quick equalization and meet the demand of charging and discharging with high current.

Study on Wireless Energy and Data Transmission for Long-Range Projectile

In this paper, a compact wireless energy and data transmission device which can be used on the remote projectile is described. This device uses magnetic field generated by the coupled coil to receive the energy and data and transfers the internal data. These functions above can be realized by operating the control circuit in different modes, which requires only one coil in the projectile. In energy transmission mode, energy can be transferred into the projectile wirelessly, as well as the external data by data encoding technology. High-frequency magnetic field is generated by internal device when the internal data is delivered to the external receiver coil. Interference between the energy and data transmission can be avoided by adjusting the data resonant frequency which is different from the energy transfer frequency by changing the resonant circuit topology and the internal capacitance. The device works at zero voltage switching mode, and produces lower loss, which is beneficial for improving the efficiency. The experiment results show that the device can transfer energy at 1MHz, and internal data at 500 kHz.

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.