Study of High-capacity Single-body Li-ion Battery Charging and Discharging System

Jianqiang Wang, Jingxin Li, Weige Zhang Department of Electrical Engineering

Beijing Jiaotong University Beijing, China

Abstract—This paper proposed a high-efficiency battery charging and discharging system for high-capacity single-body Li-ion battery. This system can work as battery formation system or battery test system. The main system structure includes a single-phase bi-directional PWM rectifier, dc-link, half-bridge converter with current-doubler synchronous rectifier as charging unit and interleaved pull-push converter as discharging unit, and control unit and communication port. The charging and discharging units were designed separately for more feasible control strategy, because several such units can be paralleled to be a bigger system through dc-link. The energy balance among batteries and the grid is controlled automatically. There are three system operation modes: all batteries charging from the grid or discharging to the grid, energy transfer among charging batteries, discharging batteries and the grid. Besides of basic functions, battery energy records were accomplished by communication with the host computer. Such records will beneficial to the battery lifetime evaluation.

Keywords—charge; discharge; PWM rectifier; push-pull converter; half-bridge converter; current-doubler synchronous converter

I. INTRODUCTION

Lithium Ion Batteries are now available in large sizes and with high discharge currents. They are rechargeable and are widely used in mangy fields, such as UPS, power supplies, electric bicycles and vehicles.

Compared with lead-acid batteries and Ni-MH batteries, Li-ion batteries are superior in terms of specific energy (the amount of available energy per unit of mass or volume) and in terms of specific power (the amount of available output per unit of mass or volume). As a result, they are seen as the most promising batteries for secondary-battery applications. [1]

The classical operating voltage of high-capacity of single-body Li-ion battery is only 3.3V, but battery capacity has been beyond 100Ah, therefore, the charging and discharging current is very large.

No matter whether battery manufacture or test, the battery should be charged and discharge repeatedly. Generally, the battery charging energy comes from the grid, and the battery discharging energy releases by the resistors. In doing so, it will be very expensive and wasteful. It is estimated that the battery

formation cost is about 20%~30% of total manufacture cost. There is a lot of energy waste in high-capacity battery formation or test processing.

This paper proposed a novel high-capacity Li-ion battery charging and discharging system. Such system can work as battery formation system or battery test system. The main characteristic is high energy utilization efficiency. The system did not include energy release resistors. It not only can achieve basic battery charging and discharging function, but also recycle battery discharging energy and feedback the surplus discharging energy to grid.

II. MAIN SYSTEM STRUCTURE

The main system structure includes a single-phase bi-directional PWM rectifier, dc-link, half-bridge converter with current-doubler synchronous rectifier as charging unit and interleaved pull-push converter as discharging unit, and control unit and communication port. The component blocks are shown in figure 1. A pair of charging and discharging unit serves a single-body Li-ion battery. One single-phase bi-directional PWM rectifier serves up to ten groups of charging and discharging pairs through dc-link. The DC link voltage is set above 350V corresponding to 220V single-phase AC grid rms voltage. The respective controllers executed unit control function and the main control unit monitors all unit operating states. The control unit includes communication port which can be connected to the upper computer.

Figure 1. main system structure This work was supported by Beijing Science and Technology Project with No. Z08010402140801.

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A. SINGLE- PHASE BI-DIRECTIONAL PWM RECTIFIER In this system, the main function of the single-phase bi-

directional PWM rectifier is to achieve the unit power factor at grid side and to maintain stable dc-link voltage when battery charging or discharging.

The control strategy is that if the input energy on dc-link is not balance to the output energy, the dc-link voltage must become higher or lower. According to this, the PWM rectifier should change the duty cycle of switch to response the dc-link voltage fluctuation. The main structure of single-phase bi-directional PWM rectifier is shown in figure 2. Its control is realized by DSP with SPWM control method. Bi-directional PWM rectifier can achieve the unit power factor no matter whether battery charging or discharging. The dc-link voltage is set to 400V for less bus current and for feeding electricity back to the grid if possible.

Figure 2. main structure of single-phase bi-directional rectifier

B. CHARGING UNIT The input of charging unit is connected to the dc-link and

the output is connected to the single-body Li-ion battery. So there is high voltage transfer ratio between dc-link voltage and the single-body Li-ion battery voltage. Overhigh transformer turn ratio is not easy to realize physically. Based on this, the charging unit topology chooses half-bridge converter with current-doubler synchronous rectifier, see figure 3.

Figure 3. main structure of charging unit

The topology shown in figure 3 is suitable to high voltage transfer ratio and low-voltage high-current output. At first, the voltage on half-bridge converter transformer is only half of the input voltage. Secondly, the output of converter transformer goes through a current-doubler synchronous rectifier. It boosts terminal output voltage to double secondary output voltage of the transformer. Consequently, the real transformer turn ratio is only 1/4 of theoretical voltage transfer ratio. The transformer T is with only one secondary winding. Furthermore, in the current-doubler synchronous rectifier, if the current-doubler inductors integrate with the secondary coil of the transformer, the volume of charging unit should be less.

The control time sequence should be noted specially. The dead time between the driving signals of two main switches on primary side must exist to avoid the short circuit caused by conducting of two main switches at the same time. And two synchronous rectification switches driving signals are also complementary and are turned on during the dead time of primary switches.

C. DISCHARGING UNIT The discharging unit is based on an interleaved push-pull

topology, which is shown in figure 4. The input of discharging unit is connected to the battery and the output is connected to the dc-link. Point to the characteristic of low-voltage high-current input and high-voltage low-current output, five push-pull converters are adopt with the parallel primary windings and series secondary windings. The transformer is with only one primary winding. The secondary voltage is rectified by fast recovery diode full bridge. In every branch of primary, two equal adjustable pulse-width, 180ºout-of-phase pulses drive two primary switches, they are all MOSFET to avoid the complex flux imbalance in the push-pull transformer.

Figure 4. main structure of discharging unit

III. OPERATION PRINCIPLEThe system has three operation modes: all batteries

charging from the grid or discharging to the grid, energy transfer among charging batteries, discharging batteries and the grid. That is charging, discharging and hybrid operation.

When all single-body Li-ion batteries are charging, they powered by the grid through the single-phase bi-directional PWM rectifier and the charging unit. At this time, the batteries have two charging modes: constant current or constant voltage. The mode choice is decided by the battery charge state.

When all batteries are discharging, the discharging energy goes through the discharging unit and the PWM rectifier, and feeds back electricity to the grid. The discharging process generally has only one choice, namely constant current discharging. But the discharging current is actually decreased

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in segmentation, and the constant current discharging occurs in every segment.

If some batteries are in charging but the others are in discharging, the system works in hybrid operation mode. At this time, the discharging energy will be transferred from the discharging ones to charging ones directly through the dc-link. Therefore, few charging or discharging energy absorbs from the grid or feedbacks to the grid by the PWM rectifier. In ideal state or occasionally, the net energy from the grid can be even about zero.

If this system is utilized in battery formation processing, hybrid operation is advised to be the most usual operation mode. Such battery formation can save more electricity by energy recycling between charging and discharging batteries.

The dc-link voltage keeps constant during system operation. This function is mainly achieved by single-phase bi-directional PWM rectifier. When discharge energy is surplus, the PWM rectifier feeds energy back to the grid in order to sustain constant dc-link voltage. Otherwise, the PWM rectifier absorbs energy from the grid for the same purpose. All the operations of this system are executed automatically by respective controllers.

IV. EXPERIMENTAL RESULT AND WAVEFORMS

The system prototype is already constructed and operating normally, but it is still in test stage when this paper was writing. Figure 5 gives a part of preliminary test waveforms of transformer in charging and discharging units. The tested high-capacity single-body Li-ion battery is rated at 100Ah.

The left is primary and secondary winding voltages of the charging unit (from above to bottom). There is oscillation existing when switch off. The classic efficiency of this unit is about 80%. This owe to the synchronous rectifier.

The right is switch driving signals, primary winding voltage and discharging current of the discharging unit (from above to bottom). There is also oscillation existing when switch off, but the oscillation Amplitude is not big as charging unit. The classic efficiency of this unit is about 75%.

(a)charging (b)disharging

Figure 5. waveforms of the transformer in charging and discharging units

V. CONCLUSION

This battery charging and discharging system is designed for high-capacity single-body Li-ion battery. In fact, this system is also applicable to other battery style, such as lead-acid batteries.

There are several power converter types in this system, they are scarcely new topology, but integrated in a system suitably. The charging and discharging units were designed separately for more feasible control strategy. In system prototype, there are ten groups of charging and discharging combination. They are connected by dc-link. The energy balance among batteries and the grid is controlled automatically.

Besides of basic functions, battery energy in charging and discharging cycles were recorded. These records are communicated to the host computer for data analysis. These records will beneficial to the battery lifetime evaluation and battery performance detection.

REFERENCES

[1] Nobuaki Takeda, Sadao Imai, Yusuke Horii, Hiroaki Yishida, “Development of High-Performance Lithium-Ion Batteries for Hybrid Electric Vehicles,” Technical Review, 2003(15):68~72

[2] Paku, R., Marschalko, R.. “Operating space of a bidirectional PWM ac-to-dc converter applied in active line-conditioning”. IEEE AQTR2008: 489~494

[3] Xuefei Xie, Liu, J.C.P., Poon, F.N.K., Man Hay Pong. “A novel high frequency current-driven synchronous converter for low voltage high current applications”. The IEEE APEC2001:469~475.

[4] Wei Chen, Zhengyu Lu, Xiaofeng Zhang, Shaoshi Ye, “A Novel ZVS Step-up Push-Pull Type Isolated LLC Series Resonant Dc-Dc Converter for UPS Systems and Its Topology Variations,” IEEE

[5] Qun Zhao and Lee, F.C., “High-efficiency, high step-up DC-DC converters,” IEEE Trans. on Power Electronics, Vol.18, Part 1, 2003: 65~73.

[6] Tsai-Fu Wu, Jin-Chyuan Hung, Jeng-Tsuen Tsai, Cheng-Tao Tsai, Yaow-Ming Chen, “An Active-Clamp Push–Pull Converter for Battery Sourcing Applications,” IEEE Trans. on Industry Applications, Vol.44, No.1, 2008:196~204

[7] Sam Ben-Yaakov, Mor Mordechai Peretz, “Modeling and Behavioral SPICE Simulation of a Self Adjusting Current-Fed Push-pull Parallel Resonant Inverter (SA-CFPPRI),” IEEE PESC2004:61~67

[8] A.Bhinge, N.Mohan, R.Giri, and R.Ayyanar, “Series Parallel Connection of DC-DC Converter Modules with Active Sharing of Input Voltage and Load Current,” IEEE APEC 2002:648~653.

[9] J.W.Kim, J.S.You, and B.H.Cho, “Modeling, control, and design of input-series-output- parallel-connected converter for high-speed-train power system,” IEEE Trans. on Power Electronics, 2001:536~544.

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