Research on Battery Charging-Discharging in New Energy Systems

As an energy storage component, the battery plays increasingly important role in new energy industry. Charging and discharging system is the vital part of the application of the battery, but the charge and discharge are always designed separately and carried by different part in the traditional application. Additionally, most battery discharge mode and method are always simplified which cannot ensure to meet the demand of power utilization. In the actual energy storage system, the design of the energy converter, which make the power storage and supply as a whole and the design of the charge and discharge method, will play an important role in efficient utilization of the battery system. As a part of the new energy system, the study makes battery and the charging and discharging system as a whole to store energy, which can store and release electric energy high efficiently according to the system state and control the bidirectional flow of energy precisely. Using TMS320F2812 as the control core, the system which integrates charging and discharging with battery monitoring can achieve the bidirectional Buck/Boost power control. It can achieve three-stage charging and selective discharging of the battery. Due to the influence of the diode reverse recovery time, current oscillation will appear. In order to eliminate the oscillation, we can set the circuit to work in critical conduction mode. The experimental result shows that the system can achieve the charging and discharging control of lead-acid battery and increase the battery life time further.


INTRODUCTION
As the structure of the global energy is developing in the direction of green, the appliance of lead-acid batteries in new energy has fast development.In wind energy, solar energy, tidal energy and other new energy systems, the power fluctuates greatly due to the impact of changes of natural conditions.So the energy storage battery is essential to stabilize the power output, especially in some large systems (Bimal, 2000).Besides, as a kind of distributed power, it can balance the new energy to ensure the system stable and reliable.The battery is not only an important energy storage device, but also a stable power supply device, such as ones in electric vehicles and other mobile devices (Xu and Sankaram, 1993;Hua and Lin, 2000) In conclusion, a battery can affect the life time of the system due to the important role it playing in new energy system.To solve the problem, the property of a battery needs to be improved, as well as the design of the charging and discharging strategy.At present the battery charging and discharging system is designed mostly for some UPS systems (Noworolski et al., 1991) and the application in large new energy system is less.Literature (Sauer and Garche, 2001) only gives the design and model of battery in photovoltaic systems.In addition most articles only study the charging method (Cope and Podrazhansky, 1999;Bo-Yuan and Yen-Shin, 2012) and present few clear design scheme of the charging circuit.The battery discharge mode and method are also not detailed and charge and discharge of battery have been usually separated into two parts (Koutroulis and Kalaitzakis, 2004).Even through Analog control chips are still used, the control precision can not satisfy the requirement of accuracy.Digital control is the trend of control method in the field of the industry.As the battery charging is electrochemical reaction, charging methods will have influence on the speed of the charge.A lot of references have presented the improvement in the traditional three-level charging method (Reid and Glasa, 1984;Guo and Huang, 2010), such as intermittent charge, pulse charge and negative pulse charge.But the realization methods are more complex and cannot ensure to make the battery charging speed improved significantly This study will study the Valve-Regulated Lead-Acid Battery (VRLA) and optimize the design of the charging and discharging strategy to improve the property of the battery.The system uses relatively simple and practical Buck/Boost converter topology.Through the improvement of this topology and the optimization of the hardware circuit and the control method, we can make the system achieve stable charging and discharging with high efficiency, which extends the life time and improves the state of charge of the battery.

BATTERY ENERGY STORAGE IN NEW ENERGY SYSTEM AND THE CIRCUIT OF CHARGING-DISCHARGING
 Battery energy storage in new energy system: Figure 1 shows the structure of the new energy system, which is composed of solar power, electric vehicles charging, batteries energy storage and grid-connected bidirectional inverter.These four parts are linked via the DC bus-bar and they can work as a whole or individually.
This study shows the design, calculation and test result of batteries energy storage (BES).The main functions of BES unit are to store energy from photovoltaic or the feedback power of electric vehicles and to discharge to supply energy to the electric vehicles when needed by linked to the system via the DC bus.Therefore, the electrical converting circuit of batteries can charge and discharge fastly, convert the working state rapidly and have high precision of stabilizing voltage/current and advanced charging and discharging control mode.
 The Bi-directional circuit for battery chargingdischarging: The components parameters in the circuit as shown in Fig. 2  In this system, the voltage of 32 cells in series is DC384V.(The measured initial voltage range of cell: 12.87-13.1V)The charging voltage of the cell is 14.4-15 V and the maximum charging current whose value is 16.25A is not larger than 25% of the rated value.
Over-discharge means that battery terminal voltage is lower than the required cut-off voltage.It causes batteries serious damage and reduces the life time of the battery.The discharging current is not larger than 3C and a cell cut-off voltage is not less than 1.3V.Meanwhile V91 and V92 are always state-off (Jun et al., 2010;Ned et al., 1995).

 The principle of buck and boost circuit: In
In the circuit, V91 and V92 mustn't be on conduction at the same time in order to avoid a short circuit.

 Characteristics of the bi-directional power conversion circuit:
The circuit in Fig. 2 combines Buck circuit with Boost circuit, so that it can achieve bi-directional power conversion through the controlling of V91 and V92.
The system contains the following advantages: o DC/DC converter has advanced control technology.o Two IGBTs-V91, V92 and freewheeling diodes D91, D92 in Buck and Boost circuits are integrated in a single module and they make the power circuit simple and easy to be installed and cool.o Controlling circuit is simple.o The switching frequency of IGBT is up to 20 kHz and the filter reactor is small and light.o Batteries work with converters as a whole, so we don't need to replace a battery like traditional charging devices.The overall efficiency of the system is high because the power supply doesn't need to be isolated by the transformer.o The energy achieves seamless bi-directional exchange under accurate calculation of the DSP.
When cells are discharged in Boost mode, a relatively high reverse current spike is discovered due to long reverse recovery time (350 ns) of IGBT.In order to avoid the damage, V92 is conducted when the inductor current drops to 0. This method can achieve the effective conduction of the IGBT so that large reverse current can be eliminated and the damage of IGBT and freewheeling diodes is avoided.
By using the critical conduction mode control strategy, V92 is conducted when the inductor current come to be 0, which makes the converter work in the critical conduction mode.Therefore, the inductor should be coupled with the coil when the main circuit is designed.The coupled circuit can output the falling edge to control the opening of IGBT when the inductor current is 0 (Huang, 2006).
 Control circuit: Figure 3 shows the overall system design diagram of the control circuit.The main control chip is the TI DSP TMS320LF2812, besides the system includes drive circuit, voltage and current detection circuit, protection circuit, keyboard display circuit and auxiliary power supply.

CONTROL ALGORITHM AND REALIZATION OF DSP
 Control algorithm: In conventional cases, as shown in Fig. 4, the charging mode is three-stage: constant current, constant voltage and floating.However, the constant charging current will increase if the DC bus voltage rises fast due to the large amount of electric energy generated by solar power.(experimental subject are ten 12V65AH valve regulated lead acid batteries).
During the transition to the second stage, the charging current should be limited in order to avoid a sudden increase of the current.So the charging mode maintains the first stage namely the charging mode of The discharging mode of the battery is the selective constant voltage or the selective constant current.The charging mode is selected by users or the system and the system can select the discharging mode when the condition is OK.
The control diagrams of the charging system and the discharging system are the same because they use the same modes, shown in Fig. 5.

Control flowchart:
The main program achieves some initialization, preparatory work before running the system and displaying when running the system.The initialization includes the system in the DSP, IO conFiguration, all the variables, AD sampling module, EV, etc.After the preparation, open the AD sampling interrupt and interrupt every 50us to sample two groups of voltage and current signals.We can get the actual value after filtering and converting the result of the sampling.Then enter into the charging and discharging subprogram respectively through the judgment of the system operating mode.The flow chart of the main program is shown in Fig. 6.The function of the charging subprogram is to achieve stable operation of the charging circuit in three  The flow chart of the discharging subprogram is shown in Fig. 8.

EXPERIMENTAL RESULTS AND ANALYSIS
 Waveform for charging: In the first stage, charge the battery with relatively large current at the beginning of the charging.We use 10 A to charge the battery and the terminal voltage of the battery gradually increases linearly and enters the second stage when the voltage increases to 140 V.
In the second stage, use 145 V to charge when entering the constant voltage mode and the charging current rapidly decreases exponentially.The third stage begins when the current reduces to 0.6 A.
In the third stage, use the smaller current 0.5 A to charge the battery, namely floating.The charging ends and the system stops running when the battery voltage rises to 155 V.
Figure 9a shows the C-E (collector-emitter)voltage of V 91 in the first stage of the charging.The Buck circuit    We can find that there are current oscillations and voltage spikes when the IGBT works.The reason for the phenomenon is that the diode reverse recovery time is too long and makes the parasitic inductance and parasitic capacitance in the circuit resonant.In order to eliminate the current oscillation, we can make the circuit operate in critical conduction mode.Therefore, it is necessary to design a zero-current detection circuit.In this study, this circuit is designed and tested by experiments.The waveform is shown in Fig. 13.When the inductive current reaches zero, the detection circuit outputs the falling edge and IGBT is controlled to open by DSP.The function of the critical conduction mode is to eliminate the reverse recovery of the freewheeling diode by turning on the active switch when the inductor current drops to zero.

CONCLUSION
The study researches the charging-discharging system of the lead-acid battery in new energy system and achieves the bidirectional Boost/Buck power circuit which combines charging with discharging.The system achieves three-stage charging and selective discharging of the battery.The experiment shows that the system can achieve the control of charging and discharging of the lead-acid battery and increase the utility ratio and the life time of the battery.To realize the safe use of the battery further, battery management system is conFigured to test a single cell so that the battery can be better maintained.In addition, the system control algorithm can also continue to be improved to make the system more stable.

ACKNOWLEDGMENT
This study is supported by the National High Technology Research and Development Program ("863"Program) of China with the project title of 'R&D on Key Technology of Micro-grid Including Distributed Power Supplies' (No. 2011AA05A107).We would also like to thank Mr. Teng Wen and Dr. Wang Yifeng for their help.Finally, we would like to thank the anonymous comments of reviewers which helped revise the study.

Fig. 2 ,
Buck circuit is composed of IGBT-V91, freewheeling diode D92 and inductance L62.Power supply in the DC bus-bar side is transported to the battery side through L62 on V91 conduction.DC output voltage is different according to the different duty cycle of V91; therefore the battery is charged by the output voltage which is controlled by V91.Meanwhile V92 and D91 are always stateoff.

Fig. 3 :
Fig. 3: Overall system design diagram of control circuit In Fig. 2, Boost circuit is composed of IGBT-V92, freewheeling diode D91 and inductance L62.L62 stores electric energy on V92 conduction.The positive voltage of L62 is in the left after the closing of V92.Energy is transported to the side of DC bus-bar after the voltage of L62 integrates with the battery voltage, so energy in the battery is released through BOOST circuit.Meanwhile V91 and V92 are always state-off(Jun et al., 2010;Ned et al., 1995).

Fig
Fig. 8: Subprogram of discharging Fig. 10: Current and voltage waveforms in the constant current stage

Fig. 13 :
Fig. 13: Zero-current detection wave of the inductor discharging mode.Discharging current is constant at 7A and the pulse duty cycle is 17.8%.We can find that there are current oscillations and voltage spikes when the IGBT works.The reason for the phenomenon is that the diode reverse recovery time is too long and makes the parasitic inductance and parasitic capacitance in the circuit resonant.In order to eliminate the current oscillation, we can make the circuit operate in critical conduction mode.Therefore, it is necessary to design a zero-current detection circuit.In this study, this circuit is designed and tested by experiments.The waveform is shown in Fig.13.When the inductive current reaches zero, the detection circuit outputs the falling edge and IGBT is controlled to open by DSP.The function of the critical conduction mode is to eliminate the reverse recovery of the freewheeling diode by turning on the active switch when the inductor current drops to zero.