DESIGN OF HIGH ENERGY LITHIUM-ION BATTERY CHARGER

Transcription

1 Australasian Universities Power Engineering Conference (AUPEC 2004) September 2004, Brisbane, Australia DESIGN OF HIGH ENERGY LITHIUM-ION BATTERY CHARGER M.F.M. Elias*, A.K. Arof**, K.M. Nor* *Department of Electrical Engineering, Faculty of Engineering University of Malaya **Department of Physics, Faculty of Science University of Malaya Abstract This paper presents the design of microcontroller-based battery charger to charge a high energy Li-ion battery pack. The charging method, balancing technique, charging control algorithm, battery protection, battery management unit and implementation of the battery charger are discussed concisely. Advantages and disadvantages of the design are also discussed as well as problems arisen. 1. INTRODUCTION Lithium-ion battery is a rechargeable battery that has the highest energy density, lightweight, small in size and long shelf life. However, Li-ion battery charging is slightly more complicated as several factors must be considered especially when the battery pack consists of series connected Li-ion battery cells. Moreover, the battery cannot be over-charged due to its chemistry limitations. Therefore, exceptional control unit is required in controlling both charging and discharging process in order to ensure that Li-ion battery is extended life. In this paper, the Li-ion battery charger is designed for high energy applications such for storage system in linear generator system, backup power system, electric and electric hybrid vehicle. The battery charger is designed to charge 42V, 60Ah Li-ion battery pack in which the battery pack comprises 100 cells of 4.2V, 6Ah Li-ion battery connected in both series and parallel. 2. DESIGN CONCEPTS 2.1 Charging Method The charging method chosen is constant current/constant voltage (CC/CV) method. The CC/CV is the best method in charging the Li-ion battery because it offers the fastest charging time to fully charge the battery. However, it is more complicated to be implemented. Charging starts with constant current (CC) mode until the battery has reached the maximum voltage of 4.2V. In this mode, train of short current pulses is applied periodically. Between current pulses, the voltage across the battery terminal is monitored to prevent overcharging and the CC mode is then switched into CV mode once the maximum voltage is reached. At this stage, the battery is about 85% of its full capacity [1]. Meanwhile, in CV mode, charging current is monitored to determine when the charging process can be terminated. Normally, the battery is considered to be fully charged when the charging current drops below 0.1C. In this design, the battery is charged at 0.5C. Figure 1 shows the typical charge/discharge profile of the Li-ion battery. I max Current 0 A CC Charging Mode Current, Voltage vs. Time Voltage Current CV Charging Mode Time CC Discharging Mode V max V min Fig. 1. Typical Li-ion Charge/Discharge Profile 2.2 Cell Balancing Cell balancing is a technique to balance battery cells and it is very important when charging a battery pack, which consists of a number of cells in a series string. Unlike lead acid and NiMH battery in which they can be naturally balanced through gassing, Li-ion battery Voltage

2 requires special circuit to balance all the cells [3]. In fact, each Li-ion battery has different internal impedance, thus leads to the unbalance of cells during charging. Several factors which may also lead to unbalanced cells such as variations in cell assembly, different charge acceptance levels, different charging rates and number of discharge available, temperature gradient across battery pack and power dissipation distribution in the system [4]. Unbalance causes unequal individual voltages when series connected battery is charged as one or more cells will reach the maximum level before the rest. In order to balance battery cells, there are several cell balancing methods with diversity in charging control algorithm which can be applied. In fact, unbalanced voltage cells can reduce the entire battery pack capacity up to 25% (typical) of the battery capacity with 150mV difference between cells at full charge condition. This causes the available battery capacity to be reduced, since the weakest cell in the string determines its effective capacity. Therefore, battery cells voltage is maintained to be equal or within acceptable difference during charging. The battery cells are considered balanced when the capacity range of each cell is within 3% [2]. I S1 B3 Bn R1 S2 R2 S3 R3 Sn Rn Fig. 2. Charge Shunting/Dissipative R i There are several methods to perform cell balancing on Li-ion battery. They are charge shuttling, charge shunting, dissipative resistor as well as energy converter. In this design, charge shunting is chosen as the method of balancing. Charge shunting is an end-of-charge cell balancing method. The charge shunting cell balancing method selectively shunts the charging current around each cell as they become fully charged. Figure 2 shows charge shunting cell balancing method [3]. In this method, charging current is shunted across the unbalanced cell so that it is charged at lower rate. This method is highly efficient but requires large power dissipating resistor and also high current switches as compared to other methods. 2.3 Charging Algorithm Charging control algorithm is a set of rules followed by the microcontroller in charging the Li-ion battery. There are several charging control algorithm which can be used and it is depends on the cell balancing method chosen. Recommended cell balancing algorithm for charge shunting method are as follows [4]: 1) Identify the unbalanced cells. 2) Enable cell balancing action during charge. 3) Stop charge periodically and measure cells 4) If cells are balanced, disconnect the balancing action and continue to charge to full capacity level. 5) If cells are not balanced, continue to charge with balancing action active for another period. 6) Measure cell every period. Continue to balance if cells are not matched; stop balancing if cells are matched. 7) If cells are not balanced after full charge, continue with balancing action during discharge. 8) Cells become unbalanced more often when the charge time is rapid. 9) Balancing action improves when cells are charged slower. 10) Avoid as much as possible rapid charges and match the balancing action to the charge period while allowing balancing time to take place. Fully charged battery with proper cell balancing algorithm should have more than 90% of its full capacity. 2.4 Protection Protection circuit is very important for battery charger especially for Li-ion or other Lithium-based battery charger. It ensures that the performance of the battery is preserved effectively and protects the battery from overcurrent, over-voltage, under-voltage and also overtemperature. Short-circuit for instance can damage the battery and therefore requires current sensing circuit to monitor the charging current. Once over-current is detected, charging must be stopped immediately, otherwise this will cause damage to both battery and the charger itself. As discussed earlier, over-voltage must be avoided, thus

3 requires precise voltage monitoring circuit to provide the cells voltage status to the microcontroller. Even though in a short run, the voltage of the battery increase as the battery is being charged over the maximum voltage, but in a long run, this can significantly cause reduction in the battery capacity [1]. Under-voltage can also significantly reduce the battery capacity as the battery is allowed to operate under the minimum voltage level of the battery. In fact, certain Li-ion battery has certain operating voltage range; 3.5V to 4.2V for instance. Therefore, the battery must be disconnected from the load and the charger must recharge the battery when the battery has reached the lower voltage limit. Another factor, which is over-temperature, must also be taken into account. Therefore, the battery is ensured to operate within specific temperature range for safety reason. Operating the battery lower than the temperature limit can cause reduction in battery capacity whereas operating the battery higher than the temperature limit increase the internal discharge and cause the battery to discharge at faster rate [4]. In fact, large temperature gradients also contribute to the cell charge mismatch during charging, hence requires temperature monitoring circuit for thermal management. 2.5 Management Unit The purpose of the battery management unit is to control the whole operation of the battery charger. management unit consists of a microcontroller and many sensors. Microcontroller contains charging algorithm, which enables it to determine when to start and stop the charging process. Sensors provide the status of the battery and send it to the microcontroller to be processed in order to charge the battery at the most optimum level. The unit will respond whenever there is any fault occurs and gives an indication of the type of fault [5]. Generally, battery management unit acts as the brain of the battery charger and determines efficiency and performance of the whole system. 2.6 Integration with Linear Generator Specifically, the battery charger is designed to charge Li-ion battery as it is used in linear generator storage system. Since the output of the linear generator is unregulated AC output, therefore it needs to be conditioned first via power conditioning devices, which consist of advanced converter. The converter output, which is regulated DC output, is then stepped down and used to power the battery charger. One of the main functions of the battery is to start the linear generator, in which the battery will draw a high current from all of the battery modules. On the other hand, it can be used as backup power for DC load and also AC load. Once the linear generator is running, it will recharge the battery to the maximum level. charger together with battery management unit should manage to ensure that the battery is always in fully charged condition and ready to be used whenever needed. LINEAR ENGINE LINEAR GENERATOR Fig. 3. Block Diagram of Integrated Linear Generator System 3. BATTERY CHARGER IMPLEMENTATION In order to charge 100 battery cells, it is divided into 10 battery modules that are connected in parallel where 1 module comprises 10 battery cells in series. At any time, only one module is charged which requires total charging current of 3A. Once completed, it will sequentially charge the rest of the battery pack. Figure 4 shows block diagram to charge a battery pack, in which the switching control is required to select a battery module. RECTIFIER 1 CHARGER 2 CC/CV Source BATTERY INVERTER 3 Fig. 4. Charging a Pack DC LOAD AC LOAD 10 In this design, there is only one cell balancing circuit required because of at any time only one battery module

4 can be charged. Therefore, cell balancing circuit will be switched to the corresponding battery module that will be charged. The balancing circuit is designed to shunt almost all charging current to balance cells faster. Thus, the value of shunting resistor must be selected properly and it should be noted that the resistor value must not too small as this will discharge the battery cells even under charging process. In this design, the value of shunting resistor is chosen so that the balancing action can be performed in both charging and discharging. The reason is that, if the battery cells are still unbalanced at the end of charging it could be done by discharging the unbalanced cells only. Balancing action is activated when the microcontroller senses voltage difference between one cell and the others and it is stopped when the voltage equal to the lowest cell voltage. Figure 5 shows charge shunting cell balancing circuit diagram where power mosfet if driven by photovoltaic isolator. the battery module that will be charged. The voltage is measured using unity gain differential amplifier. Figure 6 shows the voltage monitoring circuit diagram. It is noted that measuring the battery cells voltage during charging does not give the actual voltage of the battery due to its effective serial resistance (ESR) and effective serial inductance (ESL). Therefore, the battery cells voltage are measured between current pulses in CC charging mode, whereas in CV charging mode, the charging will be stopped periodically in order to measure the cells voltage. The average value of the ESR for Li-ion battery is small and lies between milliohms whereas the value of inductance is in nanohenries [1]. The smaller value of ESR and ESL, the smaller power dissipated when the battery discharged, and the better the battery is. CC/CV Source CC/CV Source Microcontroller Gate Driver ADC Gate Driver Analog MUX Gate Driver Fig. 5. Charge Shunting Cell Balancing Circuit Diagram The charger has voltage, current and temperature monitoring circuit in order to monitor the battery status as well as charging and discharging process. This information will be sent to the microcontroller for data processing before control action can be executed. In this design, there is also one voltage monitoring circuit used in order to reduce the complexity of the circuit. The circuit shares the same switching circuit as the cell balancing circuit so that it will always be connected to Fig. 6. Voltage Monitoring Circuit Diagram for One Pack Current sensing circuit is designed to give precise charging current especially in order to determine when the charging should be stopped. The design uses current sense chip with linear relationship between charging current and corresponding output voltage, in which the output is then converted into digital data as the output of the voltage monitoring circuit.

5 Temperature monitoring circuit consists of thermistors, which are attached at every single cell where the output voltage is also converted into digital form before it can be sent to the microcontroller. In determining the best charging control algorithm, several experiments are carried out to obtain the actual characteristics of the Li-ion battery under charging and discharging. From the results, the frequency and duty cycle of charging current pulses that will give the most effective result for cell balancing action could be determined. On the other hand, the delay time required for the battery to become stable before its voltage can be measured could also be determined. These factors are very important to design a fast battery charger while keeping all the cells balanced until the end of charging. 4. DESIGN ISSUES Efficiency in cell balancing action is very important issue where all the cells voltage must be highly balanced at the end of CC mode before CV mode is started. Charging at higher rate in CC will not significantly reduced the overall time, but it will cause the battery cells to become unbalanced faster. The charging process also can be optimized by varying the current pulse duration in both charging mode instead of fixed pulse duration. Fig. 7. Development of PIC MCU Program in MPLAB 6.5 Software Using PICBasic Pro Compiler Pulse duration can be made longer in the beginning so that charging becomes faster by reducing the frequency to monitor the cells voltage. Whereas, at the end current pulse duration can be made shorter to monitor cells voltage frequently in order to ensure that all cells are highly balanced. As discussed earlier, temperature gradient may raise safety concern when temperature of the battery increases over limit while charging. Therefore, when the temperature exceeds certain pre-assigned value, the battery can be charged at lower rate to reduce the temperature of the battery. This will require variable current source to charge battery with different charging current, in which depends on the battery condition. Another issue is that the volume and footprint of the charger system is bulky. This is mainly due to the large number of component being used. Currently available single chip solution can only supports typically up to 4 numbers of cells in a string, whereas Li-ion battery charger in the design can charge battery, which consists of 10 numbers of cells in a series string. Fig. 8. Specially Developed Program in Microsoft Visual Basic 6 To Monitor Charging Normally, there are many more issues involved in the design because of many factors need to be considered. Charging method, balancing technique, charging algorithm, battery protection, battery management and

6 AC/DC Converter (Rectifier) Current/Voltage Converter Constant Current (CC) Source Sw itch Short Circuit Protection Safety Fuse Lithium-Ion Pack Optimization Cell Balancing Circuit Unregulated AC output (Linear Generator) Constant Voltage (CV) Source Timer A/D Converter Voltage/ Temperature Sensor Over Voltage/ Under Voltage/ Over Temperature Monitor Microcontroller Fig. 9. Complete Block Diagram of Li-ion Charger implementation are the key factor that determine the performance and efficiency of the Li-ion battery charger. Figure 9 shows the complete block diagram of Li-ion battery charger. 5. CONCLUSION In charging the Li-ion battery, many factors must be considered to ensure that the battery capacity and its cycle life are preserved. The charger must be highly efficient and reliable as it affects the performance of the battery. 6. REFERENCES [1] K.K. Vijeh, Current, voltage and temperature govern Li-Ion battery charging, Online Source, National Semiconductor Corp., May 28, Available: [2] Y. Drory, C. Martinez, The Benefits of Cell Balancing, AN141, Xicor Incorporated. Available: [3] S.W. Moore, P.J. Schneider, A Review of Cell Equalization Methods for Lithium Ion and Lithium Polymer Systems, Society of Automotive Engineers, Inc., [4] Maximizing Life It Takes More than a Gas Gauge, Seminar Notes, Xicor Incorporated. Available: [5] I. A. Khan, Chargers for Electric and Hybrid Vehiches, Power Electronics in Transportation, Proceedings, Oct Page(s): [6] V.L. Teofilo, L.V. Merritt, R.P. Hollandsworth, Advanced Lithium Ion Charger. IEEE AES Magazine, November [7] Intelligent Charger Reference Design, Application Note PICREF-2 DS30451C, Microchip Technology Inc., [8] R. DelRossi, Cell Balancing Design Guidelines, AN231 DS00231A, Microchip Technology Inc., 2002.