ADVANCED HYBRID ENERGY STORAGE SYSTEM FOR MILD HYBRID ELECTRIC VEHICLES

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1 International Journal of Automotive Technology, Vol. 12, No. 1, pp (2011) DOI /s x Copyright 2011 KSAE /2011/ ADVANCED HYBRID ENERGY STORAGE SYSTEM FOR MILD HYBRID ELECTRIC VEHICLES D.-H. SHIN 1), B.-H. LEE 2), J.-B. JEONG 3), H.-S. SONG 4) and H.-J. KIM 5)* 1) Department of Mechatronics Engineering, Hanyang University, Gyeonggi , Korea 2) Department of Electronic, Electrical, Control and Instrumentation Engineering, Hanyang University, Gyeonggi , Korea 3) Korea Automotive Technology Institute, 74 Yongjeong-ri, Pungse-myeon, Cheonam-si, Chungnam , Korea 4) Department of Control and Instrumentation Engineering, Korea University, Seoul , Korea 5) School of Electrical Engineering and Computer Science, Hanyang University, Gyeonggi , Korea (Received 11 July 2008; Revised 8 June 2010) ABSTRACT Hybrid electric vehicles (HEV) utilize electric power and a mechanical engine for propulsion; therefore, the performance of HEVs is directly influenced by the characteristics of the energy storage system (ESS). The ESS for an\ HEV generally requires high power performance, long cycle life, reliability and cost effectiveness; thus, a hybrid energy storage system (HESS) that combines different types of storage devices has been considered to fulfill both performance and cost requirements. To improve the operating efficiency and cycle life of a HESS, an advanced dynamic control regime in which pertinent storage devices in the HESS can be selectively operated based on their status is presented. Verification tests were performed to confirm the degree of improvement in energy efficiency. In this paper, an advanced HESS with a battery management system (BMS) that includes an optimal switching control function based on the estimated state of charge (SOC) is presented and verified. KEY WORDS : Energy storage system, Hybrid energy storage system, Hybrid electric vehicle, Mild-HEV 1. INTRODUCTION Interest in environmentally friendly and highly efficient vehicles such as HEVs, plug-in HEVs and EVs has increased because of recent concerns with greenhouse gas reduction and rising oil prices. Since the HEV Toyota Prius was introduced in 1997, various types of HEVs such as mild, soft, hard and plug-in types have been developed and sold while meeting the regulations and/or the needs of customers (Lam and Loueya, 2006). In the case of mild-type HEVs with features such as idle stop and start, power boosting and regenerative braking (Stienecker et al., 2005), Toyota presented the first Crown 3.0 model to the Japanese market in 2002, and GM then released the Saturn Vue model to the US in 2006, which was followed by the Sierra and Silverado pickup model in Almost all of the market forecasts predict drastic and steady growth in the HEV market in the next decade. Effective HEV driving typically involves a proper combination of mechanical engine power and electric motor power to propel the vehicle; therefore, HEV driving efficiency with respect to fuel economy must be related to the electric system performance, which significantly depends on the characteristics of the energy storage system (ESS). *Corresponding author. hjkim@hanyang.ac.kr To improve the total system efficiency of a HEV, one of the essential considerations is to build an ESS that is capable of sufficiently supplying and receiving electric energy. In other words, the ESS should cope with high-power discharging and charging operations due to various driving conditions, such as starting, passing and regenerative braking. Although most of the HEVs currently available in the market are equipped with Ni-MH batteries, lead-acid batteries are attractive and are used in mild HEVs that have relatively restricted functionality and require consideration of cost effectiveness (Trinidad et al., 2003; Stienecker et al., 2006a). Lead-acid batteries exhibit both at a reasonable cost and a high reliability in automotive applications, but they generally exhibit some drawbacks in the cycle life, power characteristics, weight, etc (Henson, 2008). Numerous studies have been performed to improve their performance; furthermore, various aspects have been examined by researchers to provide alternatives to, rather than improvements in the battery (Shyai et al., 2007). One such consideration is to combine various types of energy storage devices to improve the cycle life, power characteristics and operating efficiency of an ESS by utilizing high-power storage devices such as ultracapacitors and high energy devices such as lead-acid batteries (Stienecker et al., 2006b; Cheng and Wismer, 2007; Lukic et al., 2006), i.e., a hybrid energy storage system (HESS), as shown in Figure 125

2 126 D.-H. Shin, B.-H. Lee, J.-B. JEOng, H.-S. Song and H.-J. Kim 1. Furthermore, further advancements can be achieved if the proper storage device is in harmony with the respective charging status and vehicle driving conditions, which is referred to as an advanced hybrid energy storage system (advanced HESS). In this paper, an advanced HESS with a storage device selecting function via BMS is presented, and experimental tests are presented to verify the degree of improvement in energy efficiency for mild HEV applications. 2. ADVANCED HESS 2.1. System Configuration Typical mild HEVs require an improved energy storage system because of their high current operation at a low operating voltage range. In particular, idle-stop-and-start functionality significantly increases the number of engine starts, which involves frequent high-current discharges (Karden et al., 2007). Often, the starting current reaches a 10 C to 15 C rate, which causes a decrease in the performance and cycle life. The regenerative braking function is another major factor that compromises performance because a high-charging current is applied to the energy storage device in a short time period as kinetic energy is transformed into electrical energy during braking. However, batteries that are used as energy storage devices for automotive applications require chemical reactions during energy input and output processes, which restricts their ability to charge or discharge high current in a short time period. As a means of counteracting their drawbacks and meeting requirements for ESSs in mild HEVs, the combination of a high-power storage device (ultracapacitor) with a high-energy storage device (battery) in parallel (Lajnef et al., 2007), as shown in Figure 1, was considered and is referred to as a hybrid energy storage system (HESS). Obviously, the cycle life and charge/discharge characteristics of a HESS are improved compared to a battery; however, there is still a potential for further improvement if the HESS can be controlled in accordance with the charging status of each storage device and predicted vehicle operating conditions, i.e., advanced HESS. Figure 2. Advanced HESS with dynamic control. The basic control strategy of an advanced HESS is to selectively connect the proper storage device(s) to the power-net based on the state-of-charge (SOC) information to achieve high efficiency and extended cycle life. This process is achieved by using an ultracapacitor under peak power operations, such as idle-stop-and-start mode, as well as regenerative braking and by combining an ultracapacitor with a battery under energy conservation conditions in which more electric energy can be supplied to the loads, especially during the period when the engine is off. The advantage of the advanced HESS is the minimal cost increase due to the addition of only a switch. Furthermore, the SOC of each storage device can be adjusted by considering the vehicle driving conditions, such as the vehicle speed, throttle position, engine speed, etc. Functions such as lowering the SOC of the ultracapacitor in advance to receive more recuperative energy if regenerative braking is expected and raising the SOC of the ultracapacitor if engine restarting or power boosting is predicted are suggested. To achieve these manipulating functions, an energy conversion device such as a DC-to-DC converter between the ultracapacitor and the battery can be implemented; however, this results in a relatively high additional cost. Dynamic control of the advanced HESS is performed by a battery management system (BMS) that estimates the Figure 1. HESS using a ultracapacitor and battery in parallel. Figure 3. Block diagram of the BMS controller.

3 ADVANCED HYBRID ENERGY STORAGE SYSTEM FOR MILD HYBRID ELECTRIC VEHICLES 127 SOC of the relative storage devices and controls the connecting/disconnecting switch and exchanges information with vehicle controllers via the controller area network (CAN). Furthermore, the BMS performs various functions, as shown in the block diagram in Figure Advanced HESS The advanced ultracapacitor-battery hybrid energy storage system consisted of an ultracapacitor module, a 36 V valveregulated lead-acid (VRLA) battery, a selective switch and a BMS, as shown in Figure 2. The battery was the same as the battery that was used in the Toyota Crown, which is representative of a mild-hybrid vehicle with a 42 V power-net. The battery is a valveregulated type and is made by GS-Yuasa. The electrical specifications of the battery are shown in Table 1. The ultracapacitor module consisted of 18 serial-connected 1,700F cells, which was supplied by NESSCAP, and the electrical characteristics are shown in Table 2. The switch that was used to connect or disconnect the battery to the power-net was a NAIS EB 100 relay, which was developed by Matsushita for a 42 V system. The rated current-carrying capacity of the relay is 100 A at DC 42 V with a contact resistance of 1.5 m. In the advanced HESS configuration, which is shown in Table V VRLA battery specification. Nominal Voltage Rated Capacity Energy Density Power Density (10 sec) 36 [V] 20 [Ah] 27.5 [Wh/kg] [W/kg] Table 2. Electrical Characteristics of the Ultracapacitor Module. Rated Voltage Rated Capacity DC ESR Available Energy 42 [V] 94.5 [F] 7.5 [m] 0.41 [Ah] Figure 5. Control algorithm of the advanced HESS. Figure 4, the ultracapacitor was always connected to the power-net as its primary source because of its superior power characteristics and efficiency. However, the VRLA battery was selectively connected via the switch under conditions that required high energy because it exhibits a relatively lower power density than the ultracapacitor. Because of its very low equivalent series resistance (ESR), the ultracapacitor is suitable for high-current operations under very high power conditions. Accordingly, the efficiency at high-power charge/discharge operations can be improved. The hybrid configuration is expected to improve the performance and lifetime by maximizing the use of the ultracapacitor. In the proposed advanced HESS, the BMS should have a significant role: properly controlling the switch for which the parallel connection will be configured based on the estimated SOC of each storage device to improve the efficiency and lifetime of the HESS. The BMS with the optimal switching control algorithm based on the estimated SOC was developed and applied in the verification tests. Figure 5 shows the algorithm that controls the switch according to the status of the energy storage devices estimated in the BMS of the advanced HESS. First, the BMS executes the mode (i.e., regenerative braking mode, idlestop-and-start mode, or regenerative mode) in accordance with the SOC of the storage devices, which consists of an ultracapacitor and a battery, or a control command from the VCU. Second, the SOC is compared to the control value (A-G value in Figure 5) that was previously determined. Finally, the switch is controlled or the SOC warning messages (full, empty, etc.) of each storage device are output based on logic in the BMS. 3. TEST RESULTS Figure 4. System test configuration of the advanced HESS Test Profiles The degree of improvement (with respect to the operating efficiency) of the advanced HESS containing the switch control function based on the estimated SOC was verified with a series of tests. First, an appropriate test profile was

4 128 D.-H. Shin, B.-H. Lee, J.-B. JEOng, H.-S. Song and H.-J. Kim Figure 6. ZPA efficiency test profile. Figure 7. Modified test profile. Figure 8. Cycle test results of the battery alone. defined to compare and contrast the operating efficiency. Initially, the FreedomCAR 42 V Battery Test Manual from INEEL and DOE (Barnes, 2003) was applied to the comparisons. The Zero-Power Assist (ZPA) Efficiency and Life Test Profile was chosen for the tests of both the battery and the ultracapacitor-battery parallel configuration. Figure 6 shows the ZPA cycle test profile simulating the driving pattern of a 42 V mild hybrid vehicle without regenerative function. However, some mild HEVs, such as the Toyota Crown, use a 3.5 kw belt-driven integrated starter-generator (ISG) to apply regenerative braking function with zero-power assist to increase vehicle efficiency such as fuel economy. Therefore, it is more reasonable to consider the regenerative braking condition in the ZPA test profile. Figure 7 shows the modified test profile that utilizes both idle stopstart and regenerative braking conditions, which simulates a charging and discharging pattern that represents mild HEV driving conditions. The modified test profile reflects a regenerative charging of kw for two seconds, which simulates the braking condition of a mild HEV that uses a 3.5 kw belt-driven ISG. To ensure its objectivity, this profile was discussed and approved by a representative Korean motor company that develops 42 V mild HEVs using HESSs. In the test profile shown in Figure 6 and Figure 7, the amount of charge/discharge energy was determined under the assumption that the charge/discharge efficiency of the energy storage system is 90% Results of the Cycle Tests To verify the degree of improvement of the advanced Figure 9. Cycle test results of the HESS. HESS for a 42 V mild hybrid vehicle with regenerative braking, cycle tests were conducted using the modified cycle profile shown in Figure 7. The efficiency tests for three types of storage configurations (battery only, ultracapacitor-battery parallel connec-

5 ADVANCED HYBRID ENERGY STORAGE SYSTEM FOR MILD HYBRID ELECTRIC VEHICLES 129 Figure 10. Cycle test results of the advanced HESS. tion and advanced HESS with BMS) were performed. Before the energy efficiency cycle tests were performed, the SOC for each storage system was set to 60% of its full capacity to account for the capacity to absorb recuperative energy while braking, and all of the tests were performed under a constant temperature condition of 25 o C. Initially, only the VRLA battery was tested. As shown in Figure 8, the instantaneous peak power due to engine restarting and regenerative braking causes high current discharging and charging, which results in low energy efficiency and battery degradation. From these results, the battery performance significantly declines as the modified cycle is repeated, which implies an acceleration of the drop in efficiency. Then, the HESS with the ultracapacitor-battery parallel connection without a switch was tested, as shown in Figure 9. The voltage drop is significantly reduced compared to the battery alone, which implies that the efficiency of the parallel configuration is higher than that of the battery alone. The charging and discharging current from the battery is reduced because the ultracapacitor has a lower internal resistance than the battery and can manage the instantaneous peak power burden. According to the test results, the peak discharge current decreased by about 47% (239 [A] 127 [A]). Therefore, the combination of the ultracapacitor with the battery resulted in a significant improvement in the efficiency and the battery cycle life. However, in the HESS, although the magnitude of the charging and discharging current of the battery was reduced, there is still a relatively high current that can diminish the energy efficiency and cycle life. In particular, the high recuperative charging current due to regenerative braking can cause a degradation of the battery, which significantly affects the efficiency and cycle life. A simple, parallel-connected HESS is not sufficient to manage the high current charging that is caused by regenerative braking; however, it results in some improvement in the efficiency and cycle life compared to the battery alone. Finally, the advanced HESS with the switching control shown in Figure 2 was tested, and Figure 10 shows the test results. The switch maintained a normally open condition as long as the SOC of the ultracapacitor was in the predefined range and if the battery could be disconnected from the power-net. The ultracapacitor absorbed the entire instantaneous peak charging current during braking and supplied the peak discharging current during engine restarts, which increased the efficiency because of the low ESR of the ultracapacitor. As shown in Figure 10(a), the peak charging current due to regenerative braking was eliminated, and the discharge current was also reduced. Because of the decreased peak current, the HESS performance should improve. Furthermore, the wider range of operating voltage variation shown in Figure 10(b) implies the effective utilization of the ultracapacitor, which results in an improvement in the efficiency of the HESS. In other words, as the frequency of usage of the high efficiency storage device (ultracapacitor) increases, the advanced HESS should be more efficient than the ultracapacitor-battery parallel configuration. The SOC can be measured and the changing ratio can be calculated from the results of the tests by using the follow- Table 3. Comparison of energy efficiency. Battery alone HESS Advanced HESS Fully Charged Capacity [Wh] (C Full, Estimated SOC 100%) Capacity before Cycle Test [Wh] (C SOC60, Estimated SOC 60%) Remaining Capacity [Wh] (C Remain, After Cycle Test) Changed Capacity [Wh] (C Remain - C SoC60 ) Capacity Change Ratio [%]

6 130 D.-H. Shin, B.-H. Lee, J.-B. JEOng, H.-S. Song and H.-J. Kim ing process: Step 1: Measure the full capacity of each storage system, and repeat this measurement two more times. Calculate the mean value. (= Fully charged capacity C Full ) Step 2: Set the SOC to the predefined value (60%) by constant current discharging at a rate of 1/5 C 5 for 2 hours. (= Capacity before cycle test C SOC60 ) Step 3: Perform the tests with the modified profile for 100 cycles. Step 4: Measure the remaining capacity of the storage by discharging to the cut-off voltage. (= Remaining capacity C Remain ) Step 5: Calculate the capacity change ratio of 100 cycles using Equation (1). C Capacity Change [%]= Remain C SoC (1) Table 3 shows a summary of the amount of capacity change during the cycle tests for three different types of energy storage systems: battery only, ultracapacitor-battery in parallel (HESS) and advanced HESS. According to the test results, the battery alone exhibits a capacity decrease of approximately 33%, whereas the HESS decreases less than 5%. Furthermore, the advanced HESS with a selective connecting function exhibits a capacity increase of 5.5% because the appropriate switching scheme encourages the usage of the highly efficient ultracapacitor. Therefore, by using only a switch, the advanced HESS can improve the energy efficiency by approximately 10.5% compared to the ultracapacitor-battery parallel connection, and this improvement can be further demonstrated by increasing the number of cycles as in a lifetime test. 4. CONCLUSION C Full The advanced hybrid energy storage system with a switch and a BMS was developed to improve energy efficiency and cycle life, and the system was tested to verify the degree of improvement in energy efficiency. To perform reliable verification tests, a modified cycle profile was developed based on the ZPA Efficiency and Life Test Profile from INEEL and DOE, and to ensure its objectivity, this profile was discussed and approved by a representative Korean motor company that develops a 42 V mild HEV using HESSs. Then, the energy efficiency tests were performed using three different types of ESSs: battery only, ultracapacitor-battery parallel connection and advanced HESS. The test results indicate that the advanced HESS achieves energy efficiency improvement by adopting storage selection functions that are governed by the BMS, based on the SOC estimation. The results indicate that appropriate storage combinations improve the efficiency of energy storage systems, and a proper switch control algorithm that considers the SOC of the respective storage devices and driving information is essential. REFERENCES Barnes, J. (2003). FreedomCAR 42V Battery Test Manual. INEEL and DOE/ID April. Cheng, D. L. and Wismer, M. G. (2007). Active control of power sharing in a battery/ultracapacitor hybrid source. 2nd IEEE ICIEA 23-25, Henson, W. (2008). Optimal battery/ultracapacitor storage combination. J. Power Sources, 179, Karden, E., Ploumen, S., Fricke, B. Miller, T. and Snyder, K. (2007). Energy storage devices for future hybrid electric vehicles. J. Power Sources, 168, Lajnef, W., Vinassa, J. M., Briat, O., Azzopardi, S. and Woirgard, E. (2007). Characterization methods and modelling of ultracapacitors for use as peak power sources. J. Power Sources, 168, Lam, L. T. and Loueya, R. (2006). Development of ultrabattery for hybrid-electric vehicle applications. J. Power Sources, 158, Lukic, S. M., Wirasingha, S. G., Rodriguez, F., Jian, C. and Emadi, A. (2006). Power management of an ultracapacitor/battery hybrid energy storage system in an HEV. IEEE VPPC 6 8, 1 6. Shuai, L., Gorzine, K. A. and Ferdowsi, M. (2007). A new battery/ultracapacitor energy storage system design and its motor drive integration for hybrid electric vehicles. IEEE Trans. Vehicular Technology 56, 4, Part 1, Stienecker, A. W., Flute, M. A. and Stuart, T. A. (2006a). Improved battery charging in an ultracapacitor-lead acid battery hybrid energy storage system for mild hybrid electric vehicles. SAE Paper No Stienecker, A. W., Stuart, T. and Ashtiani, C. (2005). A combined ultracapacitor-lead acid battery storage system for mild hybrid electric vehicles. IEEE VPPC 7 9, 6. Stienecker, A. W., Stuart, T. and Ashtiani, C. (2006b). An ultracapacitor circuit for reducing sulfation in lead acid batteries for mild hybrid electric vehicles. J. Power Sources, 156, Trinidad, F., Gimeno, C., Gutierrez, J., Ruiz, R., Sainz, J. and Valenciano, J. (2003). The VRLA modular wound design for 42 V mild hybrid systems. J. Power Sources, 116,