A New Battery/Ultra-Capacitor Hybrid Energy Storage System For Electric .
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.A New Battery/Ultra-Capacitor Hybrid EnergyStorage System for Electric, Hybrid and Plug-inHybrid Electric VehiclesJian Cao, Member, IEEE, and Ali Emadi*, Senior Member, IEEEElectric Power and Power Electronics Center and Grainger LaboratoriesElectrical and Computer Engineering DepartmentIllinois Institute of TechnologyChicago, IL 60616, USAE-mail: jcao10@iit.edu; emadi@iit.eduURL: http://hybrid.iit.eduAbstract—In this paper, a new battery/ultra-capacitor hybrid energystorage system (HESS) is proposed for electric drive vehicles includingelectric, hybrid electric, and plug-in hybrid electric vehicles. Compared tothe conventional HESS design, which uses a larger DC/DC converter tointerface between the ultra-capacitor and the battery/DC link to satisfythe real time peak power demands, the proposed design uses a muchsmaller DC/DC converter working as a controlled energy pump tomaintain the voltage of the ultra-capacitor at a value higher than thebattery voltage for the most city driving conditions. The battery will onlyprovide power directly when the ultra-capacitor voltage drops below thebattery voltage. Therefore, a relatively constant load profile is created forthe battery. In addition, the battery is not used to directly harvest energyfrom the regenerative braking; thus, the battery is isolated from frequentcharges, which will increase the life of the battery. Simulation andexperimental results are presented to verify the proposed system.Keywords—Battery, control, DC/DC converters, electric vehicles,energy storage, hybrid electric vehicles, plug-in vehicles, power electronics,propulsion systems, ultra-capacitor.I.INTRODUCTIONEnergy storage systems (ESS) are of critical importance inelectric, hybrid electric, and plug-in hybrid electric vehicles(EVs, HEVs, and PHEVs) [1]-[9]. Of all the energy storagedevices, batteries are one of the most widely used. However, abattery based ESS has several challenges providing the impetusto look for additional solutions [1]-[5]. In battery-based energystorage systems, power density of the battery needs to be highenough to meet the peak power demand. Although batterieswith higher power densities are available, they are typicallypriced much higher than their lower power density counterparts.A typical solution to this problem is to increase the size of thebattery. However, this also causes an increase in cost. Inaddition, thermal management is a challenge for batteries tosafely work in high power load conditions not only to cooldown the battery, but also to warm up the battery in coldtemperatures in order to reach the desired power limits. Inaddition, an issue concerning the life of the battery is thebalancing of the cells in a battery system. Without thebalancing system, the individual cell voltages tend to drift apartover time. The capacity of the total pack then decreases rapidlyduring operation, which might result in the failure of the totalbattery system. This condition is especially severe when thebattery is used to do high rate charge and discharge [6], [7]. Inaddition to these issues, applications that require instantaneouspower input and output typically find batteries suffering fromfrequent charge and discharge operations, which have anadverse effect on battery life [6], [7]. For such systems, it iscrucial to have an additional energy storage system or a bufferwhich is much more robust in handling surge current.In order to solve the problems listed above, hybrid energystorage systems (HESS) have been proposed [3]-[5], [10]-[15].The basic idea of a HESS is to combine ultra-capacitors (UC)and batteries to achieve a better overall performance. This isbecause, compared to batteries, ultra-capacitors have a highpower density, but a lower energy density. This combinationinherently offers better performance in comparison to the useof either of them alone. Several configurations for HESSdesigns have been proposed, which range from simple tocomplex circuits. Based on the use of power electronicconverters in the configurations, HESS can be classified intotwo types of passive or active. Conventional active methodsuse one or multiple full size DC/DC converters to interface theenergy storage device to the DC-link. In this case, full sizerefers to the fact that the DC/DC converter forms the sole pathfor the flow of energy in the device.In the most widely used conventional HESS designs, thebattery pack is directly connected to the DC link while a halfbridge converter is placed between the UC bank and the DClink. However, in order to utilize the power density advantageof the UC, the half bridge converter must match the powerlevel of the UC. In most cases, the half bridge converter is asignificant portion of the cost. Although this design solves theproblem of the peak power demands, the battery still suffersfrom frequent charge and discharge operations. To solve allthese problems mentioned above, a new HESS is proposed inthis paper.Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.The proposed HESS will be presented and verified in detailin this paper. The paper is organized as follows. Section II is ageneral introduction to HESS. Section III presents designconsiderations for different HESS configurations. Section IVdiscusses the topology and operating modes of the proposedHESS. Section V focuses on the case study and PSATsimulation results. A comparative analysis is presented inSection VI. Experimental verification is presented in SectionVII followed by the conclusion, which is in Section VIII.Passive paralleling as discussed in [4] and [10] is thesimplest method of combining battery and UC bank togetherbecause the two energy sources are hybridized without anypower electronic converters/inverters. Figure 1 shows the basictopology of the passive parallel method. In this method, sincethe two sources are always paralleled, VBatt VUC VDC . TheUC essentially acts as a low pass filter.II. HYBRID ENERGY STORAGE SYSTEMSBoth batteries and UCs fall under the category ofelectrochemical devices. However operating principles of boththese devices are different which make their characteristicshighly different [3], [4]. Table I lists some of the keycharacteristics for different battery types while table II showsthe same for UCs. As the tables show, batteries have arelatively high energy density of 30-200Wh/Kg, which varywith chemistry and power density. On the other hand UC has amuch lower energy density and significantly higher powerdensity. At the same time, the life of the UC is over one millioncycles, which is much higher than that of batteries. Also UCshave superior low temperature performance compared tobatteries. These characteristics allow for an optimalcombination in order to achieve an improved overallperformance.TABLE ITYPICAL CHARACTERISTIC OF BATTERY CELLSChemistryNominal CellVoltage(Volt)Lead Acid2EnergyDensityPowerDensityCycle life(Wh/Kg)(kW/kg)(Times)30-400.18Up to 800Up to 1,000Ni-Mh1.255-800.4-1.2Li-Ion3.680-1700.8-2Up to1,200Li-Polymer3.7130/2001-2.8Up to 1,000Li-IronPhosphate3.2/3.380-1151.3-3.5Up to 2,000Figure 1. Basic passive parallel hybrid configuration.Advantages of this method include ease of implementation,no requirements for control or expensive power electronicconverters. The major problem with this topology is that itcannot effectively utilize the UC stored energy. This will befurther discussed in the HESS design considerations section.B. Ultra-Capacitor/Battery ConfigurationThe ultra-capacitor/battery configuration [12] is the moststudied and researched HESS. Figure 2 shows the diagram ofthe HESS configuration. By using a bi-directional DC/DCconverter to interface the UC, the voltage of UC can be used ina wide range. However, the bi-directional converter needs to beof a larger size in order to handle the power of the UC. Inaddition, the nominal voltage of the UC bank can be lower. Thebattery is connected directly to the DC link; as a result, the DClink voltage cannot be varied.TABLE IITYPICAL CHARACTERISTIC OF ULTRA-CAPACITOR CELLSNominal kW/kg)Cycle Figure 2. Ultra-capacitor/battery configuration.The topologies of HESS have been studied over the pastyears. Here a review of the most widely used HESS topologiesis given.A. Basic Passive ParallelC. Battery/Ultra-Capacitor ConfigurationBy swapping the position of the battery and UC in theUC/battery configuration, we get the battery/UC configuration[10], [13] as shown in figure 3. In this configuration the voltageof the battery can be maintained lower or higher than the UCvoltage. The UC is connected to the DC link directly workingCopyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.as a low pass filter. If the control strategy applied to thistopology allows the DC link voltage to vary within a range sothat the UC energy can be more effectively used.F. Multiple Input Converter ConfigurationAs we discussed in the previous sub section, the cost ofmultiple converter configuration is expensive because itrequires two full size bidirectional converters to interface bothbattery and UC. Multiple input converter topologies [15] areproposed in order to reduce the cost of the overall system. Thesystem diagram of the multiple input converters method isshown in figure 6.Figure 3. Battery/ultra-capacitor configuration.D. Cascaded ConfigurationTo make a better working range of the UC of thebattery/UC configuration, another bi-directional DC/DCconverter was added between the UC bank and the DC link.This forms a cascaded converter topology as can be seen inFigure 4 [4].Figure 6. Multiple input converter configuration.III. HESS DESIGN CONSIDERATIONSWhile many design considerations have been addressed byresearchers, most discussions focus on the specific topologyused, with not much detail from the system perspective. Thissection discusses the basic design considerations that should beconsidered in the development of battery/UC HESS topologies.A. Voltage Strategy of the Two Energy SourcesFigure 4. Cascaded configuration.E. Multiple Converter ConfigurationInstead of cascaded connection of the two converters, themultiple converter method [15] parallels the output of the twoconverters. Figure 5 shows the diagram of the multipleconverter topology. The outputs of the two converters are thesame as the DC link voltage. The voltage of both the batteryand the UC can be maintained lower than the DC link voltage,less balancing problem so as incurred. The voltage of the UCcan vary in a wide range so the capacitor is fully used. Thedisadvantage of this method is that two full size converters arenecessary.In designing a battery/UC HESS, the selection of thevoltage strategy is strongly related to the characteristics of thebattery and UCs used [4], [8]. Higher voltage capacity for theenergy storage device presents a higher demand for the cellbalancing circuit. This is because cell imbalances growexponentially with the number of cells in series [6]. Oneapproach to reduce balancing needs is to use cells with lowerperformance variations (capacity, internal resistance, and selfdischarge rate). However, matched performance is essentiallyreached by cycling a big batch of cells and finding similar cellsthat can be grouped together. A better matched performancetypically indicates the need for a bigger batch of cells to selectfrom. This will result in an added cost to the total battery pack.Therefore, depending on the characteristics of the battery andUC cells, a voltage tradeoff between the storage elements needsto be made. It must be noted that in most cases, UCs are easierto balance with lower additional cost.Topology of a HESS depends significantly on the voltagestrategy selected [4], [8]. In the following discussion, VUC ,VBatt , and VDC are referring to the voltage of the UC bank,voltage of the battery pack, and voltage of DC link,respectively. If ( VUC VBatt VDC ), it indicates that a batteryFigure 5. Multiple converter configuration.pack is connected directly to the DC link and a UC connectedto the DC link through a bidirectional DC/DC converter. In thisCopyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.case, the power rating of the DC/DC converter needs to matchthat of UC in order to fully utilize the higher power capabilityof the UC. The advantage of this voltage strategy is the abilityto use the entire range of the UC where a lower voltage UCbank is needed. If ( VBatt VUC VDC ), it refers to a switch inpositions between the battery and the UC with reference to theprevious method. The UC bank is now connected to the DClink directly, while the battery is connected to the DC linkthrough a DC/DC converter. With this topology, the voltage ofthe battery can be maintained at a lower magnitude so that lessbalancing issues need to be addressed. If VBatt VUC VDC , itmeans the battery and the UC are directly paralleled andconnected to the DC link. The most significant advantage ofthis topology is that no DC/DC converter is needed. However,the working range of the UC is very small. If VBatt VUC VDC (not necessarily unequal), then both the battery and UCare connected to the DC link through power electronicconverters or other mechanisms.B. Effective Utilization of Ultra-Capacitor Stored EnergyWhile energy delivery in a battery is not a function ofvoltage, energy storage in an UC obeys the law of storage in astandard capacitor as shown in equation (1).1ECap CV 22(1)Voltage of the UC needs to discharge to half of the initialvoltage in order to deliver 75% of the energy stored. The abilityto use the UC energy storage effectively is a major criterion inevaluating HESS configurations. If the UC is connected to theDC bus via a DC/DC converter ( VUC VBatt VDC ), 100% ofthe energy can be delivered theoretically. However, a safetymargin is allowed in order to prevent a reverse charge ofunbalanced cells. 90% of the UC energy can be delivered whena voltage variation of 66% is permitted. If the battery and UCare paralleled passively, the voltage of the UC cannot change alot. Even in an aggressive discharge (within the battery powerlimits), the voltage of the battery pack can drop only up to 20%of the nominal voltage. Assuming that the UC is designed tocover the nominal voltage, VMax VNom , the total energy canbe delivered by the UC is:EffCap 112222(2)EUtilized 2 C VNom 2 C VMin VNom VMin 36%21ETotalVNom2C VNom2The actual energy available is less than 36% because amargin needs to be allowed for the UC to cover higher voltageof the battery pack during charging or regenerative braking.C. Protection of the Battery from Over CurrentAn important design concern of a battery/UC HESS is tofully utilize the significantly higher power limits of the ultracapacitor to support acceleration and fully recover energythrough regenerative braking. Unlike applications such aslaptops which draw a relatively constant and predictablecurrent from the battery, energy storage systems in automotivepropulsion applications undergo frequent charge and dischargecycles. These frequent charges are typically current surgescaused by unpredictable regenerative braking. If this surge isinjected directly into a battery without regulating, the batterycould die very quickly. This is especially true for lithium-ionbatteries [3], [6], [7]. The common engineering solution for in abattery ESS to this problem is to provide charging anddischarging power limits to the controller (usually a lookuptable with state of charge and battery temperature as inputs).This allows the hybrid system optimizer to follow power limitsin order to protect the battery. The discharging power limitensures that no additional power is drawn from the batteryduring aggressive acceleration while the charging power limitsforce the hybrid controller to activate mechanical brake earlyin order to absorb the portion of extra energy that cannot betaken by the battery. This process is a trade-off where energy isexpended for the security of the battery pack. In a Battery/UCHESS system design, it is important to utilize the much higherpower limit of the UC to not only protect the battery but alsoincrease the overall performance of the electric drive system.D. HESS Total CostCompared to a conventional battery ESS, two majorcomponents are added to a battery/UC HESS: UC and theDC/DC converter (if employed). UC technology has madegreat strides in increasing energy density. However, UC cost isstill a major component of the overall HESS system cost.Power handling capacity of the converter is anotherimportant factor that influences cost of the HESS. If a higherpower DC/DC converter is needed, a practical issue is theadditional cost. Special thermal management is also required [9]which adds complexity and increases overall cost of the system.However, a tradeoff still exists between energy stored in theUC bank and cost associated with an effective HESS solution.IV. THE PROPOSED HESSA. The Averaging ConceptConventional HESS connects the UC via a DC/DCconverter to satisfy the real time peak power demands of thepowertrain controller. This will require the DC/DC converter tohave the same power capability as the UC bank or at leasthigher than the maximum possible demand value. Theproposed HESS achieves this in a different way, which can beconsidered an application of the averaging concept. Theaveraging concept is introduced as follows.Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.Figure 7 shows the battery pack power of a mid-sizeelectric vehicle simulated in PSAT with the United StatesEnvironmental Protection Agency (EPA) Urban DynamometerDriving Schedule (UDDS). The UDDS is a driving cyclestandard which is designed to simulate city driving in theUnited States. The cycle simulates an urban route of 12.07kilometers (7.5 miles) with frequent stops. The maximumspeed is 91.2 km/h (56.7 mi/h) and the average speed is 31.5km/h (19.6 mi/h). The duration of the cycle is 1369.00 Seconds.try to maintain the voltage of the UC higher than that of thebattery. Therefore in most cases, the diode is reverse biased.Diode or Controlled ery DCLinkHigherVoltageUCInverterMotor-Figure 8. Proposed HESS configuration.Typically, the high voltage DC link is allowed to vary in a2:1 ratio. This results in a 50% of voltage discharge ratio of theUC. The voltage discharge ratio of the UC is defined as thefinal voltage over the initial voltage. There is no strict rulewhen determining the UC discharge ratio. Typically, a 50%ratio results in 75% of the stored energy being utilized. Thereason to select a 50% discharge ratio is that in a constantpower application, the decrease of VUC is in an exponentialFigure 7. UDDS simulation results for a mid-size EV (2003Honda Accord EV) with battery only ESS.As can be seen from Figure 7, the start stop nature of citydriving will result in frequent charges and discharges of thebattery at high power, which ranges from -36.8kW to 54.8kWaccording to the simulation data. However, based oncalculations, the average moving power of the battery pack isonly 5.95kW, which is about 1/10 of the peak power.The significant difference between the peak and averagepower suggests the following: ideally, if an energy storagedevice which is good at handling power is employed to work asa buffer, we only need a DC/DC converter to feed 5.95kWconstant power to charge the buffer energy storage device(BESD). In this case, the DC/DC converter size is minimizedwith the cost of an increased BESD size. However, for aspecific vehicle application, the averaging nature allows theDC/DC converter and the BESD size to be optimized in orderto achieve the same result.Based on the averaging concept, the new battery/UC HESSconfiguration is proposed and the diagram is illustrated inFigure 8. In this configuration, different from the conventionalHESS designs, the high voltage DC link is allowed to vary in apredefined ratio. The motor drive is designed to be able tohandle the current at the lower voltage. A higher voltage UCbank is always directly connected to the DC link so as toprovide peak power demands where as a lower voltage batteryis connected to the DC link via a power diode (or a controlledswitch). A reduced size bi-directional DC/DC converter isconnected between the battery and the UC to convey energy tocharge the UC. The DC/DC converter is always controlled tomanner. On the other hand, the load is demanding a constantpower which is the product of the voltage and current; therefore,the UC current will increase exponentially which will result ininefficiency.In order to explain the operation of the HESS, an allelectric vehicle is used as an example. In an electric vehicleapplication, the operation of the HESS can be separated intofour modes. They are vehicle low and high constant speedoperating modes; acceleration mode, and deceleration(regenerative braking) mode. The practical operation of theHESS is complex, but it is a combination of the above fourmodes. The four operating modes will be discussed below indetail.B. Mode I: Vehicle Low Constant Speed OperationThe constant speed operation of the vehicle was separatedinto two depending on if the power of the DC/DC converter( Pconv ) can cover the power demand ( Pdmd ). If Pdmd is equalto or smaller than Pconv , we call this operating condition thelow constant speed mode. If the vehicle is running at a higherspeed and in which Pdmd is higher than Pconv , we call it thehigh constant speed mode. Both the low and high constantspeed operating modes are ideal modes, since in practicalvehicle driving, the power demand is always changing. Theyare defined here in order to explain the operation of theproposed HESS.Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.vehicle acceleration. Figure 11 illustrates the energy flow of theacceleration mode phase I.Figure 9. Low constant speed operation energy flow.Figure 11. Acceleration mode phase I energy flow.Figure 9 shows the energy flow of the low constant speedoperation of the HESS. In the low speed operating mode, sincePconv Pdmd , the voltage of the UC ( VUC ) can be maintainedhigher than the voltage of the battery ( VBatt ); the DC linkvoltage ( VDC ) can also be maintained at any value higher thanthe battery voltage. In the constant speed mode, the UC isneither absorbing nor providing power to the electric motor.Since the UC voltage is higher than that of the battery, the mainpower diode is reversely biased. There is no energy flowthrough the diode. The battery is not providing any energydirectly to the motor inverter.With the decreasing of VUC , VUC will drop to the samelevel as VBatt . When VUC VBatt , the battery and UC becomedirectly paralleled through the diode. The system enters thehigh constant speed operating mode. In the high constant speedoperating mode, if Pdmd becomes less than Pconv , the powerdifference between Pconv and Pdmd will be used to charge theUC. The energy flow is illustrated in Figure 12.C. Mode II: Vehicle High Constant Speed OperationIn the high constant speed operating mode, Pdmd Pconv ,VUC can no longer be maintained higher than VBatt . Therefore,the main power diode is forward biased. The battery isproviding energy directly to the motor inverter. In this mode,the DC/DC converter will be turned off. Figure 10 shows theenergy flow of the high constant speed operating mode.Figure 12. Acceleration mode phase II energy flow.E. Mode IV: Deceleration (Regenerative Braking)In the deceleration mode, there are two phases. In phase I,the regenerative power will be injected into the UC only. Inphase I, the DC/DC converter might be in boost operation or nooperation depending on if VUC is less than the target ultracapacitor voltage VUC tgt . The energy flow diagrams for theFigure 10. High constant speed operation energy flow.two conditions are shown in Figures 13 and 14, respectively.D. Mode III: AccelerationAt the beginning of the acceleration mode, assume VUC VBatt . Since Pconv Pdmd , VUC will keep decreasing. Energyfrom the UC and the DC/DC converter are both supporting theCopyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.V. CASE STUDY AND SIMULATION RESULTSA case study of the proposed HESS design has been carriedout and the designed system is simulated using the PSATsoftware in order to prove the concept of the HESS.A. Simulation SetupAn all-electric vehicle PSAT model based on a 2003 HondaAccord was built. Figure 16 shows the drivetrain configurationof the vehicle.Figure 13. Regenerative braking phase I energy flow when Vuc Vuc tgt. - Figure 16. Simulated drivetrain configuration in PSAT.Figure 14. Regenerative braking phase I energy flow when Vuc Vuc tgt.Figure 15 shows the energy flow of the regenerativebraking phase II. Phase II describes the working conditions ofthe continuous regenerative braking. If continuous regenerativebraking is needed, in order to make sure VUC is within the safeoperating range, the DC/DC converter will work in buck modeto convey the energy from the UC to the battery. Whendesigning the proposed HESS, the ESS components can beproperly sized that regenerative braking phase II can be used asless as possible. This will extend the life of the battery as wellas increase the accuracy of battery SOC estimation.The design goal is to use the UC to cover the city drivingpower demands of the vehicle with the energy feeding from theDC/DC converter. Based on the existing 2003 Honda AccordEV Model in PSAT software, a new powertrain controller isdesigned in order to replace the battery only ESS with theproposed HESS. The battery of the HESS is sized in order todeliver the same range as with the EV ESS; The UC is sized inorder to satisfy the design goal which is to use the UC to coverthe city driving power demands of the vehicle with the energyfeeding from the DC/DC converter. Several DC/DC converterand UC rating combination are simulated and Table III showsthe final list of the components of the modeled vehicle.TABLE IIISPECS OF THE SIMULATED VEHICLE AND DRIVETRAIN COMPONENTSChassisVehicle WeightDrivetrain ConfigurationElectric MotorFigure 15. Regenerative braking phase II energy flow.2003 Honda Accord1737.80 kg (With the 2 energy storagesystems)2 wheel drive electric with single reductionratio of 1 6UQM PowerPhase 75Motor Power36kW continuous, 75kW peakUC Bank375V max 16.67FUC Bank Configuration150 Maxwell PC2500, 2.5V 2500F in seriesBattery Pack172.8V 180Ah Ni-MHBattery Pack ConfigurationDC/DC Converter144 1.2V 90Ah cells in series and 2 inparallelConstant efficiency, 12kW peakAccessory Power500WCopyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
Hysteresis control schemes were applied to control the VUCby managing the power of the DC/DC converter. Theimplemented DC/DC converter power strategy based on theUC open circuit voltage is shown in Figure 17. Modeling of thehysteresisVUC regulation was accomplished inUDDS Cycle VehicleSpeed (MPH)This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.6050403020100200400600800Time (s)1000120014000200400600800Time ime ime (s)100012001400UltracapacitorBank Power (kW)UltracapacitorBank Voltage (V)Matlab/Stateflow.B. Simulation ResultsThe configured vehicle was simulated in PSAT with the U.S. Environmental Protection Agency (EPA) UrbanDynamometer Driving Schedule (UDDS) . Figure 18 shows thevehicle speed, UC voltage, UC power, and battery pack powerof the configured vehicle with the DC/DC converter powerlimited to 12kW.The simulation results indicate that the designed HESS isworking as expected. During the overall drive cycle, the UCbank can cover the power needs with the 12kW DC/DCconverter pumping energy to recover the power consumption.As can be seen in Figure 18, the peak power of the UC bankranges from -38kW to 37.3kW. On the other hand, the peakpower of the battery pack is limited to 12kW
electric, hybrid electric, and plug-in hybrid electric vehicles (EVs, HEVs, and PHEVs) [1]-[9]. Of all the energy storage devices, batteries are one of the most widely used. However, a battery based ESS has several challenges providing the impetus to look for additional solutions [1]-[5]. In battery-based energy
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