Evaluation strategy of regenerative braking energyfor supercapacitor vehicle

Zhongyue Zou a, Junyi Cao a,n, Binggang Cao a, Wen Chen b

a State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi'an 710049, Chinab Division of Engineering,Wayne State University, Detroit, MI 48202, United States

a r t i c l e i n f o

Article history:Received 14 September 2013Received in revised form25 July 2014Accepted 19 September 2014Available online 11 October 2014This paper was recommended forpublication by prof. Y.Chen

Keywords:Regenerative braking energySupercapacitorsEnergy conversion efficiencyElectric vehicleMeasurement methods

a b s t r a c t

In order to improve the efficiency of energy conversion and increase the driving range of electricvehicles, the regenerative energy captured during braking process is stored in the energy storage devicesand then will be re-used. Due to the high power density of supercapacitors, they are employed towithstand high current in the short time and essentially capture more regenerative energy. Themeasuring methods for regenerative energy should be investigated to estimate the energy conversionefficiency and performance of electric vehicles. Based on the analysis of the regenerative braking energysystem of a supercapacitor vehicle, an evaluation system for energy recovery in the braking process isestablished using USB portable data-acquisition devices. Experiments under various braking conditionsare carried out. The results verify the higher efficiency of energy regeneration system using super-capacitors and the effectiveness of the proposed measurement method. It is also demonstrated that themaximum regenerative energy conversion efficiency can reach to 88%.

& 2014 ISA. Published by Elsevier Ltd. All rights reserved.

1. Introduction

One of the most important features of electric vehicles is thatthe kinetic energy of vehicle mass in the braking process can beconverted into other forms of energy and stored in the storagedevices. Those regenerative braking energy can be converted tothe kinetic energy of vehicles by controllers when starting oraccelerating again [1]. The energy regeneration system can beclassified into three categories: flywheel energy-storage system,hydraulic energy-storage system and electrochemical energy-storage system. Electrochemical energy-storage system wasproved to be a promising technical means to realize the energyregeneration in vehicles. If an electric vehicle runs at a high-speedmode, the transient current due to braking feedback in the motorbus will increase up to 200 A or more [2], and this current willcause enormous damage to traditional batteries such as lead acidand lithium batteries. In contrast to the traditional batteries, thesupercapacitors have higher power density, and it is more reason-able for the large amount of braking energy to be quickly chargedinto supercapacitors by proper transformation from kinetic energyto electrical energy. Therefore, the supercapacitors can greatlyenhance energy savings and consequently extend the drivingrange. On the other hand, supercapacitors could output huge

current instantaneously, and then reduce the power output ofthe batteries. Moreover, the accelerating capability of electricvehicles and battery life will also be improved accordingly.

As such, installing a supercapacitor as an auxiliary powersource for electric vehicles has become the latest research focus[3–13]. Different energy system architectures with supercapacitorsfor electric vehicles were proposed by Faggioli et al. in [3], andtheir research shows that application of supercapacitors to electrictraction systems can lead to substantial benefits in terms ofelectric vehicle performances, battery life and energy economy.Thounthonga et al. [4] demonstrated the application of the fuelcell and supercapacitors in electric vehicles to form a hybrid powersource. In their research work, a small-scale test bench was set upto verify the excellent performances of the proposed energymanagement, and the experimental results show that during themotor starts/stops or other significant steps, the designed hybridenergy system could provide the balance of energy and also absorbexcess energy from regenerative braking. Ortuzar et al. [5] adoptedan auxiliary energy system based on supercapacitors and evalu-ated the cost of different power support systems, and the resultsshowed that when the proposed auxiliary energy-system config-urations were included, the cost could be reduced significantlycompared with the system powered only by fuel cells. Moreover,the cost reduction was more when the supercapacitors wereemployed for this purpose. Guidi et al. [6] investigated theimpact of the addition of a power buffer using supercapacitors toa pure electric city vehicle equipped with an energy dense

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ISA Transactions

http://dx.doi.org/10.1016/j.isatra.2014.09.0110019-0578/& 2014 ISA. Published by Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ86 29 82667938; fax: þ86 29 82668601.E-mail address: caojy@mail.xjtu.edu.cn (J. Cao).

ISA Transactions 55 (2015) 234–240

Sodium–Nickel Chloride (ZEBRA) battery. Reference [6] alsoshowed remarkable loss reduction in the battery during normalcity driving was believed to result in a longer battery life. Dixonet al. [7] introduced the combination of a ZEBRA battery (highspecific energy but low specific power) with UCAPs (low specificenergy but high specific power), and this arrangement couldincrease driving range, efficiency, acceleration, and regenerativebraking capability in electric vehicles. Paladini et al. [8] arrangedthe supercapacitors as secondary energy buffers to capture asignificant portion of the braking energy and thus to improvethe fuel economy for all cycles. Takahara et al. [9] proposed analternative way of applying supercapacitors with serial and paral-lel configuration mode to a pure electric vehicle equipped withtraditional batteries. The implementation of a hybrid energysystem using supercapacitors was demonstrated by Cao and Emadi[10], Carter et al. [11], Hochgraf et al. [12], and Blanes et al. [13] toprotect the batteries on an electric vehicle from high-peakcurrents and therefore their lifetime will be extended. Based onthe above analysis, it is concluded that the supercapacitors, as partof hybrid energy sources in electric vehicles, can greatly improvevehicle performance in terms of efficiency, acceleration, drivingrange, and regenerative braking.

However, hybrid energy system with supercapacitors is alwayscomplex and its corresponding control strategy is difficult to bedesigned in applications. With the development of supercapacitortechnologies, it is possible to use them as vehicle's independentenergy sources, especially for short-distance vehicles such as citybuses, tunnel trucks and terminal trucks in wharf. The main reasonis that those vehicles with supercapacitors could qualify with fast-recharging capability, the frequent start–stop and regenerativebraking [14–19]. Hori [14] produced a supercapacitor vehicle,which could complete charging in five seconds and could reachto the maximum speed of 50 km h�1.

The literature has shown the feasibility to replace the originalstorage devices by supercapacitors in electric vehicles. However,none of them discussed the assessment of energy regenerationefficiency in details. Moreover, there are no effective methods andequipment to achieve an accurate measurement of the regenera-tive braking energy in supercapacitor vehicles. In present research,the detection algorithm for evaluating the regenerative brakingrecovery efficiency for a supercapacitor load trucks with a regen-erative braking strategy is proposed. Meanwhile, a measurementsystem for testing the regenerative braking efficiency in variousconditions is also investigated.

2. Supercapacitor vehicles and assessment equipment

2.1. Characteristics of supercapacitor vehicle

Supercapacitor vehicles can be divided into two types: puresupercapacitor electric vehicles and hybrid supercapacitor vehi-cles. A hybrid supercapacitor vehicle is composed of supercapaci-tors and batteries or fuel cells. It is obvious that hybrid electricvehicles with supercapacitors have a complex energy managementsystem. A pure electric vehicle with supercapacitors as the onlyenergy source will have a simpler energy management system,faster recharging speed and lower cost compared with hybridelectric vehicles. Therefore, pure supercapacitor electric vehicleshave more advantages in the condition of short distance andfrequent start–stop. Energy regeneration technology has beenwidely used in electric vehicles or electric motorcycles; however,the regenerative braking energy is not well utilized because thepower density of batteries like lead–acid, Ni-MH and Lithium islower than that of supercapacitors so that it is difficult to increasethe overall energy efficiency of vehicles. Furthermore, the optimal

design of control strategy over regenerative braking is alsorequired. Electric vehicles powered only by supercapacitors havebecome a recent focus of research due to their potentially highperformance. The issue is that there are no experimental evalua-tions for regenerative energy in supercapacitor vehicles at present.The accurate evaluation becomes a critical work to push thesupercapacitor vehicle technologies forward. Thus, developing aneasily implemented measurement system is necessary to achievethe regenerative-braking energy evaluation.

The supercapacitor vehicle to be used in current research isshowed in Fig. 1. The major parameters of the tested vehicle arelisted in Table 1. The electric truck powered by supercapacitors isdeveloped to freight the heavy goods in the wharf. They have thefast charging capability and high energy efficiency. Various para-meters, such as current and voltage of the supercapacitors andmotor, vehicle speed, acceleration, and brake pedal signals aremeasured to evaluate the regenerative braking efficiency.

This supercapacitor electric vehicle has an energy regenerativesystem, consisting of a general braking system, a power transmis-sion system, a motor and its control system, and an accumulatorand energy management system. When the driver steps on thebrake pedal, the brake controller will judge which conventionalbraking strategy should be applied according to the motor'sworking condition, the charging state of supercapacitors, wheel'ssliding rate and other parameters. When braking system runs inregenerative braking mode or composite braking mode, the kineticenergy of the vehicle can be transformed to electric energy by themotor, and this process is considered as the regenerative brakingenergy that will be stored into the supercapacitors. When thetruck starts rapidly or speeds up, the regenerative braking energystored earlier will be released to increase the energy efficiency andextend the driving range on a single charge.

Fig. 1. The supercapacitor truck.

Table 1Specification of the supercapacitor vehicle.

Parameters Value

Vehicle weight 10,000 kgMaximum speed (no-load) 40 kmh�1

Electric power rating 140 kWMaximum speed of motor 4000 rpmDriving range (no load) 20 kmSupercapacitor UCE15V80000AMaximum discharge current 600 AMaximum regenerative current 300–400 AVoltage 350–590 VDriving range(load) 4 km

Z. Zou et al. / ISA Transactions 55 (2015) 234–240 235

2.2. Measurement system for electric vehicles

The electric vehicle is one of the most promising cleantransportations in the twenty-first century; how to increase itsdriving range by overcoming the limitation of traditional energystorage and inverter technologies is the key. Algorithm optimiza-tion of energy regenerative braking is one of the potential meansto improve the vehicle's overall efficiency. The assessment ondifferent energy regenerative-braking algorithms is required in theprocess of optimization, which can be realized by on-line mon-itoring of various vehicle parameters such as current and voltageof supercapacitors and the motor, speed, acceleration and brakepedal signals. They are illustrated in Fig. 2. In this work, anassessment system is proposed and implemented via multi-channel signal collection and evaluation.

As shown in Fig. 3, the designed test system contains a signalacquisition card, a series of voltage and current sensors. Forconvenient measurement in the running vehicle, the portablesystem is composed of an advanced USB data acquisition card anda notebook computer. The analysis software, Matlab, is employed todeal with the huge raw data acquired from the test system. Thevoltage and current of supercapacitors and the motor are acquiredin the measurement along with braking signals, accelerator signalsetc. Both traveling state and energy-regeneration efficiency can becalculated using the real-time data.

Hall current sensors with the range of 500 A are selected tomeasure the instantaneous current signals of the supercapacitorsand motor. A 16-bit A/D conversion card is used to convert analogsignals to the digital signals. Then, the converted digital signals aresent to the portable computer. The corresponding test data can beplotted in real time on the screen. The signals of the accelerator,brake, and speed can be transformed into the correspondingvoltage signals ranging from 0 V to 5 V using the isolated voltagesensors. USB data-acquisition card can convert them into digitalsignals and store them in the hard drive of the notebook computer.Because the driving distance of the supercapacitor truck used inthis work is limited to 20 km, the proposed measuring system canacquire all real-time data in the driving range.

3. Energy regeneration efficiency

This experimental vehicle can convert the regenerative brakingenergy into electrical energy by charging supercapacitors directly.

In the process of regeneration, there are copper, iron andmechanical losses in the motor; they are represented by Ed. It isassumed that the resistance of the vehicle is f , the recyclableenergy could be described as follows:

E¼ 12mv2� f s�1

2Jeω

2�Ed; ð1Þ

Fig. 2. Required signals for the performance evaluation of the supercapacitor truck.

Fig. 3. Measuring system setup for the supercapacitor truck.

Z. Zou et al. / ISA Transactions 55 (2015) 234–240236

where E is available recyclable energy (J), m is the mass of vehicle(kg), v is the vehicle's speed before braking (ms�1), s is the brakingdistance (m), f is vehicle's moving resistance (N), Je is the momentof inertia of rotating parts in vehicle, and ω is the angular velocity.The loss Ed is always neglected in calculation of recyclable energybecause they are very small in the high efficiency motor forelectric vehicles when compared with the vehicle's kinetic energy.

Fig. 4 shows the working state of the energy regenerationsystem. In case of braking, DC motor works in the state ofgenerating power, and method of excitation is separately excited.The balance equation of current in the DC generator can bedescribed as

Uout ¼ cendϕd� IdzRa�U0; ð2Þwhere ce is back emf constant,nd is the motor speed, ϕd is the mainflux of the motor, Ra is the armature resistance, U0 is the voltage ofbattery, Uout and Idz are the armature voltage and current,respectively. When the supercapacitor truck comes into the stateof braking, the excited current is generally adjusted through thedesigned control strategy to connect the vehicle's braking torque Twith rotation speed n. That is also called the braking character-istics of a DC motor, as shown in Fig. 5 [20]. In the process ofregenerative braking, the braking characteristics of the vehicle arealways guaranteed.

For the regenerative-energy system of trucks, the ratio of thepower consumed by supercapacitor and motor resistance to thegenerating power output from motor can be expressed as

ηr ¼I2dzR

Uout Idz; ð3Þ

where R is the equivalent internal resistance of the supercapacitorpack. According to the capacitor's charging equation

Idz ¼Uout

Re

� tRC ; ð4Þ

the ratio of charging energy losses to generating electrical energyfrom vehicle kinetic energy can be given by

ηr ¼ e� tRC ; ð5Þ

Integrating Eqs. (1), (3), (4) and (5), the consuming energydistribution of the supercapacitor vehicle during regenerativebraking can be depicted in Fig. 6. The kinetic energy would beeventually consumed as the following types:

(1) Driving resistance loss,(2) DC/DC inverter loss,(3) DC motor and mechanical transmission loss,(4) energy recycled in supercapacitors, and(5) equivalent resistance loss of supercapacitor consumption.

Because the energy loss from the inverter, motor and mechan-ical transmission is slight, they may be ignored in the experi-mental analysis. Therefore, the efficiency of energy regenerationcan be obtained as

η¼ ð1�ηrÞ � 1=2mv2� f s�1=2Jeω2� �

1=2mv2; ð6Þ

However, it is very difficult to exactly test the vehicle resistanceand equivalent rotary inertia during braking. Therefore, theregenerative energy by supercapacitors can be directly calculatedusing real-time voltage and current signal. Accordingly, the effi-ciency of recycling energy can be described as:

η¼ ð1�ηrÞ � ðR U0I0dt�RI20R dtÞ

1=2mv2; ð7Þ

where I0 is the current of energy source. It is obvious that theefficiency of recycling energy can be obtained using the proposedonline measured system because the value of many parameters inEq. (7) will be recorded in the driving range as long as the capacityof the hard disk in portable computer is enough large.

4. Experimental verification

Before measuring performance of the supercapacitor truck, theregenerative braking energy, the signals of the accelerator, brakingand speeds should be measured exactly. Based on the measure-ments, the characteristics of these signals can be verified sepa-rately. The performance of the supercapacitor truck under variousexperimental conditions, such as acceleration, deceleration andregenerative braking, are evaluated.

I0

U0 U0ut

Idz

nd

Ed

IdLDC/DC

M u

Fig. 4. Schematic diagram of the energy recovery system.

Fig. 5. Braking characteristics of the vehicle. Fig. 6. Distribution of vehicle kinetic energy.

Z. Zou et al. / ISA Transactions 55 (2015) 234–240 237

The plotted curves for the accelerator, braking and speedsensor signals are showed in Figs. 7–9, respectively. Fig. 7 indicatesthat the accelerator signal is a DC voltage and its magnitude variesfrom 0 V to 9.8 V; the stronger the accelerator signal is, the fasterthe acceleration of the vehicle will be. When you press on theaccelerator pedal fully down, the voltage of output signal is 9.8 V.The braking signal in Fig. 9 is also a DC voltage and its magnituderanges from 0 V to 5.7 V. When the brake pedal is pressed downfully, the value of output signal is 5.7 V. The speed signal shown inFig. 8 is a consecutive square wave with its maximum amplitudebeing about 7.5 V. If the truck is driven one kilometer, the 15,127square waves can be obtained by the speed sensors. The speed ofthe supercapacitor truck can be calculated by the period of thesquare wave. The larger square-wave frequency corresponds to thefaster vehicle speed and vice versa; then, the speed of the truck iscomputed as shown in Fig. 10.

The frequency of the output voltage and current of the maininverter ranges from 0.5 HZ to 100 Hz, based on which thesampling frequency of the A/D data acquisition card is set up to2 kHz according to sampling theorem. The output voltage andcurrent of the main inverter can be acquired by the proposedtesting system. At the same time, the real-time power curve can becalculated. As can be seen from Fig. 11, the input current of maininverter in the vehicle is increased gradually at the beginning. Thepeak current is 221 A, and the corresponding peak power of themain power inverter increases to 90 kW. When the supercapacitortruck enters the braking state, the current of the inverter becomesnegative. At this time, the power inverter begins to convert thekinetic energy of the vehicle into electric energy. This convertingprocess is regulated and the energy is charged into the super-capacitors. The peak charging current is 180 A and the maximumcharging power is 72 kW.

When the truck starts, the input power to the main drivingmotor gradually increases to the peak power 68 kW (shown in

Fig. 12). After that peak, the motor's input power begins todecrease slowly. When the supercapacitor truck comes into thebraking deceleration state, the input power of the main drivemotor becomes negative. It is also indicated that the main drivingmotor is operated in the power generation state.

According to the calculation method of regenerative brakingefficiency proposed in Section 3, the total kinetic energy can becalculated when the vehicle begins to brake. During the brakingprocess, the main driving motor is operated in generative braking,and the electric energy converted from the kinetic energy is storedin the supercapacitors finally. While the power curve of theinverter is integrated under deceleration, the recovery energystored in the supercapacitors can be computed using Eq. (7).

0 10 20 30 400

2

4

6

8

10

T/s

Acc

elar

atio

n/V

Fig. 7. Accelerator signal.

3 3.5 4 4.5 5 5.5 6-2

0

2

4

6

8

Rot

atio

nal s

peed

/V

T/s

Fig. 8. Rotational speed.

0 10 20 30 400

2

4

6

8

T/s

brak

ing/

V

Fig. 9. Braking signal.

0 10 20 30 400

10

20

30

40

50

spee

d/(k

mh-1

)

T/s

Fig. 10. Vehicle speed.

0 5 10 15 20 25 30 35 40400

450

500

550

Vol

tage

/V

0 5 10 15 20 25 30 35 40-200

0

200

400

Cur

rent

/A

0 5 10 15 20 25 30 35 40-100

0

100

Pow

er/k

w

T/s

Fig. 11. Voltage, current and power curves for main inverter.

Z. Zou et al. / ISA Transactions 55 (2015) 234–240238

Because the energy loss in inverters and transmission is verysmall, it is ignored in the subsequent analysis.

In order to evaluate the efficiency of regenerative brakingenergy, the working scenario of the truck is: the truck is acceler-ated from zero to the maximum speed, then driven at a constantspeed and slowed down to stop. The braking time can be changedby the driver's command under experimental condition. The inputsignals with the duration time of 50 s and sampling frequency of2 kHz are shown in Fig. 12; the related variables are calculated asfollows:

(1) The total energy consumption for acceleration: 0.469634 kWh,(2) recycling energy: 0.138672 kWh,(3) kinetic energy before braking: 0.176404 kWh,(4) recycling energy/total energy consumption: 33.283953%, and(5) recycling energy/vehicle kinetic energy: 88.610559%.

To verify the evaluation strategy for regenerative efficiency, sixexperiments on the truck without loads are firstly carried out,where the emergency braking state is tested twice and slowbraking is tested for four times. The final results are listed inTable 2.

As Table 2 shows, the energy regenerative efficiency is low inthe first two experiments. The main reason is that, in an emer-gency braking situation, both the air braking and regenerativebraking of the main motor are effective and the former plays the

major role in this case. Most of the kinetic energy is lost due tofriction, making the regenerative energy is very little. The last fourtests are in the state of slow braking. The regenerative braking isdominated and the air braking does not work or just plays asupplemental role. At this circumstance, the energy recoveryefficiency is high. In addition, because the evaluated truck usesthe supercapacitors as storage devices, it has the ability to absorbhigh current and to make the recovered energy absorbed rapidlycompared with other types of batteries. It is proved that theefficiency of braking energy recovery in the electric vehiclespowered by supercapacitors is greatly enhanced than the effi-ciency by ordinary batteries.

According to the specification of the supercapacitor truck, 70 tof weight will be carried to move4 km. In case of load, theexperiment results shown in Table 3 are obtained using theproposed evaluation system. It can be seen from Table 3 that thetotal consumption energy for achieving a maximum speed of25 kmh�1 with a 70 t load is up to 1.14 kWh. While without load,the maximum consumption energy for the maximum speed40 kmh�1 is only about 0.578 kWh, which is illustrated inTable 2. The heavy load increases the total consumption energyand can not improve the regenerative efficiency. The maximumrecycling energy is only 0.1 kWh in case of 70 t load. Moreover, theaverage ratio of the recycling energy to the kinetic energy beforebraking is 14.07%. This result may likely attribute to low brakingspeed and loss of mechanical friction.

Fig. 13 demonstrates the voltage, current, and power of thesupercapacitors during the driving range on a single charge. Afterfully charged, the voltage of the supercapacitors reaches to the

0 5 10 15 20 25 30 35 40 45 500

5

10

acce

lera

tor/V

0 5 10 15 20 25 30 35 40 45 500

5

10

brak

ing/

V

0 5 10 15 20 25 30 35 40 45 50-100

0

100

pow

er/k

W

0 5 10 15 20 25 30 35 40 45 500

50

spee

d/(k

mh-1

)

T/s

Fig. 12. Main drive motor input power curve (accelerator signal, brake signal,motor input power, speed signal).

Table 2Experimental results without load.

Label Totalconsumptionenergy foracceleration(kWh)

Kineticenergybeforebraking(kWh)

Recyclingenergy(kWh)

Recyclingenergy/Totalconsumptionenergy (%)

Regenerativeefficiency (%)

1 0.557732 0.190889 0.036845 6.606 19.3022 0.480585 0.172277 0.057274 11.917 33.2453 0.498263 0.178717 0.155156 31.139 86.8164 0.472100 0.177073 0.155249 32.884 87.6755 0.339447 0.176404 0.138006 40.656 78.2336 0.534913 0.187287 0.160190 29.946 85.531

Table 3Experimental results with 70 t load.

Label Totalconsumptionenergy foracceleration(kWh)

Kineticenergybeforebraking(kWh)

Recyclingenergy(kWh)

Recyclingenergy/Totalconsumptionenergy (%)

Regenerativeefficiency (%)

1 0.968881 0.481716 0.070747 7.302 14.6862 1.066876 0.453326 0.058402 5.474 12.8833 1.142086 0.517553 0.100279 8.780 19.3764 1.139385 0.494705 0.062435 5.480 12.6215 0.816167 0.430943 0.046391 5.684 10.765

0 200 400 600 800 1000 1200 1400 1600 1800300

400

500

600

Vol

tage

/V0 200 400 600 800 1000 1200 1400 1600 1800

-500

0

500

Cur

rent

/I

0 200 400 600 800 1000 1200 1400 1600 1800-200

0

200

Power/kW

T/s

Fig. 13. Voltage, current, and power curves of the supercapacitors during thedriving range per charge.

Z. Zou et al. / ISA Transactions 55 (2015) 234–240 239

maximum value of 546.6 V. When the truck stops, the voltage ofsupercapacitors is 356 V. As shown in Fig. 13, there exist frequentstarts and stops in the whole process; the positive pulse isacceleration while the negative pulse represents the regenerativebraking. In this experiment, the total traveling time of the super-capacitor truck is 29.3 min and the total energy consumption is10.498 kWh. At the same time, the recycled energy is 2.524 kWhin the whole range. Therefore, the rate of recycled energy to totalconsuming energy is 24.0427%. This has clearly verified that thesupercapacitors can greatly improve the truck's fuel economy in allworking conditions.

5. Conclusion

It has been demonstrated that the proposed energy-regeneration detection system can effectively measure the effi-ciency of regenerative braking. The supercapacitor truck showsexcellent energy regenerative characteristics in that the brakingenergy can be absorbed efficiently and reliably, and the maximumefficiency can be up to 88%. Therefore, the electric vehicle poweredby supercapacitors can obtain higher energy-regeneration effi-ciency and also output larger power in a short time.

In addition, the regenerative-braking energy efficiency variesalong with the driver's behavior. Especially, the force imposed onthe brake pedal is one of the important factors that affect theregenerative efficiency. In case of the emergency braking, thebraking force comes mainly from front wheels; if we can establishfront and rear regenerative braking mode, then energy-recoveryeffects will be better. In short, the energy-recovery efficiency inelectric vehicles can be guaranteed by the supercapacitor technol-ogy. It is believed that the supercapacitor technologies willimprove the performances of electric vehicles, and bring electricvehicles into a new stage.

This proposed measurement system can also be easily imple-mented in the evaluation and maintenance of hybrid electricvehicles. The assignation of the braking force on the four wheelsis very vital to the vehicle's safety in the braking process. There-fore, in the further research, the function of evaluating brakingforces should be considered in the proposed system.

Acknowledgment

This project is being jointly supported by the National NaturalScience Foundation of China (Grant no. 51075317) and Program forNew Century Excellent Talents in University (Grant no. NCET-12-0453).

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