System Design of Electric Assisted Bicycle using
EDLCs and Wireless Charger
Jun-ichi Itoh, Kenji Noguchi and Koji Orikawa
Department of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp，k_noguchi@stn.nagaokaut.ac.jp，orikawa@vos.nagaokaut.ac.jp
Abstract— This paper discusses an electric assisted
bicycle which uses electric double layer capacitors (EDLCs)
as a power source. EDLCs are charged through a rapid
charger by using wireless power transmission. In this paper,
first, the energy capacity of EDLCs is designed. Next, theantenna for the wireless power transmission and thecharger are investigated. Third, this paper compares thevolume and the power loss of the three kinds of DC-DC
converters which are step-down type, boost-type and buckboost type for the charging and discharging of EDLCs. As aresult, boost-type is the most compact in the power capacityof the electric assisted bicycle. Finally, the proposed system
is experimentally verified as a prototype. As a result, theproposed system can shorten to 1/4 the charging time of theconventional system.
Keywords— Electric assisted bicycle, Electric double layer
capacitors,Wireless power transfer, System design

I.

INTRODUCTION

Recently, electric assisted bicycles have been attracted
because the safety and easy driving on bicycle. The
electric assisted bicycles use a lithium-ion battery as
energy source. The lithium-ion battery is suitable for along assist time owing to high energy density. However
the lithium-ion battery is the short lifetime and thenecessary to the long charging time.
On the other hands, the electric vehicle using electric
double layer capacitors (EDLCs) of long cycle life has
been developed as city commuters [1].This system uses
EDLCs instead of the lithium-ion battery. However, thelarge vehicle such as a car is afraid of the empty of theenergy on the load.
The electric bike with EDLCs has been also proposed[2]. The EDLCs are suitable for the electric bike owing to
the small vehicle. However, the electric bike is also afraidof the empty of the energy.
This paper proposes an electric assisted bicycle with
EDLCs [3]. The electric assisted bicycle is not afraid of
the empty of the energy because it can be driven by
pedals on the bicycle even if no electric energy. The
concept of this system has two concepts as follows; First,
it is assumed that the EDLCs are used as only the assistof starting acceleration and slopes for the electric assisted
bicycle in order to suppress the total energy of the
EDLCs; Second, the wireless power transfer system isFig. 1. Configuration of the proposed system. The proposed system
can be charged by simply park the electric assisted bicycle
to bicycle parking area.

applied to rapid charger for the EDLCs because of small
capacity and the short charging time.
In order to maximize the advantages of the proposedsystem, it is necessary to optimize the design from theperspective of the overall system in consideration of
trade-off problem of the time required to fully charge the
EDLCs and the total volume of the power conversion
system.
Therefore, as the purpose of optimum design of the
proposed system, this paper evaluates three items asfollows;
1) The energy capacity of EDLCs which is required
for assist, based on the running test,2) The antenna using a print circuit board (PCB) andcharger design for the wireless power transmission,3) The circuit topology of the interface DC-DC
converter for EDLCs in the proposed system
configuration in terms of the volume and efficiency.
Finally, the proposed system is experimentally verified
as the prototype.
II.

PROPOSED SYSTEM CONFIGURATION

Fig. 1 shows the configuration of the proposed system.
The proposed system uses a Radio Frequency (RF) power
supply at the input stage of the transmitting antenna forwireless power transmission [4-8]. Also, the proposedsystem comprises a rapid rechargeable AC-DC converterand EDLCs in the latter part of the receiving antenna. In
addition, the interface DC-DC converter for the assist

DESIGN OF ENERGY CAPACITY OF EDLCS

～
～
～ ～

III.

Height [m]performs discharging control of the EDLCs by assisting
the drivers when a brushless DC (BLDC) motor is driven.
It is noted that the BLDC motor and DC-AC converter
are commercial products [9].

A. Conditions of Assist
Fig. 2 shows the assist pattern of a driving test. The
pattern A is a driving pattern for the evaluation of theassist in the accelerating mode. The pattern B is a driving
pattern for demonstration of the assist in the climbing
slopes with a gradient (average gradient 6%).
B. Measurement of Cumulative Energy
Fig. 3 shows the method of measuring the cumulative
energy of each assist pattern. The energy consumption by
pattern A or pattern B is measured using a power meter.
The cumulative energy is measured in the conventional
system.
Fig. 4 shows the time characteristics of the cumulative
energy of each assist pattern. The energy for the assist is
716 J (the energy is 436 J during acceleration time) in thepattern A, and that of the pattern B is 10.6 kJ. From theresults in the pattern B, when the distance climbing the
distance of the hill was set to 1 km, the necessary energy
becomes 53.0 kJ. This paper designs the capacitance of
EDLCs that can only assist this energy. Further, thevolume of the EDLCs with lithium-ion battery iscompared. In addition, the total volume and power loss
for three types DC-DC converters using EDLCs as a
power source are compared. In addition, the energy of
EDLCs is designed in 12.1kJ as a prototype of the system
in Section 6.
C. Comparison of Energy
Fig. 5 shows the relationship between the energy andvolume of the EDLCs (Nippon Chemi-Con Co,
DDLE2R5LGN232KCH2S) and lithium-ion battery inthe marketplace [9-10].
The enegy E of EDLCs is calculated by (1) where
Ctotal is the total capacitance of EDLCs, Vmax is the
maximum voltage of EDLCs (The rated voltage of EDLC
per unit is 2.5 V. In this case, the maximum voltage isdesigned to 22.5 V), Vmin is the minimum voltage of
EDLCs that is designed by the designer (In this case, theminimum voltage is designed to 8 V). The number andthe output voltage range of the EDLCs are designed
when the charging and discharging energy of EDLCs
meet more than 53.0 kJ.
1⎛⎜V 2 − V 2 ⎞⎟
E= C
min ⎠
2 total ⎝ max(1)

As shown in Fig. 5, the energy E, which is supplied
from the EDLCs is designed to 56.5 kJ. Furthermore, theenergy density of the EDLCs in the marketplace is 5.16
Wh/dm3. When the energy density becomes more than
1.5 times of the present EDLCs, the energy source size ofthe proposed system can become smaller than that of theconventional systems. In other words, if the distance of
2/3 is considered in order to suppress the total energy

Fig. 2. Assist pattern of driving test.
The assist assume only two patterns.

Fig. 3. Method of measuring the cumulative energy ofeach assist pattern.

800
600
400
200
0
0

1

2

3

4

5

6 7 8
Time [s]9 10 11 12 13(a) Pattern A.

12000
10000
8000
6000
4000
2000
0
0

10

20

30
40
Time [s]

50

60

70

(b) Pattern B.
Fig. 4. Time characteristics of the cumulative energy ofeach assist pattern. The cumulative energy is measured based on theconditions of Figure 2.

700
600
500
400
300
200
100
0
0

1

2

3

Fig. 5. Relationship between energy and volume of
EDLCs and lithium-ion battery.
The request range of volume for EDLCs is shown.

4

capacity, the proposed system becomes useful. Therefore,
operation verification of the system is discussed as a
200m distance in Chapter 6.
IV.

DESIGN OF ANTENNA FOR WIRELESS POWER
TRANSMISSION AND CHARGER

A small receiving antenna is preferred in order
mounting to the electric bicycle. Therefore, the authors
have been proposed a formula spiral antenna made of a
printed circuit board, which is flat, compact and easy
design.
A. Design of Wireless Charger
Fig. 6 shows the structure and equivalent circuits of thetransmitting antenna for wireless power transmission.
The connection point of the short type and open type isthe output of a RF power supply. For the receiving
antenna, the equivalent circuit is the same. The antenna
has a two-layer structured wiring in order to increase theinductance value.
Fig. 7 shows the design flow chart of the charger andthe antenna for the wireless power transmission. From
Fig. 7, the charging power is determined by the charging
time and the charging energy. The antenna is designed
from the specifications of the antenna size and thecharging power. The charger considers the short charging
time and the volume of power conversion system for the
charging power. The target of charging time T is 60 sec,
and the charging energy E is supplied to 56.5 kJ. In
addition, the average output power of the charger E/T is 1
kW. However, the size of the circuit becomes large when
the charging circuit combines with the discharging circuit.
Therefore, the charging control should be placed in RF
power supply of the power transmission side of thewireless power transfer system is realistic.
Also, the antenna size depends on the mounting
position of the power receiving side. The antenna forwireless power transmission is experimentally compared
the short type and open type with the same size.
B. Comparison of Transmission Efficiency
Fig. 8 shows the system configuration of theexperiments. Arbitrary frequency is output using afunction generator. The output power cannot output 1
kW(The average output power of the charger) because of
capacity limit of RF power supply. Therefore, the outputof the RF power supply is scaled to 100W (10% of 1kW).
Reflected power PR is measured using a power meter inthe front stage of the power transmission antenna. Also,
the antenna of short type is connected in series with thecapacitor of capacitance 500 pF.
Fig. 9 shows the experimental results of the frequency
characteristics of the transmission efficiency. The poweris amplified by the amplifier. The antenna is designed
according to the flowchart of Fig. 7. From Fig. 9, it isconfirmed that the short type can realize the low
resonance frequency compared to the open type at thesame transmission efficiency and antenna size. In general,

Fig. 6. Structure of the antenna for wireless power transmission.
The antenna is a printed circuit board of the two layers.

Fig. 7. Design flow chart of charger and antenna forwireless power transmission. Specifications of the antenna isdetermined by the antenna size and the charging power.

Fig. 8. System configuration of the experiments.
The system of the experiment circuit is matched at 50Ω.
100
90
80
70
60
50
40
30
20
10
0
0.6

1.1

1.6

2.1

2.6

3.1

3.6

Fig. 9. Experimental results of the frequency characteristics ofthe transmission efficiency.
It is the result of the experiment under the same conditions.

iL2c
S1c
iL1c
L1c

S2c

(a) Boost-Type.

15

DESIGN OF THE INTERFACE CONVERTER FOR
EDLCSA. Circuit configuration to be compared
Fig. 10 shows the circuit configuration of three kindsof DC-DC converters. This paper investigates the stepdown type, boost-type and buck-boost type as the
charging and discharging DC-DC converter by using
EDLCs. The number and the output voltage range of the
EDLCs are designed when the charging and discharging
energy of EDLCs meet more than 53.0 kJ. Thus, the DCDC converters are designed that the charging anddischarging energy of EDLCs is as follows: the energy ofthe boost-type, step-down type and buck-boost type are56.5 kJ, 58.0 kJ and 55.2 kJ, respectively. In addition, thespecification of the input voltage of the DC-AC converter
is 24 V (Vouta, Voutb, Voutc), and the output voltage of the
DC-DC converter is controlled to 24 V at constant.
Further, the reactors of all types are designed to thecurrent ripple value (1.5 A) as same as the step-down
type. Therefore, the reactors of all types are designed thatthe current ripple of the reactor meets less than 1.5A. On
the other hand, the smoothing capacitor is designed fromthe allowable ripple current.
In this system, by the force step on the pedal of abicycle, the energy, which is used for an assist, and theload of BLDC motor are changed. It is necessary toincrease the capacitance of the smoothing capacitor inorder to reduce the variation in the output voltage of the
DC-DC converter owing to load variations. However, thevolume of the smoothing capacitor should be reduced interms of volume implementation. Therefore, when theload fluctuations occur, it is important to reduce thevolume of the smoothing capacitor by implementing thefast control response of the output voltage [11].
B. Comparison of Power Loss
Fig. 11 shows the power loss analysis of three kinds of
DC-DC converters when the voltage of the EDLCs Vin ischanged. The power loss is analyzed under the conditions
that the output power of the DC-DC converter is 384 W.

(c) Buck-boost Type.

20

10
5
0
8

15

22.5

(a) Boost-Type.
20
15
10
5
0
25.5

31

37.5

(b) Step-down Type.
20Power loss [W]

V.

C1c +
vconvcvoutc
24V(b) Step-down Type.
Fig. 10. Investigated circuit configuration.
The EDLCs of a circuit of three types are designed with the same energy.

If resonant frequency (Frequency of RF power supply) is
the low frequency, the efficiency of RF power supply ishigh. Therefore, the short type can be high efficiency ofthe system than the open type. So, the proposed system isadopted the short type.

EDLCs(12 series connection)
Capacitance : 192F
30V(voutc+6)~
vinc
ioutc 18V(voutc-6)
Energy: 55.2kJ
icc

15
10
5
0
18

24

30

(c) Buck-boost Type.
Fig. 11. Results of power loss analysis of the three kinds of
DC-DC converters.
The power loss is neglected iron loss of the reactor,
the conduction loss of the diode, recovery loss.

It is noted that dead time is neglected. As shown in Fig.
11, it is confirmed that the switching loss and the
conduction loss of the MOSFET dominates the total loss

in the boost-type. Also, it is confirmed that the switchingloss of the MOSFET is dominates the total losses in the
step-down type and buck-boost type. This is because theinput current of the boost-type is increased owing to the
input voltage, which is lower than the step-down type andbuck-boost type.
C. Comparison of Total Volume
Fig. 12 shows the relationship between the total
volume and the output power for the three kinds of DCDC converters. It is noted that the power loss ismaximum for the voltage of EDLCs in Fig. 11. In
addition, the volume of the Fig. 12 is the sum of volume
of EDLCs and DC-DC converter (Reactor, Heat sink,
Electrolytic capacitor).
In this paper, CSPI (Cooling System Performance
Index), which is a reciprocal of the product of the volume
and the thermal resistance, is introduced to estimate thevolume of cooling system. The CSPI indicates thecooling performance per unit volume of the coolingsystem. It means that a high performance cooling systemshows high CSPI. Therefore, the cooling system isminiaturized when CSPI become higher. The volume of
the cooling system volcooling is given by (2) from therelationship between the power loss and the rise intemperature [12].

volcooling =

Ploss
1
=
Rth × CSPI
T j − Ta × CSPI

(

)

3

(3)

where Kv is the constant value depending on the shapeof cores, W is the maximum energy of the reactor, Ku is
the occupancy of the window, Bm is the maximum flux
density of the core, and J is the current density of thewire.
The capacitor volume is calculated based oncommercially available electrolytic capacitors [14].The
volume of the electrolytic capacitor is proportional to therms value of the ripple current of the electrolytic
capacitor. The volume volCE of the electrolytic capacitor
is given by (4).
−1
vol CE = γ VCE
I CRMS

7
6
5
4
3
2

(a) Total volume.

8

ReactorHeatsink

Capacitor

EDLCs

7
6
5

(2)

where Rth is the thermal resistance of the coolingsystem, Tj is the junction temperature of the switching
device, Ta is the ambient temperature, Ploss is the power
loss of the switching device.
In this paper, the reactor is designed by the Area
Product concept [13] using a window area and a crosssectional area. Then volume of the reactor volL is givenby (3).

⎛ 2W ⎞ 4⎟⎟
volL = K v ⎜⎜⎝ K u Bm J ⎠8(4)

where γ-1VCE is the proportionality factor between therms value of the ripple current and the volume, and ICRMS
is the rms value of the ripple current of the electrolytic
capacitor.

4
3
2
1
0
(b) Ratio of the volume.
Fig. 12. Relationship between the total volume and the output power
for the three kinds of DC-DC converters.

From Fig. 12 (b), around the output power of 100 W,
the volume of the EDLCs is dominant in the converter of
all types. The number of EDLCs for the boost-type is the
fewest in all of other DC-DC converters. Therefore, thevolume of the boost-type is the smallest. However, if theoutput power is 1.1 kW or more than 675 W, the volumeof the step-down type is smaller than that of the boosttype and the buck-boost type. For the boost-type and thebuck-boost type, the ripple current of the smoothing
capacitor is increased when the output power is increased.
Therefore, the ratio of the volume of the smoothing
capacitor is increased. On the other hand, for the stepdown type, the ripple current of the smoothing capacitor
does not depend on the output power. Therefore, when
the output power is increased, therefore, when the output
power is increased, the volume of heat sink and reactor
which is very small compared to the volume of the
EDLCs is increased. However, the volume of theconverter does not increase too much. Furthermore, if theoutput voltage of the DC-DC converter is set to over 24V,
the cross over point of the volume of three kinds of DC-

Fig. 13. The entire circuit diagram of the proposed system.
The proposed system is designed based on the system design to chapter 5.

DC converter is shifted toward high output power. The
reason is that the volume of the capacitor is decreased
depends on its ripple current according to the high output
voltage of the DC-DC converter. Also, boost-type is themost compact in the power capacity (384W) of theelectric assisted bicycle. So, the proposed system isadopted the boost-type.
VI.

CHARGING VERIFICATION OF SYSTEM

A. Design of the whole system
Fig. 13 shows the circuit diagram of the entire system
based on the system design to chapter 5. By the systemdesign, the total volume of boost type is most minimumin three kinds of DC-DC converters at the maximum
power (384W) of electric assisted bicycle. Therefore,
boost type is adopted for the bi-directional DC-DC
converter. In the antenna for wireless power transmission,
the short type can be high efficiency of the RF power
supply than the open type. Therefore, the antenna of short
type is adopted. The running distance of a slope is morethan 200 m as prototype specifications in this paper. Thus,
the energy of the EDLCs (Nippon Chemi-Con Co,
DDLE2R5LGN701KAA5S) is designed in 12.1 kJ [10].
In addition, the specification of the charging time is 1
minute. The wireless power transfer system is adopted
the S/P system [15].
Fig. 14 shows the equivalent circuit diagram of the S/P
system. The transmitting antenna is connected to the RF
power supply. The receiving antenna is connected to therectifier.
Table I shows the specification of antenna for wirelesspower transmission. The designed coupling factor and theself-inductance are measured by a LCR meter. The
capacity C2 of the parallel capacitor is given by (5). The
capacity C1 of the series capacitor is given by (6).

C2 =
C1 =

1
L2 ω

2
0

1
(1 − k ) L1ω 02
2
0

(5)
(6)

where L2 is the self-inductance of receiving antenna,
ω0 is the resonance angular frequency, k0 is the designed
coupling factor, L1 is the self-inductance of transmitting
antenna.
Fig. 15 shows the control block diagram of wirelesscharging. In the S/P system, If the voltage of transmitting

Fig. 14. Equivalent circuit diagram of the S/P system.
The capacitor is connected to an external antenna.
Table I. Specification of antenna.
Items
ValuesRemarks

Outline (Length)

250

mm

Outline (Side)

200

mm

Number of turns (surface)

10

turn

Short type

Number of turns (reverse face)

10

turn

Short typeSpace between copper trace

5mm

Width of copper trace3

mm

Thickness of PCB

2mm

Thickness of copper trace

70μm

Self-inductance L1

58.27

μH

Self-inductance L2

58.70

μH

Mutual inductance M

21.95

μH

Designed coupling factor k0

0.375

Primary capacitance C1

586

pF

Secondary capacitance C2

500

pF

1
1 + sTiv

1
1 + sTic

1
sL1

FR-4

a

1
sC1

Fig. 15. Control block diagram.
The output voltage of bi-directional DC-DC converter is controlled
to a constant 24V.
Table II. Conditions of the experiment.
Items
ValuesRemarks
Frequency of the FG

929

kHz

Resonant frequency

929

kHz

Voltage of the FG

450

mVrms

Gain of the RF power source

100

Transmission distance

50mm

antenna is not controlled, the voltage of the powerreceiving antenna is varied by variation of the coupling
coefficient and load [16]. In this problem, the breakdown
voltage of the switching elements must be designed bytaking into account the variation of the coupling
coefficient and the load. Therefore, charging control by
the combination of two points as below is proposed.
1) The output voltage of the bi-directional DC-DC
converter is controlled to be constant.
2) The output voltage of the RF power supply is set
higher than the output voltage of the bi-directional DCDC converter.
Thus, the design of the breakdown voltage of theswitching device can be simplified. The charging
experiment is based on the experimental conditions
shown in Table II.
B. Experimental results of charging
Fig. 16 shows the experimental results of the wirelesscharging to the EDLCs.
Fig. 16(a) shows the voltage and current waveform of
the input and output for the bi-directional DC-DC
converter. From Fig. 16 (a), the output voltage of the bidirectional DC-DC converter is controlled to be constant,and the EDLCs are charged.
Fig. 16(b) shows the energy storage capacity of
EDLCs from the start of charging to the end of charging.
From Fig. 16 (b), the energy capacity of the EDLCs arecharged from 0% to 100% in about 6 minutes isconfirmed. The reason that charging time is not within 1
minute is for the capacity limit of the RF power supply
used in experiments of this paper. Therefore, thecharging can be achieved a full charging in 1 minuteusing the high-capacity RF power supply.
Fig. 16(c) shows the output voltage and current
waveforms of the RF power source, input voltage
waveform of bi-directional DC-DC converter, voltage
waveform of EDLC (The voltage of EDLC is at the 8V).
From Fig. 16 (c), the input power factor of the highfrequency power source is approximately 1 is confirmed.
Fig. 17 shows the comparison of the charging time forcharging the 12.1kJ of the proposed system andconventional system. The charging energy of theconventional system is calculated from specification of
the product. From Fig. 17, the proposed system canshorten to 1/4 the charging time of the conventional
system.
C. Wireless charging method
Fig. 18 shows the wireless charging method to theelectric assisted bicycle. The receiving antenna isconnected to the sides of the front basket of the electric
assisted bicycle. The proposed system is introduced to acharging side. There are the following advantages by thismethod.
1) The foreign body (Dust, etc) is less likely to adhere
to the power transmitting and receiving antenna, ascompared to a system for power transmission from theground (Therefore, it does not affect the transmission

30
25
20
15
10
5
0
0

50

100 150 200 250 300 350(a) voltage and current waveform of the input and output.

100
80
60
40
20
0
0

50

100 150 200 250 300 350(b) Energy capacity of EDLCs.

(c) Waveform(Voltage of EDLCs is at the 8V).
Fig. 16. Experimental results of the wireless charging to the EDLCs
The energy capacity of EDLCs is the wireless charging
in about 6 minutes.

5
4
3
2
1
0
Fig. 17. Comparison of the charging time for charging the 12.1kJ of
the proposed system and conventional system.
The proposed system can be charged rapidly thanconventional system.

efficiency)
2) The variation of transmission distance and thepositional deviation of between the transmitting andreceiving antenna can be reduced by the front wheel isfixed using bicycle parking stand.
3) It is low cost because the antenna and wiring are
not placed into the ground.

Transmitting
antenna

Receiving
antennaVII. CONCLUSION
This paper evaluated the energy capacity of EDLCs,
the antenna for wireless power transmission and thecharger, the interface converter for EDLCs in proposedsystem configuration. The proposed system was
experimentally verified as prototype.
The results showed that the short type as the antenna
for wireless power transmission can be to system ofhigher efficiency than the open type with the same size.
Also, the results showed that the total volume of theboost-type DC-DC converter is the smallest when the
output power is less than 1.1 kW. Therefore, in terms ofthe volume of the system at the minimum point, theboost-type DC-DC converter is suitable for the electric
assist bicycle. Also, the results showed that the proposedsystem can shorten to 1/4 the charging time of theconventional system.
In the future work, the proposed system will be
experimentally verified running test in an actual road.

To RF power
supply(a) Attached photo of the antenna.

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