TECHNICAL REPORT

Development of an Auxiliary Power Supply System for ElectricPower Steering and a High-heat Resistant Lithium-ion Capacitor
T. MIO Y. KOMATSUBARA N. OHMI Y. KIMOTO K. IIZUKA T. SUGANUMA S. MARUYAMA K. NISHI

To motorize the steering system of a full-size vehicle equipped with a 12 V power supply, JTEKT is developing
an auxiliary power supply system for electric power steering (EPS) using a lithium-ion capacitor. In addition, as aresult of working on expanding the temperature range of the lithium-ion capacitor in order to mount the system in afull-size vehicle free of a cooling-heating system, the world's first operating temperature range of -40 to 85 degrees
C was achieved. The developed capacitor operates stably even in high temperatures exceeding 100 degrees C by
restricting the upper limit of operating voltage.
Key Words: energy storage device, lithium-ion capacitor, heat-resistance, low temperature output, EPS

1. Introduction
Advanced Driver Assistance System (ADAS)
and autonomous driving (AD) are two major topics
concerning current and next-generation automobiles.
The 2018 Public-Private ITS Initiative/Roadmaps issued
by the Prime Minister’s Office set a goal of launchingvehicles with autonomous driving level 4 as defined by
SAE International J30162 (Sep 2016) on the market inlimited scenes such as expressways by around 20251), 2).
Even if some form of failure occurs in a vehicle, theAD function must continue to the extent that the vehicle
can be guided in the front/back, left/right directions3)-5).
As such, it is essential that functional safety is improvedin the steering systems equipped in vehicles with high AD
levels. Particularly in regards to vehicles with AD level4, where a human does not intervene with driving, there
is a need for the steering system to continue operatingeven in the event of power failure/interruption, etc. during
vehicle travel, therefore improving the functional safetyof current steering systems is essential6), 7).
Moreover, in order to realize ADAS and AD functions,
it is desirable to equip vehicles with electric power
steering that enables steering control from an external
ECU. However, hydraulic power steering systems areequipped in vehicles weighing 3 tons or more and large
vehicles with a steering rack axial force requiring 14
kN or more. If electric power steering (EPS) is used inthese large vehicles, during maneuvers such as stationary
steering and sudden steering, it would not be possible
to supply sufficient power demanded by the EPS, evenfor a brief period, from a 12 V vehicle power supply.

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)

In other words, it would exceed the maximum current
able to be supplied from a lead storage battery, therefore
creating a phenomenon where the steering wheel would
be heavy during maneuvering. This phenomenon iscalled “insufficient assistance,” and in order to solve
this phenomenon, JTEKT is developing an auxiliary
power supply system for EPS utilizing a Lithium-ion
capacitor for the energy storage device8)-10). This system
has high function expandability and can contribute to theaforementioned improvement of EPS’s functional safety.
This is explained in detail in the next section.
Moreover, in order to make such a system low costand more freely able to be equipped on a vehicle, itwould be necessary to omit a cooling system. The
operating temperature range of a conventional Lithiumion capacitor was narrow at around -20 to 60℃, therefore
non-conforming with vehicle temperature requirements
and making the use of a cooling-heating system essential.
As such, JTEKT also independently endeavored to
expand the operating temperature range of Lithium-ion
capacitors.

2. Auxiliary Power Supply System for EPS
2. 1 Concept
The auxiliary power supply system for EPS wasdeveloped with the aim of alleviating the power
insufficiency problem that occurred when an EPS is
mounted in a large vehicle, without the need to makechanges to either the EPS or 12 V vehicle power supply.
Figure 1 shows an external view of a prototype forthe developed system. As Fig. 2 shows, this system is65Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

a separate device installed between the 12 V vehicle
power supply and EPS system, and is configured from anenergy storage device and a charge-discharge controller
controlling the charging and discharging of the energy
storage device. In situations where the vehicle power
supply and EPS have low power consumption, poweris stored in this energy storage device, then used insituations where the EPS requires a high amount ofenergy, such as during stationary steering. The chargedischarge controller and energy storage device musthave the various reliable qualities demanded of a vehicle(mechanical stress, electrical stress, operating temperature
range).

requires power greater than the 12 V vehicle power
supply can provide. By adding the Lithium-ion capacitor
voltage to the 12 V vehicle power supply and increasing
the supply voltage, the EPS can be made high output (x ofFig. 3). If the voltage of the Lithium-ion capacitor drops,
the charge circuit is actuated and charging is performed
from the 12 V vehicle power supply (y of Fig. 3).

2. 3 Circuit Operation
Figure 4 shows the detailed circuit operation of thissystem. When travelling straight, etc. in the case that the
EPS power consumption is small, the FET1 and FET2
will close, and the EPS will only operate by the 12 V
vehicle power supply. In situations such as stationary
steering and rapid steering when the EPS requires alarge amount of power, FET1 and FET3 will close, andthe Lithium-ion capacitor will be connected in series tothe 12 V vehicle power supply. The cell voltage of the
Lithium-ion capacitor will be added to the 12 V vehicle
power supply, and as a result of power supply voltage
increasing, it will be possible to supply the EPS with alarge amount of power. Increasing the number of Lithiumion capacitors in series will increase the maximum output
power of the system. If the system operates and thevoltage of the Lithium-ion capacitor drops, FET4, FET5,
and the coil will be used to recharge the Lithium-ion
capacitor.

2. 2 Configuration and Overview
Figure 3 is a block diagram of an auxiliary power
supply system for EPS. This system is installed between a
12 V vehicle power supply [A] and EPS system [B]. Withconsideration to charging/discharging speed, repetitive
charging/discharging life, energy density, safeness andother factors, a Lithium-ion capacitor [C] was adopted forthe energy storage device.
The Lithium-ion capacitor and 12 V vehicle power
supply are connected by a charge circuit [D] anddischarge circuit [E]. The central processing unit (CPU)of the charge-discharge ECU [F], monitors the statusof the EPS and 12 V vehicle power supply and actuates
the discharge circuit if a situation arises where the EPS

Lithium-ion
capacitor[A]
Vehiclepower supply

Charge[D]
Chargecircuit[C]

Discharge[E]
Dischargey CPU xcircuit[F]

Charge-discharge ECU[B]
EPS
systemFig. 3 Block diagram of an auxiliary power supply system
for EPS

Fig. 1 External view of an auxiliary power supply system for
EPS (W137×D83×H80mm)

Developed system
Energy storage
deviceCharge-Discharge
Controller

Electric Power
Steering

Alter- 12 V
nator Battery
12 V Vehicle power supply

Fig. 2 System configuration

66

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

Discharging
circuit+

Lithiumion
Capacitor

BackupFET3(+6 V)

FET2

FET4
FET6

FET5

FET1
12 V
12 V
power
supplyHigh load[Developed system]
Charging
circuit

NormalEPS

System failure

Fig. 4 Circuit operation details

Positive tab
Negative tab（Al）

Li-foil

Laminate film

Li metal crimped
to Cu foil

（Resin film / Al）

Positive electrode（Cu+Ni plating）

Active carbon /Conductive assistant /Binder /Punching AI foil

Separator

Negative electrode
Electrolyte
Lithium inorganic salt
Carbonate solvent

Graphite /Thickener/Binder /Punching Cu foil

Layered product
Positive electrode /Separator / Negativeelectrode / Li-foil

DetailFig. 5 General structure of a lithium-ion capacitor

Every Lithium-ion capacitor cell has an average
operating voltage of 3 V, therefore 4 capacitors in series
will create a 12 V power supply in the system internally.
If a power failure occurs during travel, EPS will continue
being driven by the 12 V power supply configured in thissystem due to FET3 and FET5 closing. This is a usefulsystem for the realization of AD level 4, which requires
the full fail-operational of EPS including power failure.
Even in the unlikely event that some kind of problem
occurred in the circuit or FET, the system can continue
to drive the EPS with a bypass circuit due to FET6
closing. A feature of the system developed by JTEKT isthe high expandability in a single circuit which enables amyriad of functions to perform, and the effectiveness of
this system was verified in an actual vehicle evaluation,
however a Lithium-ion capacitor with high heat resistanceis necessary to equip this system on a vehicle without
using a cooling system. As such, we endeavored tosuppress the performance deterioration of the Lithium-ion
capacitor in a high temperature environment.

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)

3. Lithium-ion Capacitor
3. 1 Structure and Features
The Lithium-ion capacitor is a new energy storage
device that has characteristics midway between a lithiumion secondary battery and electric double layer capacitor,and comprises a lithium-ion secondary battery’s graphite
negative electrode and electric double layer capacitor’s
activated carbon positive electrode11), 12) (Fig. 5).
While retaining the features of an ability to instantly
supply/regenerate a large amount of power and excellent
repetitive charging/discharging resistance, which are
the merits of an electric double layer capacitor, theLithium-ion capacitor is an energy storage device with avolume energy density that is three to six times greater,
therefore it is anticipated to be useful in various industrial
domains13). However, one issue that must be addressed inorder to expand its scope of application, is the need for awider operating temperature range, and it is essential thatthe performance deterioration occurring at temperatures
above around 60℃ is suppressed.

67Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

3. 2 Grasping Deterioration Outside of Operating
Temperature Range
Through a bibliographic survey and verification test,
we elucidated the cause of performance deterioration
in a Lithium-ion capacitor as a result of what kind of
phenomenon occurring during operation at temperatures
both higher and lower than the operating temperature
range of a conventional Lithium-ion capacitor.
In the higher temperature range, irreversible increase ininternal resistance and irreversible decrease in capacitanceoccurred due to thermal decomposition of the electrolyte.
Decomposition gas of the electrolyte caused the capacitor
to expand, and ultimately led to the cell rupturing (Fig. 6).
It was observed that the decomposition of the electrolyte
occurred due to the thermal decomposition of lithium
hexafluorophosphate (LiPF 6), which is an electrolyte
commonly used in Lithium-ion capacitors and lithium-ion
secondary batteries14).

uses a chemical reaction during charging/discharging.
The speed of a chemical reaction depends on the ambient
temperature, therefore if the temperature drops, the outputof the Lithium-ion capacitor will also drop.
From the above study results, we confirmed that amajor point in broadening the operating temperature
range of a Lithium-ion capacitor was improving theelectrolyte.

3. 3 Implementation Items
Table 1 shows the implementation items for
improvement. We changed the electrolyte (lithium
hexafluorophosphate) that was the cause of theelectrolyte’s thermal decomposition at high temperatures
to an imide-type lithium compound with excellent heatresistance. Furthermore, we revised the mixing ratio of
the electrolyte solvent, etc. to a composition unique to
JTEKT whereby it does not freeze at -40℃, nor boil at
100℃. We also used a unique improvement technique tocontrol the compatibility of capacitor materials.

4. Evaluation Test Conditions
4. 1 Manufacturing of Lithium-ion Capacitors(85℃, 100 hours)

Fig. 6 Lithium ion capacitor stored in a high temperatureenvironment

In the lower temperature range, irreversible output
reduction occurs in line with the increase in internal
resistance, and once a temperature of -20℃ or below
is reached, the electrolyte freezes, making it no longer
possible to charge or discharge the Lithium-ion capacitor.
The negative electrode of the Lithium-ion capacitor isthe same material as a lithium secondary battery and

Making a positive electrode by applying activatedcarbon to aluminum foil and a negative electrode by
applying graphite to copper foil, we alternately layered
positive electrodes and negative electrodes via a separator.
With the below three criteria, we created a laminatedLithium-ion capacitor with an operating voltage of 2.2 to
3.8 V and a capacitance of 500F.
(1) Regular electrolyte (1.0 mol/L LiPF 6 / Ethylene
carbonate (EC): Ethyl methyl carbonate (EMC):
Dimethyl Carbonate (DMC) = 3: 4: 3)(2) New electrolyte (high heat-resistant electrolyte/nonaqueous solvent with a high boiling point and low

Table 1 Implementation items

Heat-resistance
improvement

Technical challenge

Implementation items

Prevention ofelectrolyte liquid decomposition

Adoption of high-heatresistant electrolyte salt

Prevention ofelectrolyte liquid boiling

Adoption ofhigh boiling-point organic solvent

JTEKT original improvements

Improvement of
low temperature output

Prevention ofelectrolyte liquid freezing

Prevention ofinternal resistance rising

68

Adoption of
low freezing point organic solvent
Optimization oforganic solvent mixing ratio
Aﬀecter identiﬁcation
Changing of positive andnegative materials

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

2. Change only electrolyte solution

3. Electrolyte solution change+ JTEKT proprietary method

80％

Threshold of IEC 62813-2015
60％

1. Conventional capacitor

40％
20％

0

200 400 600

800 1 000 1 200 1 400 1 600

85 degree C, 3.8 V float charging time, h

80％

GOOD

100％

100％
Internal resistance increase rate

GOOD

Capacity retention rate

120％

60％
Threshold of IEC 62813-2015

40％
20％
0％

0

200 400 600 800 1 000 1 200 1 400 1 600

85 degree C, 3.8 V float charging time, h(IEC 62813-2015 compliant)Fig. 7 Test result of float charging at 85 degree C

freezing point)(3) New electrolyte + JTEKT’s unique improvement

4. 2 Heat-resistance
The high temperature durability test for the Lithiumion capacitor was a 3.8 V float charging test carried out
in an ambient temperature of 85℃, as per the stipulationsof IEC 62813-2015 [High temperature continuous rated
voltage application test] of [Lithium-ion capacitors foruse in electric and electronic equipment - Test methods
for electrical characteristics]. This same standard requiresthat after 1 000 hours of float charging, the internal
resistance increase ratio is 50% or less, and the static
capacitance decrease ratio is 20% or less.

4. 3 High Current Charge/discharge Resistance
We investigated the durability of the developed
Lithium-ion capacitor against large current charging/discharging. We confirmed performance deterioration of
the capacitor by a charge/discharge cycle test where wechanged the maximum temperature from 85 to 110℃
temperature in increments of 5℃. By gradually reducing
the capacitor’s upper limit voltage from 3.8 to 3.6 V,
we performed the test approximately 10 000 times at acharging/discharging rate of approximately 900C. The
charging/discharging mode is the CC (rated current) - CV(rated voltage) mode.
After the cycle test, we measured internal resistanceand discharging capacitance in line with IEC 62813-2015,
and set a performance deterioration of around 10% or
higher as the pass/fail threshold for the test.

4. 4 Low Temperature Characteristic
We investigated the internal resistance increase rate
in a low temperature environment based on the internal
resistance at 25℃. The measurement method usedconformed with IEC 62813-2015. We investigated thecharge/discharge cycle performance of the developed
Lithium-ion capacitor at -40℃. The operating voltage

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)

range was set from 2.2 to 3.8 V, CC-CV charging/discharging mode was used, and the charging/discharging
rate was made 85C. For confirmation of Lithium-ion
capacitor performance, after returning the workpiecesurface temperature to 25℃, we measured internal
resistance and discharging capacitance under conditions
conforming with IEC 62813-2015.

5. Various Evaluation Results
5. 1 Heat-resistance
Figure 7 shows the results. For the conventional
Lithium-ion capacitor, the deterioration of both thecapacitance retention rate and internal resistance increase
rate are significant. These values fell below the IEC
standard’s requirements approximately 100 hours after
the test was begun. Gas was generated due to thermal
decomposition of the LiPF6 in the electrolyte, and the test
was terminated due to significant cell expansion in the
Lithium-ion capacitor.
In a capacitor where only the electrolyte waschanged, the capacitance retention rate improved andthe cell expansion problem was alleviated. However, no
improvement was observed insofar as internal resistanceincrease rate. In a capacitor with a different electrolyte
and JTEKT’s unique improvement measure implemented,
the capacitance retention rate and internal resistanceincrease rate requirements were satisfied even after 1 000
hours of float charging tests, as specified by the IEC
standard15). From the above results, we elucidated that,in order to increase the heat resistance of the Lithiumion capacitor, it was necessary to not only improve theheat resistance of the structural material, but also control
the compatibility of materials so that each material could
sufficiently perform.

69Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

Table 3 Capacitance reduction behavior

Capacitor upper limit voltage, V

Capacitor upper limit voltage, V

3.60

3.60

2.2%

3.7%

5.1%

0.7%

−0.2%

3.0%

2.1%

0%

2.1%

3.5%

3.5%

7.7%

Mesuring method: IEC 62813-2015

Internal resistance rise multiple（Reference value: 25 degree C）

x40

x30
Developed capacitor
Conventional capacitor

x20

x10

x0
40

20

0

20

40

Temperature, degree C

Fig. 8 Measurement result of internal resistance at low
temperature

5. 2 High Current Charge/discharge Resistance
Tables 2 and 3 show the results. By changing thelimitation on the upper limit voltage of the Lithium-ion
capacitor from 3.8 to 3.6 V, even in a 100℃ environment,
the change in Lithium-ion capacitor performance
was very minimal at less than 5%. We elucidated that
improving the heat resistance of the Lithium-ion capacitor
would contribute to suppressing the deterioration causedby self-heat generation upon repeated large current
charging/discharging.

110

0.3%

105

6.8%

3.0%
2.3%

−0.3%

100

1.8%

3.75

1.6%

−0.2%

0.7%

95

3.1%

3.70

0.8%

0.5%

0.2%

0%

90

5.7%

3.65

2.0%

0.9%

1.4%

0.6%

85

105

7.1%

3.80
Test atomospheric temp., deg. C

110

24.6%

100

3.75

95

3.70

90

3.65

85

Test atomospheric temp., deg. C

Table 2 Internal resistance increase behavior

0.0%

0.8%

0.5%

1.4%

3.80

0.2%

Mesuring method: IEC 62813-2015

resistance of the conventional capacitor at -20℃.
Figure 9 shows the results of a charge/discharge
cycle test conducted at -40℃. A 200 000 cycle test was
conducted for the developed capacitor however the
performance only deteriorated slightly. We confirmedthat the merit of the Lithium-ion capacitor, a large current
supply, was possible not only in high temperatures, butalso in low temperatures.

6. Conclusion
The auxiliary power supply system newly developed
for EPS, makes it possible to alleviate the problem ofpower insufficiency when equipping an EPS on a large
vehicle, without the need to strengthen the 12 V vehicle
power supply. Moreover, because the developed system
has excellent function expandability and also functions as
a redundant power supply during vehicle power outages
or temporary power outages, we believe it will contributeto the realization of a level 4 advanced autonomous
driving system which demands high functional safety.
Moreover, the Lithium-ion capacitor with a greater
operating temperature range is helpful in realizing asystem that can be equipped on a vehicle without using a
cooling-heating system. We anticipate it will be used ona broader scale, in not only the auto industry, but also avariety of industrial domains.

5. 3 Low Temperature Characteristic
Figure 8 shows the measurement results for internal
resistance. In contrast to the conventional Lithiumion capacitor, whereby the electrolyte freezes at -30℃
or lower making charging/discharging impossible, theelectrolyte of the developed capacitor did not freezeeven at -40℃, and charging/discharging was possible.
The internal resistance of the developed capacitor at atemperature of -40℃ was practically equal to the internal

70

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

Internal resistance increase rate, %Capacitance reduction rate, %

100
80
60
40
20
0

0

50 000 100 000 150 000 200 000
Charge/discharge cycle, times

100
80
60
40
20
0
0

50 000 100 000 150 000 200 000
Charge/discharge cycle, times(-40℃, 85C cycle test)

Fig. 9 Test result of charge-discharge test at low temperature

References
1)

2)

3)

4)

5)

6)

7)

8)

9)

Japan prime minister’s office: Advanced information andtelecommunications network society promotion strategy
headquarters (IT Comprehensive Strategy, Headquarters),
Public-private ITS concept road map 2018, http://www.
kantei.go.jp/jp/singi/it2/kettei/pdf/20180615/siryou9.pdf(in Japanese).
SAE International: Automated Driving - Levelsof Driving Automation are Defined in New SAE
International Standard J3016 (2016), Available online:
http://www.sae.org/misc/pdfs/automated_driving.pdf.
International Organization for Standardization (ISO):
Road Vehicles‒Functional saFETy, ISO 26262, ISO
Geneva, Switzerland (2011).
International electrotechnical commission (IEC):
Functional saFETy of electrical/electronic/programmableelectronic saFETy-related systems, IEC 61508-6 Annex
B, IEC Geneva, Switzerland (2010).
T. KANEKO, H. NAKAMURA, R. FUKASAWA:
SaFETy design for Automated Driving Systems, JARI
Research Journal (2016) 20161003.
M. MURATA: JTEKT’s perspective on steering systems
for autonomous driving, Automotive Mirai Summit
Spring, Tokyo, Japan 11 April (2019) (in Japanese).
M. OHASHI: 2-Drive motor control unit for electric
power steering, Denso Technical Review, Vol. 21(2016),
48-53 (in Japanese).
S . S H I N O DA , M . H I G A S H I , T. S U G I YA M A :Development of Auxiliary Power Supply System forElectric Power Supply Steering, JSAE Annual Congress(Spring) Proceedings (2014) 20145065.
F. SATO, M. HIGASHI, T. SUGIYAMA: Developmentof Auxiliary Power Supply System for Electric Power
Supply Steering, JSAE Annual Congress (Spring)
Proceedings (2015) 20155307.

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)

10) T. HIBINO, F. SATO, S. SHINODA, T. SUGIYAMA:
Auxiliary Power Supply System for Electric Power
Supply Steering, JSAE Annual Congress (Spring)
Proceedings (2018) 20180195.
11) H. TANIZAKI, K. MATSUI, S. TASAKI, N. ANDO,
Y. HATO, H. SHIBUTANI: Development of Lithiumion Capacitor, The 47 th Battery symposium in Japan
proceedings (2006) 3E04.
12) S. TASAKI, H. TANISAKI, R. MIYAKAWA, A.
SHIRAKAMI, H. TAGUCHI, K. MATSUI, N. ANDO,
O. HATOZAKI, Y. HATO, H. SHIBUYA: Developmentof Lithium-ion Capacitor, The 47th Battery symposium inJapan proceedings (2006) 3E05.
13) R. MIYAKAWA, Y. HATO, M. INAGAWA, K. INOUE,
D. SEKI: Development of high-power Lithium-ion
Capacitor, NEC Technical Report, Vol. 63, No. 4 (2010)70-74.
14) D. AURBACH, A. ZABAN, Y. EIN-ELI, I. WEISSMAN,
O. CHUSID, B. MARKOVSKY, M. LEVI, E. LEVI,
A. SCHECHTER, E. GRANOT: Recent studies on thecorrelation between surface chemistry, morphology,
three-dimensional structures and performance of Li and
Li-C intercalation anodes in several important electrolyte
systems, Journal of Power Sources, Vol. 68, Issue 1(1997) 91-98.
15) Development of high heat-resistant Lithium-ion
Capacitor, JSAE Annual Congress (Spring) Proceedings(2018) 20180194.

71

Development of an Auxiliary Power Supply System for Electric Power Steering and a High-heat Resistant Lithium-ion Capacitor

T. MIO*

Y. KOMATSUBARA*

Y. KIMOTO*

K. IIZUKA*

S. MARUYAMA*

K. NISHI**

72

N. OHMI*T. SUGANUMA*

Business Reform Electrical Power Storage Device Dept.

JTEKT ENGINEERING JOURNAL English Edition No. 1017E (2020)