Furukawa Battery

Technological Trends in Lead-Acid Batteries for Automotive
Applications
by Kenji Nakano *, Syuhei Takeshima *2 and Jun Furukawa *2

In recent years performance requirements with respect to lead-acid batteries forautomotive applications are beginning to undergo major changes as the vehicles
themselves change. In addition to the traditional “SLI functions” (starting, lighting and ignition),
there is also a need to provide advances in technologies for environmental improvement, batteries for accessory applications (power steering, stabilizers, etc.), new functions such as alternator
control for charging controls aimed at suppressing gaseous emissions and improving fuel economy, stopping of idling, and regenerative braking. Furukawa Battery is engaged not only in basic
technology and product development for lead-acid batteries to meet these requirements but
also in developing the “Ultrabattery”--a new type of lead-acid battery that will meet the sophisticated requirements of next-generation vehicles.

ABSTRACT1.

INTRODUCTION

More that 50 years have passed since the time whenlead-acid automotive batteries changed from 6-V to thecurrent 12-V system. During this period, car batteries
were optimized for SLI duty, and because they were
mounted in the engine compartment, they were required
to withstand high temperatures. Furthermore, when left in
the discharged condition, lead-acid batteries also undergo a process known as sulfation, in which lead sulfate
crystals, the product of discharge, grow larger, so that theprocess becomes irreversible and service life is shortened. Thus the batteries have to be kept at full charge
and need to have overcharge resistance. Progress has
been made in reducing maintenance by suppressing
electrolysis of the water in the electrolyte by using Pb-Ca
grid alloy instead of Pb-Sb alloy, and in improving thecorrosion-resistance of the positive grid. By these means
the life of lead-acid batteries has been extended, and topping of water has been made virtually unnecessary 1).
In recent years however, there have been significant
changes in the environment in which automotive leadacid batteries are used. In addition to the traditional “SLI
functions”, there is also a need to provide advances intechnologies for environmental improvement, batteries foraccessory applications (power steering, stabilizers,etc.)--the so-called “Micro-HEV” functions such as alternator control for charging controls aimed at suppressing
gaseous emissions and improving fuel economy, idling-stop and regenerative braking for the next generation of
automobiles 2)-4).
Up until the present, what is known as charge control
consisted of limiting the overcharging carried out to maintain a lead-acid battery at full charge, and by keeping theamount of charge low, to reduce fuel consumption to generate electricity. This means that there is a greater chanceof the battery’s state of charge being less than 100 %.
In addition, regenerative braking, in which the energy
previously dissipated by friction is converted to rotary
motion of the alternator thereby generating power tocharge the battery, is the function that has come to be
seen as the panacea for cutting fuel consumption. In
order for the electrical energy that has been generated to
be stored efficiently, it is necessary that the battery be
kept at a partial state of charge (PSOC) of from 60 % to
90 %. This means it must be operated under conditions
which in the past presented difficulties owing to the occurrence of sulfation. It is also necessary to develop diagnostic equipment that not only maintains the state of charge
under specified conditions but also, due to the need toguarantee starting performances after idling-stop shutdowns, is able to detect the battery’s failure status 3).
At Furukawa Battery, we are carrying out developmentof basic technology for lead-acid batteries suited to theseneeds, battery development, and development of status
detection technology, as well as pursuing the development of the Ultrabattery--a capacitor hybrid lead-acidbattery that provides for the sophisticated requirements of
the next generation of automobiles. In this paper we will
introduce some aspects of technologies addressing theneed for lead-acid batteries of increasing sophistication.

* Automotive Battery Div., Furukawa Battery*2 Research & Development Div., Furukawa Battery

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Technological Trends in Lead-Acid Batteries for Automotive Applications

DEVELOPMENTAL ACTIVITIESFor a start, Figure 1 is a graph showing the relationshipbetween typical failure modes of lead-acid storage batteries and life span in terms of number of cycles. These
modes are identified as premature capacity loss modes:
PCL-1, -2 and -3. In PCL-1, a non-conductive layer composed of lead sulfate and lead oxide forms at the interface
between the positive grid and the active material, leading
to capacity loss. In PCL-2, changes in the microstructureof the positive active material (PAM) cause softening andshedding, resulting in capacity loss. PCL-1 and -2 are
failure modes that have been clarified by using Pb-Ca
alloy to replace Pb-Sb alloy in the positive grid--the antimony-free effect, and understanding of the mechanismand measures to address it are progressing 5). PCL-3 is a
failure mode that first became evident in PSOC applications. It has been found that a fine lead sulfate film isformed on the surface of the negative electrode, inhibiting
ion movement and reducing capacity, and countermeasures are being developed worldwide 6).
Positive grid corrosion and growth are also failuremodes that are typical of lead-acid batteries, and by
developing new alloys, we have achieved improvements
in life span not equaled elsewhere.
In Section 2.1, therefore, we introduce technology for
improving positive grid durability that has attracted noticeworldwide, and in Section 2.2 we describe a uniqueapproach to status detection. In Sections 2.3 and 2.4 weintroduce lead-acid batteries for next-generation automobiles, incorporating advanced PCL-3 solutions. Then in
Section 2.5 we outline the developmental status of the“Ultrabattery” a capacity hybrid lead-acid battery that is
the focus of world attention.
Highly Corrosion-Resistant Alloy for Positive
Grid 7)-11)
The primary failure mode in conventional lead-acid automotive batteries has been positive grid corrosion, and thiscorrosion was particularly rapid in vehicles whose engine
compartments reached high temperatures and in regionsof high temperature. Corrosion also gave rise to growth-creep caused by the tensile strain of its products. The grid
plays a role as current collector, and a role as supporterfor the active material, so if grid corrosion proceeds it will

Capacity

2.1

PCL−1
interface
PAM/Grid

PCL−2
PAM
s oftening

PCL−3
SulfationGridcorrosion/ Growth

lead to an increase in electrical resistance due to a reduction in the current-carrying area and to a drop in discharge performance resulting from poorer contact
between the grid and the active material caused by grid
growth, and this will reach the service life limit. Higher
temperatures in the operating environment result in synergistic acceleration of corrosion and growth, and this will
have an important impact on battery service life.
As a means of countering high temperatures, alloys
with 0. 01 to 0. 03 wt % of Ag added to Pb, Ca and Sn
have come into wide use in other industrial countries
since the 1990s, and it is reported that as a result the concentration of Ag impurities in lead for recycling has risen
rapidly from about 15 ppm to around 50 ppm 12). Since not
only JIS but the industrial standards of many other countries have set 50 ppm as the upper limit of Ag concentration, there are fears for the effect on battery performance.
As an alternative, attention has been given to alloys withadded Ba, but these have not come into general use 13).
We have taken another look at Ba as an additive element that does not build up through recycling, and by
performing a large number of experiments and evaluations of alloys combining it and other additives, we havedeveloped a new alloy that in terms of corrosion-resistance and creep-resistance is far superior to what hascome before 7)-11). In the belief that this alloy will achievecustomer satisfaction in the 21st century, we have named
it the “C-21 alloy”.
We compared other representative positive grid alloys
and the C-21 alloy. Table 1 shows the composition of thealloy samples used in the comparison. Figure 2 shows
the result of constant-potential corrosion tests using
these alloys, and Figure 3 shows the result of high-temperature creep time-to-rupture tests (100°C, 16. 5 MPa,
constant load). It can be seen that the C-21 alloy exhibited clear superiority to the other positive grid alloys in
Table 1

Failure modes of lead-acid batteries.

Additive element

Conventional (a)High Sn conc. Pb-Ca alloy

C-21 (b)Pb-Ca-SnBa

Ag added (c)Pb-0.04Ca-0.60Sn

0.03 AgBa added (d)Pb-0.06Ca-1.60Sn

0.016 Ba

-

35
30
25
20
15
10
5
0

CyclesFigure 1

Composition of test alloys (wt %).
Base alloy

Corrosion weight loss（mg/cm2）

2.

Figure 2a）Conventionalb）C-21c）Ag addedd）Ba added

Test alloys

Corrosion weight loss for selected alloys at constant potential (720 hr, 1350 mV vs. Hg/Hg2SO4, 4.9M H2SO4, 60°C).

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Technological Trends in Lead-Acid Batteries for Automotive Applications

terms of corrosion-resistance and growth-resistance inhigh-temperature environments.
We then made a prototype battery using the C-21 alloy
and carried out the JIS shallow cycle life test at 75°C.
Figure 4 shows the result, and Figure 5 shows the appearance of the positive grids after the test. These results
demonstrate that the C-21 alloy is superior in terms ofcorrosion- and growth-resistance, and significantly
extends service life over batteries using the conventionalalloy. Batteries using the C-21 alloy were brought to aftermarket in October 2002, and since then have been widelyadopted by automobile manufacturers and overseas
recipients of technical assistance.
2.2 Status Detection Technology 3)
2.2.1 Methodology
Status detection for lead-acid batteries has two objectives: 1) diagnosis of failure status to determine the timeof battery replacement; and 2) determination of state ofcharge (SOC), state of health (SOH) and state of function(SOF) in Micro-HEVs and other next-generation vehicles.

Time to rupture（hour）

35
30
25
20
15
10
5
0

a）Conventionalb）C-21c）Ag addedd）Ba added

Test alloys

Figure 3

Voltage at 5 sec（V）

10.0

Time to rupture for selected alloys in constant creepload test at 100°C.

Test battery : 12V 90D26

9.5
9.0
8.5
8.0

a）Conventional alloyb）C-21 alloy

7.5
7.0

In 2) is incorporated the measurement of battery temperature, open-circuit voltage, charge and discharge current,
voltage, AC impedance, etc. , and methods of diagnosis
through simultaneous logging and correlation of thesevalues. Since lead-acid batteries used in Micro-HEVs
provide charging through regeneration while assuring
engine restarting performance, they need to be regulated
to a specific PSOC. They also need to provide precise
sensing, based on information on current, voltage, temperature etc. , of the frequent changes in battery status
while running. From the standpoint of the problem of electrolyte stratification that is inherent to lead-acid batteries,
the world’s manufacturers of automobiles, electricalaccessories and batteries are in the process of developing status detection algorithms and sensors, and some ofthese have appeared in commercial form.
2.2.2 Lead-acid Batteries with Built-in Tester Functions 14)
The diagnosis as to when to replace a car battery has
generally been made using testers installed in service stations, but more recently batteries have appeared thatthemselves incorporate a diagnostic function. Here we
present the world’s first practical car storage battery having a tester function built in.
In the past the main type used was the load tester,which determines the drop in voltage when subjected tointentional discharge, but that had the disadvantage that
diagnosis was not possible in the discharged state. It wasalso necessary to visit a service station equipped withsuch a tester. To eliminate these failings, the “FGUARD®”
lead-acid storage battery with built-in tester function,
capable of diagnosing the failure status any time, any
where, was introduced to the aftermarket in October of2003. The FGUARD incorporates in its lid diagnostic circuitry based on the conductance method, which wasdeveloped by America’s MIDTRONICS. Both SOC and
SOH are determined by pressing the tester button, whichthen measures open circuit voltage, battery temperatureand conductance under application of pulse current, andchecks against a control algorithm. The result is shownby LED display. Figure 6 shows the appearance of the
FGUARD battery. Even now, three years after it went onsale, the FGUARD is well regarded by customers.
2.3

0

1000

Figure 4

2000

3000

Cycles

5000

6000

7000 8000

Result of JIS shallow cycle life test at 75°C.

a）Conventional alloy

Figure 5

4000

Lead-Acid Battery for Idling-Stop Vehicles(Micro- HEVs) 15)
Idling-stop, the shutting down of an internal combustion
engine instead of idling, is a measure to counteract global
warming that has begun to find favor by some taxis andb）C-21 alloy

Appearance of positive grids after JIS shallow cycle
life test at 75°C.

Figure 6

Appearance of FGUARD battery.

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Technological Trends in Lead-Acid Batteries for Automotive Applications

sulfates, allowing the lead sulfates to be more readily
reduced 17), 18). In this idling-stop cycle life test, however,
the effect obtained was inadequate, so we attempted toachieve further suppression of negative electrode sulfation by the use of a new additive. When the test was
carried out on a D23 size lead-acid battery using the newadditive, negative electrode sulfation was significantly
suppressed and negative lug thinning was not observed.
The life span was 75 % greater than with the conventional
battery, but did not reach the 30,000 cycle target.
The significant factor in the life of this battery with suppressed negative electrode sulfation, however, changes
to softening of the active material of the positive electrode, so for further extension of cycle life, improvementof the positive electrode was required. Accordingly inorder to suppress this softening, we proceeded to optimize active material density and develop a new additive,
as well as to develop a new type of separator. The
improvement obtained by these means was confirmed by
a JIS heavy-load test that reflects the influence of softening, and life was extended by 35 %.
We therefore manufactured prototypes of D23 size
lead-acid batteries for idling-stop that implemented these
improvements. In terms of 5-hour rate capacity factor,
high-rate discharge characteristics at -15°C, and the like,
performance equivalent to conventional lead-acid batteries was maintained. Figure 8 shows the results of idlingstop cycle life tests. In the product under development,
the drop in electrolyte specific gravity was reduced, and it
was possible to extend the service life significantly beyond
the target of 30,000 cycles. Even in a tear-down examination after testing, virtually no negative electrode sulfationor negative lug thinning was observed. Currently this battery is undergoing in-service testing in taxis and ownerdriven cars, and as we accumulate practical data we arepreparing for commercial introduction.
2.4

Lead-Acid Batteries for High-Voltage Hybrid
Vehicles (Medium-HEVs)
Valve-regulated lead-acid (VRLA) batteries use a separator in the form of a mat made of fine glass fibers, and

50,000Charge45,000100 A

40,00014.0 Ｖ：Number of cycle

1.28

35,000

Cycle number

60 sec1.30：Specific gravity

30,0001.26

25,00020,0001.24

15,00010,0001.22

Electrolyte speciﬁc gravity

transit buses. It has been reported that in a transit bus
idling-stop test done by the Tokyo Metropolitan authorities, fuel economy was increased by 14 %. Idling-stop
was also proposed in 2002 by the Outline for Promotion
Effects to Prevent Global Warming as one of the measures in its transportation sector, and they have beenworking to extend its implementation among Japan’s
automobile manufacturers. Against this background methods for testing the cycle life of lead-acid batteries fromvehicles offering the idling-stop feature were investigatedjointly by the Society of Automotive Engineers of Japan(JSAE) and the Battery Association of Japan (BAJ), and in2006 was established as a Standard of the BAJ. (SBA
S0101 Idling-Stop Life Cycle Testing).
In conventional lead-acid car batteries, however, themain factor affecting the service life cycle was positiveelectrode failure, while in the idling-stop cycle life test,
reduction in the specific gravity of the electrolyte and sulfation of the negative electrode proceeded preferentially
and, when they become far advanced, the phenomenonof thinning of the lugs that serve as current collectors at
the negative electrode appeared. We at Furukawa Battery
have been able to clarify the reasons for the thinning ofnegative lugs, and by controlling negative electrode sulfation and further improving the positive electrode, havedeveloped a battery with greatly extended idling-stop
cycle life.
Figure 7 shows the cycle life test profile for idling-stopby SBA S0101. This charge/discharge profile is repeatedfor 3,600 cycles, after each of which it is left 40 to 48
hours, after which the cycle is repeated. Since the testing
environment is a 25°C air chamber that limits the impinging of wind on the battery, battery temperature rises due
to Joule heating. The target cycle life with this test is
30,000 cycles or more.
When the test was carried out on a conventional leadacid battery of JIS D23 size, the life was 15,000 cycles orless. The failure mode was negative electrode sulfation,
but negative lug thinning was also observed. Methods ofsuppressing negative electrode sulfation have included
increasing the amount of carbon additive to the negativeelectrode to form a conductive network around the lead

5,000

45 A

0

59 sec1.20

Conventional

300 A（1sec）

New additive
for NAM

Improvement

Discharge

Figure 8
Figure 7

Cycle life test profile for idling-stop by SBA S0101.

Improvements in idling-stop cycle life and electrolyte
specific gravity for the product under development.

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Technological Trends in Lead-Acid Batteries for Automotive Applications

acid battery for motor cycles, the battery developed here
had a life span about four times longer.
2.5 Development of the Ultrabattery 20)-23)
In Micro-HEVs having the idling-stop and discharge regulation features, and in mild hybrid vehicles and high-voltage type hybrid vehicles, which also have regenerative
braking and power-assisted functions, the state of chargeof the battery is constantly kept in the partial state. They
are also subjected to large-current pulse charge and discharge, and other extreme conditions of use that were notpreviously encountered 3), 4), 6), 9).
In these applications, lead-acid batteries are beset by
the inherent problem that lead sulfate recrystallization
cannot be avoided, but charge/discharge power characteristics must also be improved. As a means of improving
such performance, proposals have been made for battery
capacitor modules in which lead-acid batteries and highcapacitance electric double-layer capacitors are connected in parallel 24)-26). However there are a number of problems with respect to configuring practical battery capacitor modules, including the high cost of electric double
layer capacitors, their low volume energy density, and thefurther cost imposed by the need for control circuitry forhybridization with the batteries.
In order to achieve a complete solution to the PSOC
issue, Furukawa Battery, working together with Australia’s
CSIRO, has developed the Ultrabattery--a hybrid leadacid battery in which battery and supercapacitor are integrated at the electrode plate level.

350

Input（％）

300
250
200
150
100

for HEVs

50
0

for motor cycles

0

20

40

60

80

100

DOD（％）

Figure 10 Input power characteristics.

110

Voltage（％）

since the electrode plates are kept under high compression while the electrolyte is limited to the amount impregnated in the electrode plates and separator, they are lightin weight and have superior charge/discharge characteristics. For this reason they are better suited than wet-typebatteries to vehicles with idling-stop (Micro-HEVs) andmild hybrid vehicles (Mild-HEVs). Furukawa Battery is
also proceeding with the development of long-life 12and 36-V VRLA batteries for this type of application. Herewe will present an outline of the development of the
FT7C-HEV VRLA battery that is installed in the Suzuki“Twin”, a high-voltage type hybrid sub-compact vehicle
that went on sale in January 2003. Figure 9 shows theappearance of the battery. It is a 12-V battery having a
6.3-Ah (3 HR) capacity, featuring compact size, high-density structure, excellent input-output performance, extended PSOC cycle life, and so on. A battery pack, consistingof 16 of these connected in series, is installed in the Twin.
To allow this battery to be mounted on a narrow base, adesign was adopted that minimized the space inside thebattery, achieving a 10 % reduction in volume compared
to a VRLA battery of the same capacity for motor cycles.
The terminals used a high clamping strength structure toprovide reliability when connected.
For the positive electrode plates, an alloy of high corrosion resistance was used for the grid, and active material
density was maximized with respect to charge/discharge
profile and depth of discharge (DOD) for the Twin.
Further, since in high-voltage type hybrid vehicles thebattery generally undergoes frequent repetition of largecurrent charge and discharge, lead-acid batteries suffer adrop in charge/discharge performance and regenerative
charging performance due to sulfation of the negativeelectrode. Accordingly we revisited the question of additives for the negative electrode active material, and by
maximizing the amounts of added carbon, lignin, and barium sulfate, we managed to reduce sulfation. Also in battery packs, each individual battery needs high reliability,
so that the role of the separator is important. Thus reliability was enhanced by adopting a separator having a levelof resistance to short-circuiting that was greatly improvedby the optimization of fine glass fiber and additives.
Figure 10 compares input power characteristics for thebattery under development here and a VRLA battery for
motor cycles. At a DOD of 50 %, the battery developed
here showed more than double the input power of thebattery for motor cycles. Figure 11, for its part, shows theresults of the PSOC cycle life test. Compared to the lead-for HEVs

100
90
for motor cycles

80
70

100

200

300

400

Cycle（％）

Figure 9

Appearance of the FT7C-HEV battery.

Figure 11 Cycle life test results.

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Technological Trends in Lead-Acid Batteries for Automotive Applications

Capacitor electrodewithout additives

15

10
Capacitor electrodewith additives
5
LA negative0- 0.90- 1.00- 1.10- 1.20- 1.30- 1.40

Negative - plate potential / V

Figure 13 Tafel plots of hydrogen gas evolution at capacitor
electrodes with and without additives, and at leadacid negative electrode.

160

Discharge power40

with supercapacitor

120

30
without supercapacitor

80

20

40
Regenerative power

0

0

20

10

Operational range

40

60

Regenerative power（w）

2.5.3 Ultrabattery Performance
We constructed a prototype 2- V valve- regulated
Ultrabattery, and compared it with a conventional VRLA
battery in terms of input and output performance and ofcycle life characteristics.
Figure 14 shows the input and output characteristics. In
mild hybrid vehicles the batteries are used at DOD ranging

20

H 2 current density/mA g -1

2.5.2 Capacitor Electrodes
Since the capacitor electrodes are made of porous carbon, there is a need to suppress evolution of hydrogengas during charging, and additives for this purpose weredeveloped. Figure 13 shows the results of measurementsof the amount of hydrogen evolved as potential was varied. In contrast to the capacitor electrode without additives, the amount of hydrogen evolved at the capacitor
electrode with additives was substantially the same as at
the lead electrode.

from 30 % to 70 %, but the Ultrabattery shows outstanding
input and output characteristics over a wide DOD range,demonstrating its suitability for PSOC applications.
We carried out cycle life tests at a PSOC based on areliable, highly optimized lead-acid battery (RHOLAB)
profile that simulated a combination of high-speed andhill-climbing running conditions for a mild hybrid vehicle.
Figure 15 shows the results. It can be seen that the life
span achieved was outstanding--about four times longer
than a lead-acid battery.

Discharge power（w）

2.5.1 Ultrabattery Structure
Figure 12 shows the structure of the Ultrabattery, which
comprises a lead-acid battery and asymmetrical capacitor combined in a single cell. The lead-acid battery has apositive electrode of lead dioxide and a negative electrode of spongy lead. The asymmetrical capacitor, for its
part, has the same positive electrode as the battery--leaddioxide, and a negative electrode of porous carbon. Sinceboth have a common positive electrode, the lead negativeelectrode and the capacitor negative electrode can beconnected in parallel and housed in the same cell withthe positive electrode. This results in the capacitor electrode bearing a portion of the load of the lead storage
electrode. This holds promise that it can be used in applications involving PSOC characteristics and large-current
charge/discharge, which could not heretofore be adequately addressed by lead-acid batteries. What is more,since the Ultrabattery requires no special electronic control circuitry it has advantages in terms of cost.

0
100

80

DOD（%）‒+

Separator

PbO2
PbO2

Capacitor
electrodePb

Lead-acidbattery+iFigure 14 Relationship between depth of discharge and charge/
discharge power.

1200

i1

i

‒

Pb
i2

Asymmetrical
capacitor

Capacitor
electrode

PbO2

Separator

Ultrabattery

Figure 12 Structure of the Ultrabattery.

Ultrabattery

Ultracells

1000

No. of cycles

‒

+

800
600
400
200
0

Control battery
Control cells

C1

C2

CB

UC1

UC2

UC3

UB

Figure 15 Comparison of life spans under RHOLAB profile.

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Technological Trends in Lead-Acid Batteries for Automotive Applications

As has been confirmed above, the Ultrabattery is clearlysuperior to lead-acid batteries at PSOC, and has gained
worldwide notice for the promise it holds for use insophisticated next-generation vehicles, for which leadacid batteries have heretofore been considered inapplicable.

3.

CONCLUSION

At a time when measures to counter global warming havetaken on new urgency, automobiles are beginning to faceemission controls of unprecedented severity. We areindebted to the automobile industry for allowing us toattain a leading position for quality and technology as asupplier of automotive lead-acid batteries. In addition to
the advanced technology for lead- acid batteries
described in this paper, we are committed to further technological development, and by curtailing the emission ofgreenhouse gases from next-generation vehicles, we willcontribute to the continued development of society.

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Capacitor, FB Technical News, 59 (2003), 22. (in Japanese)25) H. Ollhauser: “Alcoa-Audi micro-hybrid architecture”, 6thInternational Advanced Automotive Battery Conference, (2006)
Baltimore.
26) A. Schwake: “EC-funded “SUPERCAR”: Ultracapacitor modules
for mild hybrid applications”, 2nd International Symposium on
Large Ultracapacitor (EDLC) Technology and Application, (2006)
Baltimore.

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