AU2016200589B2 - Lead-acid battery - Google Patents

Abstract

OF THE DISCLOSURE A lead-acid battery includes a negative electrode material containing graphite or carbon fiber. The ratio of the mass of the negative electrode material to the mass of a positive electrode material is 0.62 or more. [1/4] FIG. 1 16 18 16 16 1 14:> FIG. 2 A~ x50time 1cycle ..... tDisharg,N ...... I 5times

Description

[1/4]

FIG. 1

16 18 16 16

1

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FIG. 2

A~ x50time 1cycle .....

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LEAD-ACID BATTERY FIELD

The present invention relates to a lead-acid battery, and particularly

relates to a lead-acid battery which is used in an environment involving deep

discharge.

BACKGROUND

With the advent of idling-stop vehicles, lead-acid batteries have been

deeply discharged more often than before. For example, lead-acid batteries

of idling-stop vehicles are based on the premise that they are used in a

partial state of charge (PSOC). Lead-acid batteries for cycle applications

like those for forklift trucks have been used at a deep depth of discharge

(DOD). When a lead-acid battery is used in a partial state of charge, its life

is reduced due to accumulation of lead sulfate in a positive electrode or

sulfation of a negative electrode. In the partial state of charge, agitation of

an electrolyte solution by gas evolution is insufficient, and thus the

electrolyte solution is easily stratified. This further reduces the life of a

lead-acid battery.

Meanwhile, when the lead-acid battery transitions into an

overdischarged state from the partial state of charge because, for example,

the vehicle is left unattended for a long period of time, metal lead passes

through the separator, and thus a permeation short circuit, in which both

positive and negative electrode plates are short-circuited, easily occurs.

The concentration of sulfate ions in the electrolyte solution decreases due to

overdischarge, and correspondingly, the concentration of lead ions in the

electrolyte solution increases. The lead ions are reduced on the negative electrode plate during charge, and metal lead dendrite grows through pores inside the separator, and thus passes through the separator to short-circuit the positive electrode plate and the negative electrode plate.

The applicant proposed improvement of the life of a lead-acid battery

in the PSOC by including graphite in a negative electrode material. For

example, Patent Document 1 (WO 2011/90113) discloses that a negative

electrode material is made to contain 0.02 to 2.20 mass% of graphite, 0.5

mass% of barium sulfate, and 0.02 to 2.20 mass% of carbon black. Patent

Document 2 (WO 2011/52438) discloses that a negative electrode material is

made to contain 0.5 to 3.0 mass% of expanded graphite and 0.6 mass% of

barium sulfate. Among documents other than those by the applicant, for

example, Patent Document 3 (JP5584216 B) discloses that a negative

electrode material is made to contain 1 to 3 mass% of graphite, 0.8 mass% of

barium sulfate, and 0.1 to 2 mass% of carbon black.

Patent Document 4 (JP5596241 B) discloses that a mass ratio MN/MP

is in a range of from 0.70 to 1.10 where Mp is the mass of the positive active

material, and MN is the mass of the negative active material, per cell

chamber.

SUMMARY

Graphite particles form a path for electrons to lead sulfate, and thus

facilitate charge at a negative electrode. The inventors have found, in the

process of conducting studies on improvement of the PSOC life, that graphite

in a negative electrode material causes a permeation short circuit. This can

be because when a graphite particle is exposed to, or protrudes from, a

surface of a negative electrode plate, the exposed portion or the like of the

graphite particle serves as a center of precipitation of metal lead. As a result, metal lead dendrite may grow from an exposed graphite particle, and then pass through the separator to cause a short circuit. The fact that graphite in the negative electrode material causes a permeation short circuit was not known, and was first found by the present inventors. The present inventors have also found that, when the negative electrode material contains carbon fiber, the carbon fiber in the negative electrode material causes a permeation short circuit, similarly.

An object of the present invention is to provide a lead-acid battery

that is:

• unlikely to be subjected to a permeation short circuit caused by

graphite or carbon fiber, and

• excellent in life performance in an environment involving deep

discharge, such as a PSOC.

An aspect of the present invention provides a lead-acid battery

including a negative electrode plate, a positive electrode plate, and an

electrolyte solution, wherein the negative electrode plate includes a negative

electrode material containing graphite or carbon fiber, and a ratio N/P

(hereinafter referred to as "N/P ratio") of a mass N of the negative electrode

material to a mass P of the positive electrode material, per lead-acid battery,

is 0.62 or more.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a sectional view of a part of a lead-acid battery in an

example.

Fig. 2 is a diagram showing a PSOC life test in the example.

Fig. 3 is a characteristic diagram showing the influence of the

content of graphite (content of barium sulfate: 0.6 mass%, N/P ratio: 0.95).

Fig. 4 is a characteristic diagram showing the influence of the N/P

ratio (content of barium sulfate: 0.6 mass%).

Fig. 5 is a characteristic diagram showing the influence of the N/P

ratio (content of barium sulfate: 0.6 mass%).

Fig. 6 is a characteristic diagram showing the influence of the

content of barium sulfate (content of graphite: 0.5 mass%, N/P ratio: 0.62).

Fig. 7 is a characteristic diagram showing the influence of the

average particle size of graphite (content of graphite: 2.0 mass%, content of

barium sulfate: 1.2 mass%, N/P ratio: 0.78).

Fig. 8 is a characteristic diagram showing the influence of the

content of carbon black (content of graphite: 2.0 mass%, content of barium

sulfate: 1.2 mass%, N/P ratio: 0.62).

Fig. 9 is a characteristic diagram showing the influence of the

concentration of aluminum ions (content of graphite: 2.0 mass%, content of

barium sulfate: 1.2 mass%, N/P ratio: 0.62).

Fig. 10 is a characteristic diagram showing the influence of the

concentration of lithium ions (content of graphite: 2.0 mass%, content of

barium sulfate: 1.2 mass%, N/P ratio: 0.62).

DESCRIPTION OF EMBODIMENTS

An aspect of the present invention provides a lead-acid battery

including a negative electrode plate, a positive electrode plate, and an

electrolyte solution, wherein the negative electrode plate includes a negative

electrode material containing graphite or carbon fiber, and a ratio N/P

(hereinafter referred to as "N/P ratio") of a mass N of the negative electrode

material to a mass P of the positive electrode material, per lead-acid battery,

is 0.62 or more.

The graphite may not only be scalelike graphite or expanded graphite

of Example, but may also be natural graphite such as scaly graphite or

earthy graphite, or artificial graphite, or expansion graphite, etc. Scalelike

graphite and expanded graphite are preferable, with scalelike graphite being

particularly preferable. Expanded graphite is graphite that has been

expanded. Carbon fiber has an effect similar to that of graphite. Carbon

fiber to be used has, for example, a length in a range of 5 im or more and 500

im or less.

Graphite or carbon fiber (hereinafter referred to as "graphite or the

like") in the negative electrode material facilitates reduction of lead sulfate

deposited on a lower portion of a plate of a lead-acid battery, and thus

improves life performance in a non-fully charged condition of the lead-acid

battery, such as a PSOC life. A content of graphite or the like in the

negative electrode material of 0.5 mass% or more significantly improves the

PSOC life, and is thus preferred. On the other hand, it has been found that

inclusion of graphite or the like in the negative electrode material causes a

permeation short circuit to easily occur. It was not known that inclusion of

graphite or the like in the negative electrode material of a lead-acid battery

causes a permeation short circuit to easily occur.

Thus, the present inventors conducted studies on inhibition of

occurrence of a permeation short circuit while graphite or the like is included

in the negative electrode material to improve the PSOC life. As a result, it

has been found that even when the negative electrode material contains

graphite or the like, an N/P ratio of 0.62 or more can inhibit a permeation

short circuit.

Inclusion of graphite or the like in the negative electrode material,

and an N/P ratio in a range of 0.62 or more and 0.95 or less significantly improves the PSOC life, and thus the N/P ratio is preferably in a range of

0.62 or more and 0.95 or less. An N/P ratio in a range of 0.62 or more and

0.78 or less more significantly improves the PSOC life, and thus the N/P

ratio is more preferably in a range of 0.62 or more and 0.78 or less.

A content of graphite or the like in the negative electrode material of

2.5 mass% or less can inhibit a permeation short circuit, and is thus

preferred. A content of graphite or the like in the negative electrode

material of 2.0 mass% or less can further inhibit a permeation short circuit,

and is thus more preferred.

Inclusion of barium sulfate in the negative electrode material can

inhibit a permeation short circuit, and is thus preferred. Inclusion of 1.2

mass% or more of barium sulfate in the negative electrode material can

markedly inhibit a permeation short circuit, and is thus more preferred.

Note that elemental barium or a barium compound such as barium

carbonate may be used instead of barium sulfate. This is because elemental

barium or a barium compound added to the negative electrode material

changes to barium sulfate after the addition. Elemental barium or a

barium compound is preferably added in an amount equivalent to 1.2 mass%

or more of barium sulfate with respect to the mass of the negative electrode

material in a fully-charged state. In terms of content equivalent to

elemental barium, the addition is preferably performed so that the content of

barium in the negative electrode material will be 0.7 mass% or more.

Inclusion of more than 3.0 mass% of barium sulfate in the negative

electrode material markedly reduces the PSOC life. This cancels the effect

of improving the PSOC life produced by inclusion of graphite or the like in

the negative electrode material. Thus, the content of barium sulfate in the

negative electrode material is preferably 3.0 mass% or less. In terms of content equivalent to elemental barium, the addition is preferably performed so that the content of barium in the negative electrode material will be 1.75 mass% or less.

Inclusion of 2.5 mass% or less of graphite or the like in the negative

electrode material, inclusion of 1.2 mass% or more of barium sulfate in the

negative electrode material, and an N/P ratio of 0.62 or more can yield a

lead-acid battery having excellent PSOC life performance and excellent

permeation short circuit resistance performance, and are thus preferred.

Inclusion of 2.0% or less of graphite or the like in the negative electrode

material, inclusion of 1.2 mass% or more of barium sulfate in the negative

electrode material, and an N/P ratio of 0.62 or more can yield a lead-acid

battery having particularly excellent permeation short circuit resistance

performance, and are thus more preferred.

Even when the negative electrode material contains 2.5 mass% or

less of graphite or the like, the negative electrode material contains 1.2

mass% or more of barium sulfate, and the N/P ratio is 0.62 or more, a

permeation short circuit may not be completely prevented. Thus, the

present inventors have conducted studies on further inhibiting a permeation

short circuit.

An average particle size of graphite in the negative electrode

material of 300 pm or less can further inhibit a permeation short circuit, and

therefore, the average particle size of graphite in the negative electrode

material is preferably 300 pm or less.

An average particle size of graphite in the negative electrode

material of 10 pm or more improves the PSOC life, and thus the average

particle size of graphite in the negative electrode material is preferably 10

pm or more.

Preferably, the negative electrode material contains carbon black.

By including carbon black in the negative electrode material, an effect of

further inhibiting a permeation short circuit is obtained. The effect of

carbon black of inhibiting a permeation short circuit is noticeable when the

content of carbon black in the negative electrode material is 0.05 mass% or

more, and thus the content of carbon black in the negative electrode material

is preferably 0.05 mass% or more.

A content of carbon black in the negative electrode material of 0.1

mass% or more produces a larger effect of improving the PSOC life than

when the content of carbon black in the negative electrode material is less

than 0.1 mass%. Thus, the content of carbon black in the negative electrode

material is preferably 0.1 mass% or more. A content of carbon black in the

negative electrode material of 0.5 mass% or more produces an especially

large effect of improving the PSOC life. Therefore, the content of carbon

black in the negative active material is more preferably 0.5 mass% or more.

A content of carbon black in the negative electrode material of more than 1.0

mass% causes the negative active material paste to be too hard to fill the

current collector. Therefore, the content of carbon black in the negative

electrode material is preferably 1.0 mass% or less.

Inclusion of aluminum ions in the electrolyte solution improves the

PSOC life. Inclusion of 0.02 mol/L or more of aluminum ions in the

electrolyte solution significantly improves the PSOC life. Thus, the

concentration of aluminum ions in the electrolyte solution is preferably 0.02

mol/L or more. Inclusion of 0.2 mol/L or less of aluminum ions in the

electrolyte solution significantly improves the PSOC life. Thus, the

concentration of aluminum ions in the electrolyte solution is preferably 0.2

mol/L or less.

Inclusion of lithium ions in the electrolyte solution can further

inhibit a permeation short circuit. This effect is significant when the

electrolyte solution contains 0.02 mol/L or more of lithium ions. Thus, the

concentration of lithium ions in the electrolyte solution is preferably 0.02

mol/L or more.

Inclusion of 0.1 mol/L or more of lithium ions in the electrolyte

solution improves the PSOC life. Thus, the concentration of lithium ions in

the electrolyte solution is preferably 0.1 mol/L or more. Moreover, inclusion

of 0.2 mol/L or less of lithium ions in the electrolyte solution improves the

PSOC life. Thus, the concentration of lithium ions in the electrolyte

solution is preferably 0.2 mol/L or less.

Most preferably, the electrolyte solution contains aluminum ions and

lithium ions.

The lead-acid battery of the present invention has excellent PSOC

life performance and excellent permeation short circuit resistance

performance, and is thus suitable as a lead-acid battery, such as for use in an

idling-stop vehicle. The lead-acid battery of the present invention can be

used not only for idling-stop vehicles etc., but also for cycle applications of,

for example, forklift trucks. In Example, the lead-acid battery is a

flooded-type lead-acid battery, but may also be a valve-regulated lead-acid

battery. The lead-acid battery of the present invention is preferably a

flooded-type lead-acid battery. The lead-acid battery of the present

invention is less likely to cause a permeation short circuit even when it is

used in a partial state of charge, and therefore, is suitable as a lead-acid

battery to be used in a partial state of charge.

The best mode example according to the invention of the present

application will be shown below. In embodying the present invention, the example can be appropriately changed based on common knowledge of a person skilled in the art and on disclosures of prior arts. In the example, the negative electrode material is also referred to as a negative active material, and the positive electrode material is also referred to as a positive active material. The negative electrode plate includes a negative electrode current collector (negative electrode grid) and a negative active material

(negative electrode material), and the positive electrode plate includes a

positive electrode current collector (positive electrode grid) and a positive

active material (positive electrode material). Solid components other than

the current collectors belong to the active materials (electrode materials).

[Example]

The negative active material paste was produced by mixing a

predetermined amount of graphite, a predetermined amount of barium

sulfate, lignin as a shrink-proofing agent, and synthetic resin fibers as a

reinforcing material with a lead powder produced by a ball milling method.

Carbon black was further included in the resultant mixture to prepare

another negative active material paste. Hereinafter, a content is expressed

in a concentration (mass%) in a formed negative active material after full

charge. The term "full charge" refers to a state in which charge has been

performed at a 5-hour rate current until the terminal voltage during charge

as measured every 15 minutes exhibits a particular value (0.01 V) three

times in succession.

The content of graphite was changed in a range of from 0 to 3.0

mass% with respect to the mass of the negative active material in a

fully-charged condition. Although scalelike graphite and expanded

graphite were used as graphite, other graphite such as earthy graphite and

artificial graphite, and carbon fiber may also be used. Both graphite and carbon fiber have high electrical conductivities, and are composed of particles larger than those of carbon black or the like. Actions thereof in the negative active material are also considered similar to each other. Among graphite and carbon fiber, scalelike graphite or expanded graphite is preferred. In particular, scalelike graphite or expanded graphite having an average particle size in a range of 10 pm or more and 300 pm or less is preferred. The scalelike graphite and expanded graphite used had resistivities as measured by a four-terminal method in a range of from 0.001

Q-cm to 0.01 Q-cm (resistivities under pressure of 2.5 MPa).

The content of barium sulfate was changed in a range of from 0.6

mass% to 4.0 mass% with respect to the mass of the negative active material

in a fully-charged condition. The barium sulfate used had an oil absorption

capacity of 12.5 mL/100 g, an average primary particle size of 0.79 pm, and

an average secondary particle size of 2.5 pm. However, properties of barium

sulfate, such as the particle sizes, the oil absorption capacity, etc. are not

limited. Barium sulfate has an average primary particle size of, for

example, 0.3 pm or more and 2.0 pm or less, and an average secondary

particle size of, for example, 1.0 pm or more and 10 pm or less. While the

content of lignin was 0.2 mass%, the content is not limited. A synthetic

shrink-proofing agent such as a condensate of a sulfonated bisphenol may be

used in place of lignin. While the content of the reinforcing material was

0.1 mass%, the content and the type of the synthetic resin fibers are not

limited. The method for producing a lead powder, the content of oxygen,

and so on are not limited. Other additives such as a water-soluble synthetic

polymer may be contained.

The above-mentioned mixture was formed into a paste with water

and sulfuric acid. An expanded type of negative electrode grid (110 mm

(height) x 100 mm (width) x 1.0 mm (thickness)) primarily composed of an

antimony-free Pb-Ca-Sn-based alloy was filled with this paste, which was

then cured and dried. The negative electrode grid may also be a cast grid, a

punched grid, or the like. The mass of the negative active material of each

negative electrode plate was made the same by dividing the mass of the

negative active material per cell shown in Tables 3 to 6 by the number of

negative electrode plates. In addition, the mass of the negative active

material per cell was made the same for all the cells. The density of the

negative active material after formation is in a range of, for example, 3.6

g/cm3 or more and 4.0 g/cm 3 or less. The amount of the filling negative active material per one negative electrode plate is in a range of, for example,

30 g or more and 70 g or less.

Synthetic resin fibers of a reinforcing material in an amount of 0.1

mass% with respect to the mass of the formed positive active material in a

fully-charged condition were mixed with a lead powder produced by a ball

milling method, and the resulting mixture was formed into a paste with

water and sulfuric acid, and was used as the positive active material paste.

An expanded type of positive electrode grid (110 mm (height) x 100 mm

(width) x 1.2 mm (thickness)) primarily composed of an antimony-free

Pb-Ca-Sn-based alloy was filled with this paste, which was then cured and

dried. The type of the lead powder and production conditions are not

limited. The grid may also be a cast grid, a punched grid, or the like. The

composition (components other than lead dioxide) of the positive active

material is not limited. For example, the positive active material may

contain antimony. The mass of the positive active material of each positive

electrode plate was made the same by dividing the mass of the positive active

material per cell shown in Tables 3 to 6 by the number of positive electrode plates. In addition, the mass of the positive active material per cell was made the same for all the cells. The density of the positive active material after formation is in a range of, for example, 3.5 g/cm 3 or more and 4.8 g/cm 3 or less.

An unformed negative electrode plate was wrapped with a

polyethylene separator with ribs protruding from a base, and seven

unformed negative electrode plates and six unformed positive electrode

plates were alternately layered. The negative electrode plates and the

positive electrode plates were connected to one another with a strap to

prepare an element. The separator is primarily composed of, for example, a

synthetic resin. The thickness of the base, the total thickness, and the like

are not limited. The thickness of the base of the separator was 0.25 mm,

but may be in a range of, for example, 0.15 mm or more and 0.25 mm or less.

The interval between a positive electrode plate and a negative electrode plate

is in a range of, for example, 0.5mm or more and 0.9mm or less. Six

elements connected in series were placed in a cell chamber of a container,

and sulfuric acid with a specific gravity of 1.230 at 20°C was added to

perform formation in the container, thereby preparing a flooded-type

lead-acid battery having a B20 size and a 5-hour rate capacity of 30 Ah.

Fig. 1 shows a part of a lead-acid battery 2. Reference numeral 4

denotes a negative electrode plate, reference numeral 6 denotes a positive

electrode plate, reference numeral 8 denotes a separator, and reference

numeral 10 denotes an electrolyte solution mainly composed of sulfuric acid.

The negative electrode plate 4 includes a negative electrode grid 12 and a

negative active material 14. The positive electrode plate 6 includes a

positive electrode grid 16 and a positive active material 18. The separator 8

is in the form of a bag including a base 20 and ribs 22. The negative electrode plate 4 is stored in the bag, and the ribs 22 face the positive electrode plate 6 side. However, the positive electrode plate 6 may be stored in the separator 8 with the ribs 22 facing the positive electrode plate. The separator is not required to be in the form of a bag as long as it isolates the positive electrode plate and the negative electrode plate from each other; the separator may be, for example, a leaflet-like glass mat or retainer mat.

The mass of the positive active material and the mass of the negative

active material, of the lead-acid battery 2, are determined as follows. The

lead-acid battery 2 in a fully-charged condition is disassembled, and the

negative electrode plate 4 is washed with water, and then dried, to remove

the sulfuric acid component. Thus, the negative active material 14 is

collected, and the mass of the negative active material per one negative

electrode plate is measured. In a similar manner, the positive active

material 18 is collected, and the mass of the positive active material per one

positive electrode plate is measured. The ratio of the sum of the mass of the

active material of all the negative electrode plates to the sum of the mass of

the active material of all the positive electrode plates is deemed as the ratio

N/P of the mass N of the negative electrode material to the mass P of the

positive electrode plate per lead-acid battery.

The content of barium contained in a formed negative active material

is quantitatively determined as follows. The lead-acid battery 2 in a

fully-charged condition is disassembled, and the negative electrode plate 4 is

washed with water, and then dried, to remove the sulfuric acid component.

Thus, the negative active material 14 is collected. The negative active

material is crushed, and hydrogen peroxide water having a concentration of

300 g/L is added thereto in an amount of 20 mL per 100 g of the negative

active material, and (1+3) nitric acid that is obtained by diluting 1 part by volume of 60 mass% concentrated nitric acid with 3 parts by volume of ion-exchange water is further added. The mixture is then heated with stirring for five hours to allow lead to dissolve in the form of lead nitrate.

Barium sulfate is further dissolved. The concentration of barium in the

resultant solution is quantitatively determined by atomic absorption

spectrometry, and the result is converted into the content of barium in the

negative active material. The content of barium in the negative active

material can then provide the content of barium sulfate in the negative

active material.

The contents of graphite and of carbon black contained in the formed

negative active material are quantitatively determined as follows. The

lead-acid battery 2 is in a fully-charged condition is disassembled, and the

negative electrode plate 4 is washed with water, and then dried, to remove

the sulfuric acid component. Thus, the negative active material 14 is

collected. The negative active material is crushed, and hydrogen peroxide

water having a concentration of 300 g/L is added thereto in an amount of 20

mL per 100 g of the negative active material, and (1+3) nitric acid that is

obtained by diluting 1 part by volume of 60 mass% concentrated nitric acid

with 3 parts by volume of ion-exchange water is further added. The

mixture is then heated with stirring for five hours to allow lead to dissolve in

the form of lead nitrate. Barium sulfate is further dissolved. Filtration is

then performed to separate graphite, carbon black, and the reinforcing

material.

Solid components obtained by filtration (graphite, carbon black, and

the reinforcing material) are dispersed in water. Using a sieve that retains

the reinforcing material, such as, for example, a sieve having an aperture of

1.4 mm, the dispersion solution is sieved twice, and then washed with water to remove the reinforcing material, thereby separating carbon black and graphite.

An organic shrink-proofing agent, such as lignin, is added to the

negative active material paste together with carbon black and graphite.

Due to the surface-active effect of the organic shrink-proofing agent, carbon

black and graphite remain in a non-aggregated state in the negative active

material even after formation. However, since the organic shrink-proofing

agent is dissolved in water and lost in the series of separation operations,

carbon black and graphite are dispersed in water, the organic

shrink-proofing agent is then added, and the mixture is stirred to

disaggregate again the aggregates of carbon black and of graphite, after

which the following separation operation is performed.

The organic shrink-proofing agent is not limited as long as it can be

added to a lead-acid battery. In Example, a lignin sulfonate Vanillex N

(manufactured by NIPPON PAPER INDUSTRIES CO., LTD.) was used. In

Example, 15 g of the organic shrink-proofing agent was added to 100 mL of

water, and a stirring operation was performed.

After the aforementioned operation, carbon black and graphite were

separated from each other by making suspension containing carbon black

and graphite pass through a sieve that does not substantially allow graphite

to pass therethrough, but allows carbon black to pass therethrough. In

Example, a sieve with an aperture of 20 pm was used. Even when graphite

having a particle size of less than this aperture is used, graphite having a

particle size of 3 pm or more causes clogging of the sieve, and thus such

graphite will not substantially pass through the sieve. After this operation, graphite remains on the sieve, and carbon black is contained in the liquid

that has passed through the sieve. Graphite and carbon black separated by the series of operations are washed with water and dried, and are then each weighed. Carbon fiber can be separated in a manner similar to that of graphite.

The average particle size (volume average size) of graphite is

measured by a light scattering method while the separated graphite is

dispersed again in water to which an organic shrink-proofing agent has been

added. A region corresponding to a particle size of less than 3 1m, if any, is

ignored as that of an impurity such as carbon black. Aluminum ions and

lithium ions in the electrolyte solution are quantitatively determined by

extracting the electrolyte solution, and then using ICP emission

spectrometry.

A permeation short circuit accelerating test and a PSOC life test

were conducted on the lead-acid battery 2 in a fully-charged condition.

Details of the PSOC life test are shown in Fig. 2 and in Table 1. The

denotation 1 CA means, for example, 30 A for a battery having a 5-hour rate

capacity of 30 Ah. The term 40 0 C air indicates that the test was conducted

in an air bath at 400 C. The number of cycles until the terminal voltage

reaches 1.2 V/cell in the test pattern in Table 1 is defined as a PSOC life.

Details of the permeation short circuit accelerating test are shown in Table 2.

This test is a test that is conducted under conditions that promote occurrence

of a permeation short circuit. Thus, the rate of occurrence of a permeation

short circuit is significantly higher than that under the actual conditions of

use of the lead-acid battery. Five cycles of the permeation short circuit

accelerating test pattern shown in Table 2 were conducted. After the 5

cycles, the lead-acid batteries were disassembled to determine the ratio of

lead-acid batteries in which short circuits have occurred. The term 250 C

water indicates that the test was conducted in a water bath at 25C. In

Tables 1 and 2, CC discharge, CV charge, and CC charge respectively mean

constant current discharge, constant voltage charge, and constant current

charge.

[Table 1] Test conditions Step Details Current and voltage Stop condition Temperature

1 CC discharge 1 CA 59 sec. 40°C air 2 CC discharge 300 A 1 sec. 40°C air 3 CV charge 2.4 V/cell, max. 50 A 10 sec. 40°C air 4 CC discharge 1 CA 5 sec. 40°C air 5 Repeat steps 3 and 4 5 times 40°C air 6 Repeat steps 1 - 5 50 times 40°C air 7 CV charge 2.4 V/cell, max. 50 A 900 sec. 40°C air 8 Repeat steps 1 - 7 72 times 40°C air 9 Rest 15h 40 0 Cair 10 Return to step 1 40 0 Cair

[Table 2] Test conditions Step Details Current and voltage Stop condition Temperature

1 CC discharge 0.05 CA 1.0 V/cell 25 0 C water

2 Left with Resistance 10 2 28 days 25 0 C water connected 3 CV charge 2.4 V/cell, max. 50 A 10 min. 25 0 C water 4 CC charge 0.05 CA 27 hours 25 0 C water 5 Repeat steps 1 - 4 5 times 25 0 C water

The results of the PSOC life test and of the permeation short circuit

accelerating test are shown in Tables 3 to 7, in which the percentages of the

contents are each a percentage by mass (mass%). Table 3 shows the results

when scalelike graphite was used, and Table 4 shows the results when

expanded graphite was used. In both cases, the negative active materials

did not contain carbon black, and the electrolyte solutions did not contain

aluminum ions nor lithium ions. The data of the PSOC life in Tables 3 and

4 are shown in relative values with respect to the values of the samples listed

on top of the respective tables, with these values defined as 100%.

[Table 3]

Mass of Mass of Scalelike graphite BaSO4 PSOC life Permeation Battery positive negative Average Number of short circuit no active active N/P ratio Content particle Content cycles (ratio occurrence material material (%) size (%) to Al defined rate (g/cell) (g/cell) (pm) as 100) (%) Al 338 320 0.95 100 40 A2 375 294 0.78 106 50 A3 420 261 0.62 0.5 150 0.6 110 60 A4 427 256 0.60 84 100 A5 331 325 0.98 80 40 A6 338 320 0.95 80 20 A7 375 294 0.78 84 20 A8 420 261 0.62 0 - 0.6 86 20 A9 427 256 0.60 78 20 A10 331 325 0.98 70 20 All 338 320 0.95 109 40 A12 375 294 0.78 2.0 150 0.6 114 60 A13 420 261 0.62 121 60 A14 338 320 0.95 110 100 3.0 150 0.6 A15 420 261 0.62 120 100 Bl 1.0 100 40 B2 1.2 104 0 B3 338 320 0.95 0.5 150 2.0 106 0 B4 3.0 106 0 B5 4.0 87 0 B6 1.2 107 0 B7 375 294 0.78 0.5 150 2.0 109 0 B8 3.0 109 0 B9 1.0 110 60 B10 1.2 113 0 Bll 420 261 0.62 0.5 150 2.0 114 0 B12 3.0 114 0 B13 4.0 90 0 B14 427 256 0.60 0.5 150 3.0 86 60 B15 331 325 0.98 0.5 150 3.0 84 0 B16 338 320 0.95 0 - 3.0 84 0 B17 420 261 0.62 0 - 3.0 89 0 B18 338 320 0.95 1.5 150 2.0 109 0 B19 1.0 109 40 B20 1.2 112 0 338 320 0.95 2.0 150 B21 3.0 115 0 B22 4.0 91 0 B23 1.2 120 0 375 294 0.78 2.0 150 B24 3.0 122 0 B25 420 261 0.62 1.5 150 2.0 121 0 B26 420 261 0.62 2.0 150 1.0 121 60

B27 1.2 125 10 B28 3.0 126 0 B29 4.0 96 0 B30 38 320 0.95 3.0 150 3.0 124 40 IB31 1 20 1 261 1 0.62 1 3.0 1 150 1 3.0 1 124 1 60

[Table 4] Mass of Mass of Expanded graphite BaSO4 POlie Permeation positive negative Average PS~ie short circuit Battery Number of cycles ocrec n. active active N/P ratio Content particle Content (ratio toDl occrrtenc material material N% size N% defined as 100) ____ (g/cell) (g/cell) _ ___ (Jim) ________ N____

Dl 338 320 0.95 100 30 D2 375 294 0.78 106 50 D3 420 261 0.62 0.5 150 0.6 108 50 D4 427 256 0.60 82 100 D5 331 325 0.98 _ __ ___ 78 40 D6 338 320 0.95 108 40 D7 375 294 0.78 2.0 150 0.6 116 50 D8 420 261 0.62 _ __ ___ 120 50 D9 338 320 0.95 30 10 .6109 100 D10 420 261 0.62 ____ ___ ___ 118 100 Dli 5 93 20 D2 338 320 0.95 0.5 10 0.6 103 D13 300 100 30 D14 ___________ 500 _ __ 91 50

D5 375 294 0.78 0.5 0.6 154 D16 ____________ 50 ___ 103 70 D17 5 110 40 D8 420 261 0.62 2.0 10 0.6 155 D19 300 116 60 D20 500 108 70 El 1.0 100 30 E2338 320 0.95 0.5 150 1.100 E3 3.0 lOS 0 E4 _______ 4.__0 84 0

E5375 294 0.78 0.5 150 1.100 E6 ____ _______ 3.0 110 0 E7 1.0 108 50 E8420 261 0.62 0.5 150 1.il0 E9 3.0 112 0 E10 1___ 4._______0 86 0 Eli 427 256 0.60 0.5 150 3.0 81 60 E12 331 325 0.98 0.5 150 3.0 80 0 E3 338 320 0.95 2.0 150 10154 E14 ____ ____ ____ ____ ___ 1.2 106 0

E15 3.0 102 0 E16 4.0 85 0 E17 1.2 115 0 375 294 0.78 2.0 150 E18 3.0 115 0 E19 1.0 117 50 E20 1.2 119 0 420 261 0.62 2.0 150 E21 3.0 120 0 E22 4.0 91 0 E23 338 320 0.95 3.0 150 3.0 118 40 E24 420 261 0.62 3.0 150 3.0 124 60 Fl 1.2 96 0 5 F2 3.0 95 0 F3 1.2 103 0 10 F4 3.0 105 0 338 320 0.95 0.5 F300 1.2 102 0 F6 3.0 102 0 F7 50 1.2 97 0 500 F8 3.0 95 0 F9 1.2 114 0 5 F10 3.0 115 0 F11 1.2 121 0 10 F12 3.0 120 0 420 261 0.62 2.0 F13300 1.2 119 10 F14 3.0 120 0 F500 1.2 111 10 F16 1 1 3.0 109 0

Fig. 3 and Table 3 show that inclusion of graphite in the negative

active material improves the PSOC life, and that inclusion of 0.5 mass% or

more of graphite significantly improves the PSOC life. On the other hand, it can be seen that inclusion of graphite in the negative active material

causes a permeation short circuit to easily occur. It was not known that

inclusion of graphite in the negative active material causes a permeation

short circuit to easily occur as described above.

Thus, the present inventors conducted studies on inhibition of

occurrence of a permeation short circuit while graphite is included in the

negative active material to improve the PSOC life. As a result, it has been

found that even when the negative active material contains graphite, an N/P ratio of 0.62 or more can inhibit a permeation short circuit (Fig. 4).

Meanwhile, when the negative active material does not contain graphite, the

effect of inhibiting a permeation short circuit cannot be obtained even when

the N/P ratio is 0.62 or more (Fig. 4). Thus, the effect of inhibiting a

permeation short circuit due to an N/P ratio of 0.62 or more is obtained only

when the negative active material contains graphite. It was not known that

graphite in the negative active material relates to a permeation short circuit,

nor that the N/P ratio relates to a permeation short circuit. Thus, it was not

foreseeable that using an N/P ratio of 0.62 or more when the negative active

material contains graphite could produce an effect of inhibiting a permeation

short circuit, and it is not easy for a person skilled in the art to get an idea of

using an N/P ratio of 0.62 or more to inhibit a permeation short circuit,

which occurs more easily by including graphite in the negative active

material.

Fig. 4 shows that when the negative active material contains

graphite, changing the N/P ratio from 0.6 to 0.62 markedly decreases the

permeation short circuit occurrence rate. It was obviously not foreseeable

that the N/P ratios of 0.6 and 0.62 have distinctly different effects of

inhibiting a permeation short circuit as described above, and it can be said

that an N/P ratio of 0.62 or more has a meaning as a critical point.

Fig. 5 shows that inclusion of graphite in the negative active material

improves the PSOC life, and that an N/P ratio in a range of 0.62 or more and

0.95 or less improves the PSOC life. More particularly, considering the fact

that inclusion of graphite in the negative active material and an N/P ratio in

a range of 0.62 or more and 0.95 or less together markedly improve the

PSOC life, it is found that the combination of inclusion of graphite in the

negative active material and an N/P ratio in a range of 0.62 or more and 0.95 or less can provide a synergetic effect (Fig. 5). An N/P ratio in a range of

0.62 or more and 0.78 or less further significantly improves the PSOC life.

Fig. 3 shows that a content of graphite in the negative active

material of 2.5 mass% or less can inhibit a permeation short circuit. Since

it was not known that graphite in the negative active material relates to

occurrence of a permeation short circuit, it is not easy for a person skilled in

the art to get an idea of using a content of graphite in the negative active

material of 2.5 mass% or less to inhibit a permeation short circuit. In

addition, Fig. 3 shows that a content of graphite in the negative active

material of 2.5 mass% or less markedly decreases the permeation short

circuit occurrence rate. Thus, it can be said that a content of graphite in the

negative active material of 2.5 mass% or less has a meaning as a critical

point. A content of graphite in the negative active material of 2.0 mass% or

less has a larger effect of inhibiting a permeation short circuit.

Table 3 and Fig. 6 show that inclusion of barium sulfate in the

negative active material produces an effect of inhibiting a permeation short

circuit. Since it was not known that barium sulfate in the negative active

material relates to a permeation short circuit, it was an unexpected result

that inclusion of barium sulfate in the negative active material produced an

effect of inhibiting a permeation short circuit. In particular, a content of

barium sulfate in the negative active material of 1.2 mass% or more

markedly improves the effect of inhibiting a permeation short circuit (Fig. 6).

Thus, it can be said that a content of barium sulfate in the negative active

material of 1.2 mass% or more has a meaning as a critical point.

Table 3 and Fig. 6 show that inclusion of barium sulfate in the

negative active material of more than 3.0 mass% markedly decreases the

PSOC life. Thus, the content of barium sulfate in the negative active material is preferably 3.0 mass% or less.

It can be seen that a content of graphite in the negative active

material of 2.5 mass% or less, a content of barium sulfate in the negative

active material of 1.2 mass% or more, and an N/P ratio of 0.62 or more can

yield a lead-acid battery having PSOC life performance higher than that of a

lead-acid battery not containing graphite in the negative active material,

and having excellent permeation short circuit resistance performance (e.g.,

samples B2 to B4 of Table 3). If any one of the content of graphite and the

content of barium sulfate in the negative active material and the N/P ratio is

outside the value range described above, a permeation short circuit cannot

be sufficiently inhibited.

For example, even when the content of barium sulfate in the negative

active material is 1.2 mass% or more, and the N/P ratio is 0.62 or more, a

permeation short circuit may not be sufficiently inhibited if the content of

graphite in the negative active material is more than 2.5 mass% (e.g.,

samples B30 and B31 of Table 3). In addition, even when the content of

graphite in the negative active material is 2.5 mass% or less, and the N/P

ratio is 0.62 or more, a permeation short circuit may not be sufficiently

inhibited if the content of barium sulfate in the negative active material is

less than 1.2 mass% (e.g., samples B1and B9 of Table 3). Similarlyeven

when the content of graphite in the negative active material is 2.5 mass% or

less, and the content of barium sulfate in the negative active material is 1.2

mass% or more, a permeation short circuit may not be sufficiently inhibited

if the N/P ratio is less than 0.62 (sample B14 of Table 3). Thus, concludingly, the combination of three configurations, that is, a content of graphite in the

negative active material of 2.5 mass% or less, a content of barium sulfate in

the negative active material of 1.2 mass% or more, and an N/P ratio of 0.62 or more, is required to sufficiently inhibit a permeation short circuit.

Since it was not known that graphite in the negative active material

relates to a permeation short circuit, it is not easy for a person skilled in the

art to get an idea of using a content of graphite in the negative active

material of 2.5 mass% or less to inhibit a permeation short circuit.

Moreover, since it was not known that barium sulfate in the negative active

material relates to a permeation short circuit, it is not easy for a person

skilled in the art to get an idea of using a content of barium sulfate in the

negative active material of 1.2 mass% or less to inhibit a permeation short

circuit. Furthermore, since it was also not known that the N/P ratio relates

to a permeation short circuit, it is not easy for a person skilled in the art to

get an idea of using an N/P ratio of 0.62 or more to inhibit a permeation short

circuit. Since it is not easy for a person skilled in the art to get each of the

ideas of using a content of graphite in the negative active material of 2.5

mass% or less, using a content of barium sulfate in the negative active

material of 1.2 mass% or less, and using an N/P ratio of 0.62 or more, it is

very difficult for a person skilled in the art to conceive of combining these

three configurations.

Table 4 shows that also when expanded graphite was used as

graphite, the results obtained were approximately the same as those

obtained when scalelike graphite was used as graphite.

Table 3 shows that even when the content of graphite in the negative

active material is 2.5 mass% or less, the content of barium sulfate in the

negative active material is 1.2 mass% or more, and the N/P ratio is 0.62 or

more, a permeation short circuit may not be completely inhibited (e.g.,

sample B27 of Table 3). Thus, the present inventors have conducted studies

on further inhibiting a permeation short circuit.

[Table 5] Mass of Mass of Scalelike graphite BaSO4 PSOC life Permeation Battery positive negative Average Number of short circuit n. active active N/P ratio Content particle Content cycles (ratio occurrence material material N% size %) to Aldefined rate (g/cell) (g/cell) _____ (Jim) as 100) N%

A16 5 96 30 Al7 10 101 40 Al 338 320 0.95 0.5 150 0.6 100 40 A18 300 100 40 A19 ________ ____ 500 95 50 A20 5 108 50 A2 375 294 0.78 0.5 150 0.6 106 50 A21 _______ ___ 500 107 70 A22 5 115 50 A23 10 119 60 A13 420 261 0.62 2.0 150 0.6 121 60 A24 300 120 60 A25 500 113 70 Cl 1.2 99 0 C2 _____ 3.0 98 0 Ca 1 1.2 104 0 C4 _____ 3.0 106 0

B2338 320 0.95 0.5 150 1.100 B4 _____ 3.0 106 0 CS 0 1.2 104 0 CC ____ 3.0 lOS 0 C7 50 1.2 100 0 C8 ___ ________ ___ 3.0 99 0 C9 5 1.2 110 0 ClO10__ 3.0 109 0 Cli 10 1.2 119 0 C12 _____ 3.0 121 0 B23 37 9 .8 20 10 1.2 120 0 B24 _____ 3.0 122 0 Ci330 1.2 120 0 C14 ___ 3.0 121 0 C1550 1.2 108 10 C16 ___ ________ ____ 30107 0 C17 5 1.2 119 0 C18 ___ 3.0 120 0 C19 10 1.2 123 0 C20 420 261 0.62 2.0 _____ 3.0 125 0 B27 10 1.2 125 10 B28 _____ 3.0 126 0 C21 ________ ___ ____ 300 1.2 123 10

C22 3.0 124 0 C23 1.2 118 10 C24 3.0 118 0

Table 5 and Fig. 7 show effects of the average particle size of graphite

in the negative active material. It can be seen that an average particle size

of graphite in the negative active material of 300 pm or less can inhibit a

permeation short circuit. It was an unexpected result that use of an

average particle size of graphite in the negative active material of 300 pm or

less can inhibit a permeation short circuit. More particularly, a content of

graphite in the negative active material of 2.5 mass% or less, a content of

barium sulfate in the negative active material of 1.2 mass% or more, an N/P

ratio of 0.62 or more, and an average particle size of graphite in the negative

active material of 300 pm or less can together yield a lead-acid battery in

which occurrence of a permeation short circuit is almost completely inhibited.

It was not known that the average particle size of graphite in the negative

active material relates to a permeation short circuit. Thus, even if a person

skilled in the art attempts to obtain a lead-acid battery having excellent

permeation short circuit resistance performance, a large number of

trial-and-error steps are required to get the ideas of using a content of

graphite in the negative active material of 2.5 mass% or less, using a content

of barium sulfate in the negative active material of 1.2 mass% or more, using

an N/P ratio of 0.62 or more, and using an average particle size of graphite in

the negative active material of 300 pm or less. Such a combination of ideas

cannot occur easily.

Table 5 and Fig. 7 show that an average particle size of graphite in

the negative active material of 10 pm or more markedly improves the PSOC

life.

[Table 6] Graphite BaSO4 CrbonPSOC life Permeation pos negatfblack Number of short Battery positive negative N/P Average cycles (ratio circuit no. ate ate ratio Content particle Content Content to Al occurrence (%) size (%) (%) defined as rate (g/cell) (g/cell) (pm) 100) (%)

B27 0 125 10

Gi 0.05 127 0

G2 0.1 128 0 420 261 0.62 Scalelike 2.0 150 1.2 G3 0.3 134 0

G4 0.5 137 0

G5 1.0 137 0

G6 0 120 10

G7 375 294 0.78 Scalelike 1.5 300 1.2 0.05 123 0

G8 0.5 131 0

Table 6 and Fig. 8 show effects of carbon black in the negative active

material. It can be seen that inclusion of carbon black in the negative active

material can inhibit a permeation short circuit. It was an unexpected result

that inclusion of carbon black in the negative active material can inhibit a

permeation short circuit. More particularly, it can be seen that a content of

graphite in the negative active material of 2.5 mass% or less, a content of

barium sulfate in the negative active material of 1.2 mass% or more, an N/P

ratio of 0.62 or more, and inclusion of carbon black in the negative active

material can together yield a lead-acid battery in which occurrence of a

permeation short circuit is almost completely inhibited. It was not known

that carbon black in the negative active material relates to a permeation

short circuit. Thus, even if a person skilled in the art attempts to obtain a

lead-acid battery having excellent permeation short circuit resistance

performance, a large number of trial-and-error steps are required to get the

ideas of using a content of graphite in the negative active material of 2.5 mass% or less, using a content of barium sulfate in the negative active material of 1.2 mass% or more, using an N/P ratio of 0.62 or more, and including carbon black in the negative active material. Such a combination of ideas cannot occur easily.

The effect of carbon black of inhibiting a permeation short circuit is

noticeable when the content of carbon black in the negative active material is

0.05 mass% or more (Fig. 8). Meanwhile, inclusion of carbon black in the

negative active material of more than 1.0 mass% made the active material

paste too hard to fill therewith the negative electrode current collector.

A content of carbon black in the negative active material of 0.1

mass% or more produces a larger effect of improving the PSOC life than

when the content of carbon black in the negative active material is less than

0.1 mass% (Fig. 8). A content of carbon black in the negative electrode

material of 0.5 mass% or more produces an especially large effect of

improving the PSOC life. Note that the electrolyte solutions of Table 6 did

not contain aluminum ions nor lithium ions. Scalelike graphite was used as

graphite in Table 6, but similar results were obtained when expanded

graphite was used.

C-~ ~ C ~ S C C).~ C)~ ) C)QN NC) C - C - C - C - ; C C C C C C N a C) Q~ 4 ______

C) - C - C ~ Qc

in ~ ci at in in r- r- ~ C C

cit~ ~ S Nj~ C)

ci 2 a5 at 6 C C C 6 0

6 C C

C I~______ -

C ~

C C C C

6 6

tfl ~NR ci c~ C U

C)

bE

C) ;~. .~ -~- 8 C)

~-1

C) C C ~ Cz~ ci C) C N Q

C) C) C)

cit

C ci ~C) N 6 C)

C- ~

~C) ~C) CO C)bL ci bE C) C C)

o (flC)C)C) C C) ci ~:~~ !

C C) - C)

ci - ci ~c in cc ~ C

Table 7 shows effects of aluminum ions and effects of lithium ions in

the electrolyte solution. In Table 7, the negative active material did not

contain carbon black. Scalelike graphite was used as graphite in Table 7, but similar results were obtained when expanded graphite was used.

Inclusion of aluminum ions in the electrolyte solution improves the PSOC

life (samples H1 to H4 of Table 7, and Fig. 9). Inclusion of 0.02 mol/L or

more of aluminum ions in the electrolyte solution significantly improves the

PSOC life (samples H1 to H4 of Table 7, and Fig. 9). Inclusion of 0.2 mol/L

or less of aluminum ions in the electrolyte solution also significantly

improves the PSOC life (samples H1 to H4 of Table 7, and Fig. 9).

Table 7 and Fig. 10 show that inclusion of lithium ions in the

electrolyte solution can inhibit a permeation short circuit. More

particularly, a content of graphite in the negative active material of 2.5

mass% or less, a content of barium sulfate in the negative active material of

1.2 mass% or more, an N/P ratio of 0.62 or more, and inclusion of lithium

ions in the electrolyte solution together yielded a lead-acid battery in which

occurrence of a permeation short circuit was almost completely inhibited.

Even if a person skilled in the art attempts to obtain a lead-acid battery

having excellent permeation short circuit resistance performance, a large

number of trial-and-error steps are required to get the ideas of using a

content of barium sulfate in the negative active material of 1.2 mass% or

more, using an N/P ratio of 0.62 or more, and including lithium ions in the

electrolyte solution. Such a combination of ideas cannot occur easily. The

effect of inhibiting a permeation short circuit by lithium ions is noticeable

when the concentration of lithium ions in the electrolyte solution is 0.02

mol/L or more.

Inclusion of 0.1 mol/L or more of lithium ions in the electrolyte solution improves the PSOC life (samples H5 to H8 of Table 7, and Fig. 10).

In addition, inclusion of 0.2 mol/L or less of lithium ions in the electrolyte

solution improves the PSOC life (samples H5 to H8 of Table 7, and Fig. 10).

Inclusion of both aluminum ions and lithium ions in the electrolyte

solution can inhibit a permeation short circuit, and significantly improves

the PSOC life (samples H9 and H10 of Table 7).

In Example, a lead-acid battery which is excellent in PSOC life and

suffers little permeation short circuit is obtained, and a glass mat etc. may

be used as a separator to produce a valve-regulated lead-acid battery.

Throughout this specification and the claims which follow, unless the

context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a

stated integer or step or group of integers or steps but not the exclusion of

any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should

not be taken as, an acknowledgement or any form of suggestion that the

prior art forms part of the common general knowledge in Australia.

Claims (12)

What is claimed is:

1. A lead-acid battery comprising:

a negative electrode plate;

a positive electrode plate; and

an electrolyte solution,

wherein the negative electrode plate includes a negative electrode

material containing graphite or carbon fiber,

a ratio N/P of a mass N of the negative electrode material to a mass P

of a positive electrode material per lead-acid battery is 0.62 or more,

the negative electrode material contains 1.75 mass% or less of

elemental barium in a fully-charged condition,

the ratio N/P is 0.95 or less, and

the negative electrode material contains 2.5 mass% or less of

graphite or 2.5 mass% or less of carbon fiber.

2. The lead-acid battery according to claim 1, wherein the negative

electrode material contains barium sulfate.

3. The lead-acid battery according to claim 1, wherein the negative

electrode material contains elemental barium.

4. The lead-acid battery according to claim 2, wherein the negative

electrode material contains 1.2 mass% or more of barium sulfate.

5. The lead-acid battery according to claim 1, wherein the negative

electrode material contains 0.7 mass% or more of elemental barium.

6. The lead-acid battery according to any one of claims 2 to 5, wherein

the negative electrode material contains 3.0 mass% or less of barium sulfate.

7. The lead-acid battery according to any one of claims 1 to 6, wherein

the negative electrode material contains carbon black.

8. The lead-acid battery according to any one of claims 1 to 7, wherein

the graphite or the carbon fiber is graphite having an average particle size of

300 pm or less.

9. The lead-acid battery according to any one of claims 1 to 8, wherein

the graphite or the carbon fiber is graphite having an average particle size of

10 pm or more.

10. The lead-acid battery according to any one of claims 1 to 9, wherein

the electrolyte solution contains aluminum ions.

11. The lead-acid battery according to any one of claims 1 to 10, wherein

the electrolyte solution contains lithium ions.

12. The lead-acid battery according to any one of claims 1 to 11, wherein

the ratio N/P is 0.78 or less.