2.0 Electrode Materials

Although a number of basic materials are used to produce all types of lead-acid batteries, significant differences exist among battery types in both materials and design. Batteries are built to satisfy specific application-based demands. Each particular duty cycle prescribes its own design, materials and methods of construction. For example, engine-starting batteries use lead-calcium alloys for their high conductivity needed for high cranking rates. Motive-power batteries use lead-antimony alloys for their renowned cycling capability. Different separator and container materials meet the needs of a wide variety of performance life and environmental conditions.

2.1 Lead

Lead is the basic raw material used to produce the lead oxide and alloys used to make lead-acid batteries. The industry standard for pig lead is ASTM Designation B29-79(84), which covers two grades used to produce oxide for active material or for mixing with other elements to produce alloys. The specifications are shown in

 

Table 2-1.

Element

Corroding Lead % Impurities

Common Lead % Impurities

Silver

0.0015

0.005

Copper

0.0015

0.0015

Silver/Copper (combined)

0.0025

---

Antimony, Arsenic, Tin

  (combined, maximum)

0.002

0.002

Zinc (maximum)

0.001

0.001

Iron (maximum)

0.002

0.002

Bismuth (maximum)

0.050

0.050

Lead (by difference)

99.94

99.94

Table 2-1: Chemical requirements for ASTM designation B29-79(84)

 

The only difference between these designations is the permissible concentrations of silver and the aggregate of silver and copper. Both elements increase evolution of hydrogen from the negative electrode. Therefore, corroding lead or lead of even greater purity is generally used in battery types where low water loss is required.

Most battery manufacturers use their own specifications and can specify lower limits and additional elements not included in Table 2-1. The Battery Council International (BCI) has also developed specifications for Standard Reference Materials used by lead-acid battery manufacturers. They include antimony alloys and calcium alloys with and without tin. They are shown in Table 2-2.

 

Element

 

Antimony        % Element in Alloy 

 

Calcium

Calcium-Tin

1%

3%

6%

Aluminum

<0.0005

<0.0005

<0.0005

0.019

0.018

Antimony

1.08

2.70

5.98

<0.0003

<0.001

Arsenic

0.136

0.151

0.169

<0.0003

<0.0005

Bismuth

0.0103

0.0073

0.0046

0.0105

0.010

Calcium

<0.0005

<0.0005

<0.0005

0.111

0.085

Copper

0.0107

0.053

0.057

0.0003

0.0004

Iron

<0.0003

<0.0003

<0.0003

<0.0003

<0.0005

Nickel

0.0003

0.0004

0.0004

<0.0002

<0.0002

Selenium

0.0142

<0.0005

<0.0005

<0.0002

<0.0001

Silver

0.0018

0.0012

0.0010

0.0017

0.0017

Sulfur

0.0015

0.0037

0.0037

<0.0005

<0.0005

Tin

0.19

0.165

0.306

<0.001

0.272

Tellurium

<0.001

<0.001

<0.002

<0.0003

<0.0001

Zinc<0.0003

<0.0003

<0.0003

<0.0003

<0.0005

<0.0003

Table 2-2: Chemical analyses of BCI standard reference materials

 

The BCI standards provide general guidance to the industry. However, as in the case for lead metal, battery manufacturers frequently use their own specifications for alloys. Standards have also been developed for battery containers and separators.

2.2 Lead Oxides as Starting Material

Lead oxides have been used in lead-acid batteries as starting materials for plates since the early days, particularly lead monoxide (PbO) and red lead (Pb3O4). It was customary to use a mixture of lead monoxide and red lead in the positive plates and pure lead monoxide in the negative plates. In contrast to today, these oxides contained very little free lead, as most battery producers then believed that it lowered the quality of the product.

The development of two processes, Ball Mill and Barton, for oxide production resulted in a dramatic change in the oxides used for battery manufacture. Both processes considerably reduced the cost of lead monoxide, but yielded a material containing a significant amount of free lead. The material is generally called leady oxide. Prompted by the reduced cost, battery companies developed paste formulations and new processes, i.e., curing, that permitted leady oxide use. Starting in the 1930s, leady oxide became the material of choice. Its principle advantages are:

 

 

Some disadvantages of leady oxide exist, for instance, no batch-to-batch exact repeatability in the manufacturing process. This causes variability in the preparation of pastes and thereby variability in plates. Free lead remains in the plates following pasting and subsequently has to be removed during the curing process. This extends the curing time, increases the inventory of plates and, perhaps most seriously, prevents development of continuous paste mixing-pasting-drying-assembly processes.

With the advent of valve-regulated batteries and battery applications that demand a high degree of reproducibility among cells, non-leady oxides, particularly pure lead monoxide and red lead, have had an upsurge in use. These materials can be made to exact chemical compositions, eliminating most of the variability associated with leady oxide. They also reduce curing time and allow continuous plate-making technology. In addition, red lead reduces formation time and improves initial capacity.

An intriguing development of the late 1990s was the use of basic lead sulfates or partially sulfated oxides to make plates. These materials have the same benefits as non-leady oxides and also reduce the amount of heat developed when the cells are filled with electrolyte. The need for cooling is thereby reduced, as is the time between filling with electrolyte and placing on formation.

Leady oxide still remains the most widely used active material for battery plate production. The Barton and Ball Mill processes are still used to make leady oxide.

Barton processing begins with feeding molten lead and air into a large pot equipped with a rotating paddle to agitate the lead. Represented by the equation Pb + O2 → PbO, the formed lead oxide is drawn off by an air stream, classified with a cyclone and blown to a hopper. Often, it is also passed through attrition mills to produce different particle size distributions for different battery applications.

Varying the conditions under which the reactor operates permits controlling the degree of lead oxidation, particle size distribution and the ratio of αPbO to βPbO in the product. For example, high-temperature-reactor operation reduces the amount of free lead in the oxide and increases the amount of βPbO. By reducing temperature, free lead is increased, as is the ratio of αPbO to βPbO. Air-flow rate through the reactor also affects characteristics of the oxide. Increased air flow increases both free lead and oxide particle size as shown in Table 2-3. Conversely, decreasing air flow reduces both free lead content and particle size. The Barton system is extremely versatile, which is why it is so widely used by the battery industry as shown in Table 2-4.

Attrition mill oxide, also leady oxide, is more commonly produced in Europe and Asia. Barton oxide is more common in the United States. In the attrition mill process, lead balls or ingots are tumbled in a rotating drum. The lead is abraded through friction and oxidized by air passed through the drum. The oxide produced this way is generally finer than Barton oxide. It has a greater acid absorption value, usually in the range of 220-240 mg/g. The oxide contains no β modification, since the temperature of the reaction in the attrition mill is only about 100°C.

 

 

 

Free Lead

Apparent

Density

Acid

Absorption

Particle

Size

 

Output

Air Flow

Higher

Higher

Higher

Lower

Coarser

Higher

 

Lower

Lower

Lower

Higher

Finer

Lower

Temperature

Higher

Lower

Lower

No effect

No effect

Higher

 

Lower

Higher

Higher

No effect

No effect

Lower

Table 2-3: Variable conditions and effects in the Barton process

 

Characteristic

Automotive Batteries

Industrial Batteries

Free Lead (% by weight)

18-24

22-28

Apparent Density (g/in.3)

18-24

23-29

Acid Absorption (mg/g)

170-210

130-155

Particle Size: Median (μm)

 % below 1 μm

3.0

10-15

3.5

5-10

Alpha: Beta Ratio

96:4

96:4

Table 2-4: Barton process leady oxide characteristics for different battery applications most important to battery manufacturers

2.3 Pure Lead Monoxide

Pure lead monoxide is becoming widely used by battery manufacturers, particularly of valve-regulated batteries. The most important way it differs from leady oxide is its absence of free lead. Particle size is greater and acid absorption lower than required for automotive batteries, but is about equivalent to the leady oxide used in industrial batteries. The absence of free lead improves the reproducibility of paste mixing and curing. As noted above, with leady oxide some free lead is oxidized during paste mixing and more oxidation occurs during pasting and curing. Since the amount of oxidation is variable in each step, inconsistent paste characteristics result; therefore, there is reduced reproducibility in the finished plates. The result is further variability in the cells made of these plates. This is a major problem in valve-regulated batteries where cell-to-cell voltage repeatability is important for proper long-term operation under oxygen recombination conditions. Another advantage of lead monoxide is that no lead-oxidation-elimination step is required in the curing process. This reduces time and permits design of process conditions for producing the optimum ratio of tribasic and tetrabasic lead sulfates in the plates.

 

Characteristic

Value

Free Lead (% by weight)

0.10

Apparent Density (g/in.3)

26-32

Acid Absorption (mg/g)

125-150

Particle Size: Median (μm)

           %<2 μm

           %<1 μm

4.5

20

5

βPb0 (% by weight

90

Table 2-5: Typical characteristics of battery grade lead monoxide

2.4 Red Lead

Red lead is beneficial in improving the electrochemical performance of lead-acid batteries. The lead is in a higher oxidation state, at a ratio of lead:oxygen = 1.33, than in lead monoxide and its electrical conductivity is also higher. Batteries made with red lead in the positive paste can be formed quicker. They also have an initial capacity higher than those made from leady oxide pastes.

Red lead is normally used in one of two ways: a high-percentage red lead (~80% % by weight Pb3O4) can be blended into a paste mixture to produce the desired amount in the finished paste, or a lower-percentage red lead (~25% % by weight Pb3O4) can be substituted for leady oxide. Either way, paste-mixing and plate-pasting processes are very similar to those with leady oxide. Particle size decreases and acid absorption increases as the percentage of Pb3O4 in the red lead is increased. Thus, if a paste mix is produced containing 25% by weight Pb3O4, a better result is generally obtained when it is added as the higher percentage material due to its higher reactivity.

 

Characteristic

25% by Weight Red Lead

80% by Weight Red Lead

% by weight Pβ3O4

25 3

80 3

% by weight PβO

75 3

20 3

% by weight Pβ

2.5 max.

0.5 max.

Apparent Density (g/in.3)

19-25

16-19

Acid Absorption (mg/g)

170-200

200-230

Median Particle Size (μm)

3.0

2.0

Table 2-6: Typical characteristics for battery-grade red lead

2.5 Oxide Production Methods

Oxide production methods for leady oxide, Pb0 and red lead are illustrated in Figure 2-1. Due to its low cost and flexibility, most lead oxide in the United States is produced by the Barton process. Barton pot oxide is also used as the feedstock for producing pure lead monoxide (Pb0) and red lead.

As shown in Figure 2-1, lead of the required purity is melted in a kettle and then fed into a reactor vessel equipped with a rotating paddle. Air is drawn through the reactor and leady oxide is drawn off by the air stream and classified by conventional methods. By careful control of temperature, air flow and paddle speed, oxide of various characteristics can be produced. Generally, the temperature will influence the amount of free lead and the alpha:beta ratio in the oxide. The velocity of the air flow will influence the amount of free lead and particle size of the material.

Pure Pb0 and red lead are produced in calcining furnaces, in which the raw material is agitated while being heated at the optimum temperature for oxidation to the required product. For Pb0, the temperature is held at 600°C. For red lead, a temperature of 450°C to 500°C is used. For red lead, the furnace is discharged when the desired amount of Pb3O4 is reached. Oxide with 25% by weight red lead can be made in 5-6 hours, while an oxide with >80% Pb3O4 may require 16-18 hours of calcining.

 The most important physical property of the oxide is particle size distribution, influencing both battery performance and life. Though the Barton process is capable of a great degree of flexibility through process parameter adjustments, a system with a hammer mill provides greater control of particle size. In a hammer mill, a variable-speed rotating shaft is fitted with a number of hammers. An air stream is drawn through the mill and controlled by a damper setting. Oxide dwell time in the mill can be varied depending on air velocity, while the amount of grinding is controlled by the number of hammers and rotation speed. The oxide being fed to the mill can be ground to a wide range of particle size distributions. Process settings have been established that allow a high degree of control over median particle size and amount of fine particle in the product. Through a combination of equipment and process, oxide particle size distribution can meet the desired specifications of the battery manufacturer.

 

Figure 2-1:  Schematic diagram of process for production of leady oxide, lead monoxide and red lead

2.6 Lead Sulfates as Starting Material

Other active materials than lead oxides are used by some battery manufacturers as the starting material for the active mass. In the late 1990s, battery designs included electrodes wound into a spiral. These batteries have a very high plate-surface area, resulting in delivery of very high power output. A problem with this type of construction is the heating that occurs when cells are filled with electrolyte. A reaction between sulfuric acid and unreacted oxide in the paste results. The manufacturer may need to install cooling systems to remove the heat before the battery is formed. In extreme cases, the reaction is so rapid that the specific gravity of the electrolyte drops to a dangerously low level, leading to dissolution of the lead sulfate. When the cells are placed on formation and the specific gravity of the electrolyte again rises, the lead sulfate can precipitate in the separator and short circuit the cell.

The problem can be reduced or eliminated by use of pastes composed of lead sulfate and water. Depending on battery application, tribasic lead sulfate, tetrabasic lead sulfate or a mixture can be used. The paste can be made from pure lead sulfates or oxide sulfated to a higher than usual degree. Cells made with highly sulfated plates evolve less heat when they are filled with electrolyte and do not require cooling. Short circuiting problems are also eliminated.

2.7 Sulfuric Acid

Sulfuric Acid plays a dual role in a lead-acid battery, both as a reactant and an electrolyte, i.e., an ionic conductor. It is important that the acid used in the electrolyte is pure and does not contain any impurities that can affect battery performance. The battery industry considers sulfuric acid that meets Federal Specification O-S-801-b to be acceptable for electrolyte manufacture. The impurity limits in Federal Specification O-S-801-b are shown in Table 2-7.

 

Impurity

Maximum Allowed (%)

Organic Matter

0.0

Non-Volatile Matter

0.0250

Alumina

0.0006

Iron

0.0050

Arsenic

0.0001

Antimony

0.0001

Calcium Oxide

0.0006

Chromium

0.0001

Copper

0.0001

Lead

0.0001

Manganese

0.00005

Nickel

0.0001

Platinum

0.00001

Selenium

0.0001

Tellurium

0.00005

Chloride

0.0010

Nitrate and Nitrite

0.0010

Table 2-7: Maximum impurity limits for sulfuric acid in lead-acid battery electrolyte

 

The concentration of sulfuric acid that is used in the various processes for production of lead-acid batteries can span a wide range. Thus, it is usually purchased as 93.19% H2SO4 (1.835 s.g.) and then diluted to the required strength for either the process or the battery type.

2.8 Grids

For the majority of lead-acid batteries, the positive electrode controls performance and life. Battery failures are generally caused by degradation of the positive plate. Based on surveys of failed automobile batteries, the predominant causes of failure have been:

 

 

In the late 1990s, some negative-plate-based battery failure has been identified in valve-regulated batteries in float service. Though not fully understood, the phenomenons most likely cause relates to self-discharge resulting from insufficient polarization while charging.

As noted above, positive plates have to meet different missions depending on battery application. This has resulted in considerable differences among the materials, design and processing of these plates for automotive, industrial and standby applications.

Positive grids in all batteries act as both support structure and electrical conductor. They must be capable of withstanding all handling operations during production, such as casting, trimming, pasting, stacking and welding. They must also have adequate electrical conductivity to reduce Ohmic losses during operation and resist corrosion so that adequate service life will be obtained. Positive grids or, more properly, support structures can be made from pure lead or from lead alloys. While the vast majority of batteries have some kind of grid structure to hold the paste, in the newer spiral-wound designs the paste is applied to a lead foil instead of a grid.

Grids can be made in a variety of ways, for example:

 

 

In book mold casting, the grid is cast in a mold directly from molten lead alloy. In continuous casting, the molten alloy is dispensed onto a rotating wheel and engraved with the grid pattern. In expanded metal and stamping, the grid is formed mechanically from a cast or rolled alloy strip. Book mold casting is more suited to small production runs as the molds on the casting machines can be changed easily and quickly. The other methods, requiring longer set-up times, suit long production runs of the same grid.

2.9 Grid Alloys

Grid alloys used most widely are lead-antimony (Pb-Sb), lead-calcium (Pb-Ca) and lead-calcium-tin (Pb-Ca-Sn). The alloying metals increase the mechanical strength of the lead, increase its creep resistance and improve its castability. These alloys are less corrosion-resistant than pure lead. Thus, other additives are used to modify the grain structure and reduce corrosion. Lead-antimony alloys are generally used in batteries that require deep discharges and long cycle lives. Lead-calcium(tin) alloys are used where the duty cycle requires high-rate discharges and shallow cycling. Some applications of the two classes of alloys are shown in Table 2-8.

 

Battery Application

Alloy

Characteristics

Engine Starting

Lead-calcium(tin)

Good high-rate performance

Automobiles

 

Low gassing rate

Trucks

 

Low self-discharge rate

Lawn Mowers

 

Poor recovery from deep discharge

Recreation Vehicles

 

 

Aircraft

 

 

Motive Power

Lead-antimony

Good cycle life

Materials Handling

 

Higher gassing rate

Airport Tugs

 

Higher self-discharge rate

Golf Cars

 

Good deep-discharge recovery

Mining Vehicles

 

 

Standby Power

Lead-calcium(tin)

Good high-rate performance

 

 

Low float currents

 

 

Low water loss

 

 

Low maintenance

 

Lead-antimony

Good choice for applications where  

  duty cycle involves cycling

Valve-Regulated

Lead-calcium-tin

Good high-rate performance

 

 

Some cycle capability

 

 

Low water loss

 

 

Low maintenance

Table 2-8: Applications of lead-antimony and lead-calcium(tin) alloys

 

Lead-calcium alloys are used in lead-acid battery grids, as calcium has proven to be a beneficial hardening agent. Figure 2-2 is a phase diagram to aid understanding of the hardening process. A phase diagram shows the different phases that occur in a mixture of metals as functions of the relative amounts of the metals and the temperature. Lead-calcium alloys are strengthened by precipitation of a calcium-rich intermetallic compound from a supersaturated solid solution. Supersaturation occurs when the molten alloy is cooled rapidly, for example, during grid casting. Considering the region marked α in Figure 2-2, the strengthening results from a dispersion of the second phase throughout the matrix. As revealed in the diagram, above 0.07% Ca, Pb3Ca crystals are formed. Below 0.07% Ca, Pb3Ca crystallites are found in the alloy, contributing to the strength by impeding grain boundary mobility.

 

Figure 2-2: Lead-calcium phase diagram

 

Lead-antimony alloys are also used in lead-acid batteries, as antimony is another effective hardening agent for lead. The phase diagram for the lead-antimony binary system is shown in Figure 2-3. The diagram reveals that for antimony concentrations lower than 3.5%, the alloys are age-hardenable and can be strengthened by formation of supersaturated solutions. This occurs when the alloy is cooled rapidly during grid casting. In this respect, antimony and calcium alloys display similarities both are hardenable by the formation of supersaturated solutions. In antimony alloys, precipitation of antimony occurs from the supersaturated solution. Between antimony concentrations of 3.5% and 11.1%, a two-phase structure of the α plus eutectic is formed when cooled to room temperature. The eutectic acts as a stiffening framework for the solid solution. As the amount of antimony is increased, increasing amounts of eutectic (α +β) are formed up to the eutectic composition of 11.1% antimony. At this point, the alloy reaches its maximum strength. This framework of eutectic in the solid solution can cause the alloy to become brittle and prone to cracking.

 

Figure 2-3: Lead-antimony phase diagram

 

When tin is added to lead-antimony alloys, it increases the strength by forming an antimony-tin precipitate in the solid solution.

Age hardening refers to the fact that hardening by precipitation of phases from a supersaturated solution is a time-dependent process hardness increases progressively with time after casting. The newly cast grids are soft and flexible, requiring aging for a certain amount of time before they are strong enough to be pasted.

It is common to age harden lead-calcium grids for up to three days and lead-antimony grids for up to two days before pasting.  Figure 2-4 shows the ultimate tensile strength of a 0.08% calcium alloy after various aging times.

     

Figure 2-4:  Aging characteristics of lead-0.08% calcium alloys

 

The air-cooled alloys reach about 80% of their fully aged strength within 24 hours of casting and then slowly increase in strength over time, reaching a level of about 5,000 psi. If the grids are quenched in water, they are considerably harder, reaching a fully aged hardness of about 6,300 psi. This is probably due to a greater degree of supersaturation in the solid solution that results in increased precipitation and strengthening.

The effect of adding tin to the alloy is shown in Figure 2-5.

     

Figure 2-5:  Aging characteristics of cast, air cooled, lead-calcium-tin alloys

 

Alloy compositions used by battery manufacturers are different for each application.

Automotive battery-grid alloys, since the 1970s, have been the lead-calcium(tin) type almost without exception: a result of the introduction of maintenance-free designs where it is either difficult or impossible to add water. Thus, alloys that minimized water loss during charging were required. For some years, alloys containing a lower amount of antimony were used, but since the 1970s they have been gradually phased out in favor of lead-calcium(tin) systems. Table 2-9 lists examples of widely used low-antimony and lead-calcium alloys. The first calcium alloys did not incorporate aluminum. It was added in the late 1970s to prevent calcium loss in the melt pot. Positive plate-grid alloys have also been developed with higher tin levels to provide improved conductivity of the grid/active material interface. An example is shown in Table 2-10.

 

Lead-Antimony

 

Lead-Calcium

Element

Concentration (%)

 

Element

Concentration (%)

Antimony

2.50-3.00

 

Calcium

0.120-0.150

Tin

0.20-0.35

 

Aluminum

0.020-0.030

Arsenic

0.15-0.25

 

Tin

0.010 max.

Copper

0.050-0.080

 

Antimony

0.0010 max.

Silver

0.01 max.

 

Arsenic

0.0015 max.

Bismuth

0.05 max.

 

Silver

0.0050 max.

Iron

0.001 max.

 

Bismuth

0.025 max.

Nickel

0.0015 max.

 

Copper

0.0025 max.

 

 

 

Iron

0.0010 max.

 

 

 

Nickel

0.0003 max.

 

 

 

Tellurium

0.003 max.

 

 

 

Zinc

0.0010 max.

Table 2-9: Typical maintenance-free alloys used in automotive battery positive grids

 

 

Element

Concentration (%)

Calcium

0.085-0.100

Aluminum

0.020-0.030

Tin

0.50-0.60

Antimony

0.0005 max.

Arsenic

0.0005 max.

Silver

0.005 max.

Bismuth

0.025 max.

Copper

0.001 max.

Iron

0.001 max.

Nickel

0.0003 max.

Tellurium

0.0001 max.

Zinc

0.0005 max.

Table 2-10: Positive grid alloy with enhanced conductivity of the grid/active material interface

     

The use of low-antimony alloys in maintenance-free automotive batteries was greater in Europe than in the United States. To reduce gassing to the lowest level possible consistent with good castability, an alloy was developed with antimony in the concentration range of 1.3%-1.5%. It was used in both positive and negative grids. A typical specification for the alloy is shown in Table 2-11. Selenium is added to the alloy as a grain refiner to improve castability.

 

Element

Concentration (%)

Antimony

1.30-1.54

Tin

0.15-0.25

Arsenic

0.10-0.20

Selenium

0.013-0.023

Copper

0.02 max.

Sulfur

0.0015 max.

Silver

0.01 max.

Bismuth

0.05 max.

Iron

0.002 max.

Nickel

0.0015 max.

Table 2-11: Typical specification for very-low-antimony alloy used in automotive battery positive and negative grids

 

Calcium alloys were used in automotive battery negative grids before being used in positive grids, because grid corrosion is insignificant at the negative plate; thus, there is no problem with a passivating-corrosion-layer build-up between the grid and the active material. A common specification for a negative-grid alloy with rapid-hardening characteristics and with aluminum to prevent calcium loss is shown in Table 2-12.

 

Element

Concentration (%)

Calcium

0.120-0.150

Aluminum

0.020-0.030

Tin

0.010 max.

Antimony

0.0010 max.

Arsenic

0.0015 max.

Silver

0.0050 max.

Bismuth

0.025 max.

Copper

0.0025 max.

Iron

0.001

Nickel

0.0003

Tellurium

0.0003

Zinc

0.001

Table 2-12: Negative-grid alloy for rapid hardening for automotive batteries

 

Recently, automotive battery manufacturers have started to use elevated silver levels in positive-grid alloys to reduce corrosion and increase creep resistance at high temperatures. This practice is becoming widespread as under-hood temperatures continue to increase. An example of a calcium/tin/silver alloy is shown in Table 2-13.

 

Element

Concentration (%)

Calcium

0.0300-0.0550

Aluminum

0.008-0.018

Tin

0.550-0.800

Antimony

0.002 max.

Arsenic

0.002 max.

Silver

0.0300-0.0350

Bismuth

0.025 max.

Copper

0.0025 max.

Iron

0.001

Nickel

0.0003

Tellurium

0.0003

Zinc

0.001

Table 2-13: Calcium/tin/silver alloy for high-temperature corrosion and creep resistance

 

Automotive battery plates are welded together in the cells using the cast-on-strap process. A good cast-on-strap alloy has a moderate amount of eutectic (27%-45%) to facilitate easy bonding to the lugs and terminals. The alloy has a large slushy region making it ideal for joining materials. It also contains moderate amounts of copper and selenium as nucleants. A cast-on-strap alloy is also often used for small parts casting. Three cast-on-strap alloys are shown in Table 2-14. The first is general purpose, the second has greater ductility to resist vibration and the third uses selenium in lieu of copper and sulfur as nucleants.

 

General Purpose Alloy

High-Ductility Alloy

Selenium Alloy

Element

Concentration (%)

Element

Concentration (%)

Element

Concentration (%)

Antimony

2.90-3.25

Antimony

2.95-3.25

Antimony

3.00-3.30

Tin

0.15-0.25

Tin

0.10-0.15

Tin

0.04-0.07

Arsenic

0.15-0.30

Arsenic

0.10-0.20

Arsenic

0.04-0.07

Copper

0.050-0.065

Copper

0.02 max.

Selenium

0.012-0.018

Sulfur

0.0005-0.0020

Sulfur

0.0015 max.

Copper

0.05 max.

Silver

0.06 max.

Silver

0.01 max.

Sulfur

0.001 max.

Bismuth

0.05 max.

Bismuth

0.05 max.

Silver

0.004 max.

Calcium

0.0015 max.

Iron

0.002 max.

Bismuth

0.03 max.

Iron

0.0015 max.

Nickel

0.0015 max.

Iron

0.001 max.

Nickel

0.001 max.

 

 

Nickel

0.001 max.

Table 2-14: Examples of cast-on-strap alloys

 

Motive-power batteries use antimonial-alloy positive grids to achieve long cycle lives. The antimony concentration varies from 5%-6%. Table 2-15 shows examples. Note the use of selenium as a grain refiner in the 5% alloy.

 

6% Antimony

5% Antimony

Element

Concentration (%)

Element

Concentration (%)

Antimony

5.75-6.25

Antimony

4.90-5.30

Tin

0.45-0.55

Tin

0.15-0.40

Arsenic

0.10-0.15

Arsenic

0.10-0.20

Copper

0.05-0.07

Selenium

0.02-0.03

Sulfur

0.006-0.008

Copper

0.03 max.

Silver

0.01 max.

Sulfur

0.002 max.

Bismuth

0.045 max.

Silver

0.01 max.

Iron

0.002 max.

Bismuth

0.05 max.

Nickel

0.001 max.

Iron

0.002 max.

 

 

Nickel

0.0015 max.

Table 2-15: Motive-power antimonial positive grids

 

In tubular plate design of motive-power batteries, positive-plate spines are cast from an alloy with a higher amount of antimony to improve flow characteristics into the tube mold. The eutectic composition of 10.5% antimony is often used. Table 2-16 shows the composition of a typical spine alloy.

 

Element

Concentration (%)

Antimony

10.25-10.75

Tin

0.03-0.05

Arsenic

0.25-0.35

Copper

0.02-0.05

Sulfur

0.002-0.006

Silver

0.015 max.

Bismuth

0.05 max.

Iron

0.005 max.

Nickel

0.002 max.

Table 2-16: 10.5% antimony alloy for tubular plate spines

 

Standby-power and valve-regulated batteries use lead-calcium alloys for both positive and negative grids. Batteries designed for cycling service have alloys with a significant amount of tin, while batteries designed for float service contain less tin. Examples are shown in Table 2-17.

 

Batteries for Cycling Service

 

Batteries for Float Service

Element

Concentration (%)

 

Element

Concentration (%)

Calcium

0.085-0.100

 

Calcium

0.060-0.066

Aluminum

0.020-0.030

 

Aluminum

0.011-0.019

Tin

1.40-1.60

 

Tin

0.55-0.61

Antimony

0.001 max.

 

Antimony

0.001 max.

Arsenic

0.0005 max.

 

Arsenic

0.0015 max.

Silver

0.005 max.

 

Silver

0.005 max.

Bismuth

0.025 max.

 

Bismuth

0.025 max.

Copper

0.001 max.

 

Copper

0.0025 max.

Iron

0.001 max.

 

Iron

0.001 max.

Nickel

0.0003 max.

 

Nickel

0.0003 max.

Tellerium

0.0001 max.

 

Tellerium

0.0003 max.

Zinc

0.0005 max.

 

Zinc

0.001 max.

Table 2-17: Lead calcium grids for standby-power and valve-regulated batteries