3.0 Electrode Paste Manufacturing

Grids are the support structure for the active material. In most lead-acid batteries, the active material is made from a paste of leady-oxide water, sulfuric acid with added fibers for greater strength and, for the negative plate, expanders. The paste formulas and properties are different for positive and negative pastes, each reaching its optimum performance at different densities.

The paste manufacturing process is very important as battery performance and life are determined by the properties of the paste. The way the paste is applied to the grid is also important to reduce the variability in both the weight of paste in the grid and the thickness of the pasted plate.

Before any grid can be pasted, it must be hard enough to withstand the stresses of the pasting operation. As noted above, both calcium and antimony alloys are soft after casting and require some time to harden. Hardening is achieved most commonly by stacking the cast grids on pallets for about three days at room temperature. Hardening can be accelerated with heat. Some battery manufacturers employ controlled-temperature heating chambers. At 150°C (302°F) the grids can be aged in less than 24 hours. The shorter aging time reduces manufacturer inventory of grids in process.

A battery paste is a complex chemical mixture. Its composition depends on the materials used and the nature of the mixing process. A freshly made paste is not at equilibrium and is still undergoing chemical and physical reactions. If not used immediately, it will become hard and unworkable. Thus, in a battery plant, plate mixing and pasting processes are usually coupled so that fresh paste is constantly delivered to the pasting machine and used right away.

3.1 The Chemistry of Paste

When water is added to lead oxide, it first fills the porous structure of the particles and then begins to coat them. The oxide continues to absorb water until all the particles are coated and water has displaced the air that was among the particles. At this point, the paste is a firm mass which will not flow unless force is applied to it. The addition of more water forces the oxide particles apart, causing bulking of the paste and an improvement of flow characteristics. Eventually, water fills all open space among the particles and the paste reaches its maximum plasticity. If water is added beyond this point, the paste becomes loose and solid/liquid surface energy and heat evolve. The pH of the paste at this point is between 9 and 10, indicating that a reaction is taking place between oxide and water. A probable result is the formation of lead hydroxide, releasing hydroxyl ions to the solution.

When acid is added to the mix, heat is evolved and the paste stiffens considerably. Work done by Ritchie et al (E.J. Ritchie et al, ILZRO Projects LE-82 and LE-84, final report, 12/31/71) indicates that the stiffening is due to an increase in the amount of water held in the colloid sheath around the particles. Since the stiffer paste requires more energy for mixing, more power is absorbed and its temperature increases. As more acid is added, the paste changes plasticity and texture and temperature continues to rise. This is due to the heat of reaction, the heat of dilution of the sulfuric acid and the mechanical work of mixing the paste.

When the acid first contacts the wet PbO, it probably reacts to form normal lead sulfate (PbSO4), which reacts with PbO to form basic lead sulfates. At temperatures below 160°F (71.1°C), the stable basic sulfate is tribasic lead sulfate (TRB). At higher temperatures, tetrabasic lead sulfate is formed by loss of water and the reaction with PbO.

4PbO + H2SO4 → PbSO43PbOH2O            (below 160°F)

PbSO43PbOH2O + PbO → PbSO44PbO + H2O          (below 160°F)

Ritchie et al speculate that other basic lead sulfates may be formed as intermediates in these reactions.

The chemical nature of the lead sulfate in the paste is important since it affects the facility with which the plate can be formed, its electrochemical performance and its durability under discharge/charge cycling. TRB is the preferred compound for engine-starting battery plates. It is readily converted to PbO2 during formation and confers high initial performance.

TTB is considerably more difficult to form, particularly in sulfuric acid solutions of concentrations greater than 15%, but it contributes to the formation of a strong and stable morphology in the crystal structure of the plate. In battery types where TTB is preferred, it is more common to produce this during the curing process than during paste mixing.

Leady oxide made by the Barton process contains up to 10% orthorhombic (β) PbO. It has lower stability than tetragonal (α) modification and is partially converted to the tetragonal phase during mixing. The finished paste, at the point of completion of pasting, will contain the following compounds:

      αPbO                                              PbSO44PbO

      βPbO (Barton oxide only)               Pb(OH)2

      PbSO43PbOH2O                           H2O

Paste for Engine-Starting Batteries

Typical paste formulas for the positive and the negative plates are:

 

 

Positive Paste*

 

Leady oxide

2200 lbs

 

Water

110 liters

50 ml/lb oxide

Sulfuric Acid (1.400 s.g.)

80 liters

36 ml/lb oxide

 

 

 

Negative Paste*

 

Leady oxide

2200 lbs

 

Water

110 liters

50 ml/lb oxide

Sulfuric Acid (1.400 s.g.)

66 liters

30 ml/lb oxide

*Plastic fibers are usually added to both positive and negative pastes to increase their dry strength. These are added at about 0.5-0.7 pounds per ton of oxide. Expanders are added to the negative paste at 0.5% to 1.00% of the oxide weight.

 

Most paste mixers are similar in design and operation. The procedure described below is usually followed. Oxide is dispensed into the mixing bowl. Other dry ingredients (fiber and expander) are added. As rapidly as possible, water is introduced. The water paste is mixed for 2-4 minutes to form a uniform mixture. Sulfuric acid is then added. To avoid hot spots, the acid is added slowly and distributed evenly over the paste, using a multi-nozzle dispenser. The sulfuric acid reacts with the lead oxide, causing the paste to get hot. For engine-starting batteries, the temperature of the paste should not be allowed to rise above 150°F (66°C). This prevents the formation of tetrabasic lead sulfate in the paste that would reduce battery performance. After adding the acid, the paste is cooled as rapidly as possible to prevent drying. It is cooled to 120°F (49°C) before being discharged into the pasting machine hopper.

The cooling is necessary to reduce water loss from the paste while it is consumed by the pasting machine. It takes about 20 minutes for the machine to use a ton of paste and considerable drying can occur if the paste is too hot. Excessive temperature can affect the pasting characteristics of the paste and its density, resulting in a variation of plate weights.

Paste consistency is usually measured by determining its density and plasticity. Density is measured by packing the paste into a cup with a volume of two cubic inches. The weight of the paste in the cup is converted to a density reading, which in the U.S. is curiously expressed in units of grams per cubic inch. Elsewhere, paste density is expressed as grams per cubic centimeter. A consensus on the correct density for positive and negative paste does not exist. However, paste density falls generally in ranges as shown below.

 

Engine-Starting Battery Positive & Negative Paste Density

 

Positive Paste

Negative Paste

gram/cubic inch

gram/cc

gram/cubic inch

gram/cc

66-70

4.0-4.3

70-72

4.3-4.4

 

Plasticity of the paste is usually measured by a Globe #1 penetrometer. It is dropped from a height of six inches (152 mm) above the top of the density cup. Penetrometer readings for paste mixes are shown below.

 

Engine-Starting Battery Positive & Negative

Penetrometer Values

 

Positive Paste

Negative Paste

26-28

24-26

Pastes for Industrial Batteries

Industrial batteries, in this book, include those designed for motive power, standby power and valve-regulated types. Such batteries are designed for long life under arduous operating conditions that can include repetitive deep cycling and long periods of float charging. Under these conditions, plate active material can soften and shed from the plate. Thus, the paste has a higher density to give it more strength. Different paste formulas are used for industrial batteries as shown below.

 

 

Positive Paste

 

Leady oxide

2400 lbs

 

Water

135 liters

56.25 ml/lb oxide

Sulfuric Acid (1.400 s.g.)

50 liters

20.8 ml/lb oxide

 

 

Negative Paste

 

Leady oxide

2400 lbs

 

Water

120 liters

50 ml/lb oxide

Sulfuric Acid (1.400 s.g.)

60 liters

25 ml/lb oxide

 

For industrial-battery pastes a temperature cap of 150°F (66°C) is not critical. It is desirable for industrial-battery positive plates to contain tetrabasic lead sulfate (TTB), which begins to form at temperatures above 160°F (71°C). The presence of TTB crystals in the plates can act as sites for the further growth of TTB during the curing process. Cooling the paste rapidly after all acid is added, to avoid drying out during pasting, is as important for industrial batteries as for engine-starting types.

Note that TTB is not formed in the negative-paste mix due to the inhibiting effect of the expander.

The penetrometer readings and densities for typical industrial pastes are:

 

Positive-Paste Densities & Penetrometer Readings for Industrial Pastes

Density

 

Penetrometer (Globe 1)

g/cubic in.

g/cc

 

70-72

4.27-4.39

24-26

 

 

 

 

 

 

Negative-Paste Densities & Penetrometer Readings for Industrial Pastes

Density

 

Penetrometer (Globe 1)

g/cubic in.

g/cc

 

72-74

4.27-4.51

23-25

 

 

 

 

 

Occasionally, due to variations in the oxides characteristics and the ambient conditions, the paste density and penetrometer readings may fall outside the specified range. If density is too high, it is acceptable to add water to the mix while it is still in the paste mixer. Water is never added after the paste mix has been discharged into the cone feeder or the pasting hopper. Water added at this stage does not mix properly into the paste, resulting in inconsistent pasting machine performance and variations in pasted plates. If the density is too low, oxide cannot be effectively added to the paste mix in either the mixer, the cone feeder or the hopper. The oxide never becomes properly incorporated into the paste. The paste formula is adjusted to add more initial water during paste preparation until the density is in the desired range.

3.2 Curing and Hydrosetting

Pasted plates are placed on pallets as they are removed from the pasting line. To prevent them from sticking together, they are flash dried in a tunnel dryer to a moisture content of 8%-9%. Afterwards, if they were made from leady oxide, they will contain about 12%-15% residual free lead. The free lead must be removed and the plates fully dried before they can be used in the assembly process. Curing is the process for accomplishing lead removal and full drying. Another function of curing is to provide the proper chemical structure in the plate for the battery application.

To prepare for curing, pasted plates are either stacked or racked. For stacking, they are placed on pallets in stacks of about ten-inches high separated by air spaces. Up to three layers of stacks can be placed on a pallet. When plates are racked, they are placed on serrated or notched rails separated by about an eighth of an inch. Racking allows better air circulation around the plates, but requires more space and more labor.

Curing can be accomplished in a number of ways. The most common method used in the late 1990s involved placing the pallets in specially constructed curing chambers. They were large ovens in which the temperature and relative humidity of the air could be controlled. Air inside the chamber was circulated to ensure temperature uniformity and to bring oxygen into contact with the stacks of plates.

Oxidation of free lead is embedded in the paste by:

 

 

A curing chamber can hold over 20 pallets of plates at 8,000 plates per pallet, for a total of 160,000 plates. If each contains 60 g of paste containing 12% free lead, 1152 kg (2,537 lbs) of free lead is in the chamber. The amount of air required to oxidize the amount of lead can be calculated from:

 

 

Pb

+ - O2 →PbO

 

207.2 g

+ 16 g → 223.2 g

Therefore, 1152 kg of lead require:

(1152/0.2072) x 0.016 kg = 88.95 kg of oxygen

 

Since 32 kg of oxygen occupy a volume of 22.4 liters at standard temperature and pressure,

 

88.95 kg occupy 22.4 x (88.95/0.032) = 62,265 liters

 

Since air is only 21% oxygen, the equivalent amount of air equals 296,500 liters (10,377 cubic feet) an amount greater than the volume of most curing chambers.

Based on the calculations, it appears unlikely that oxidation of free lead takes place entirely by diffusion of oxygen into the mass of plates. Additional oxidation can be explained as the result of electrochemical corrosion, as illustrated in Figure 3-1.

 

Figure 3-1: Electrochemical corrosion of lead particles during curing

 

The reaction depends on the development of anodic and cathodic areas on the lead particle and the presence of hydroxyl ions from water surrounding the particle. Lead ions dissolve from the particle and go into solution where they are precipitated as lead hydroxide. Therefore, the lead particle becomes negatively charged and attracts hydrogen ions onto its surface. Oxygen diffusing into the plate from the atmosphere is reduced by the hydrogen ions. Areas where oxygen concentration is highest maintain a cathodic state, while areas having less oxygen become anodic. At the anodic areas, lead goes into solution, combines with the hydroxyl ions and is precipitated as lead hydroxide. Thus, the lead particle is progressively converted to lead hydroxide.

This process explains why water is essential for the oxidation of free lead to occur. If the paste is too dry, the reaction stops due to the absence of water to provide the hydrogen and hydroxyl ions. If the paste is too wet, the reaction is inhibited due to the retardation of the diffusion of oxygen through the plate. Oxidation of free lead takes place fastest when water concentration in the paste is between 9% and 4%. Then, the paste contains enough water to supply the reaction, while at the same time allowing diffusion of the oxygen from the air into the plate.

Development of Crystal Structure in the paste occurs during the curing process. A paste in which tribasic lead sulfate predominates works best for engine-starting batteries. For industrial batteries, a paste with a considerable amount of tetrabasic lead sulfate is required. The optimum ratio of tribasic to tetrabasic lead sulfate in industrial-battery paste ranges around 1:1, with the tetrabasic lead sulfate content running from as low as 40% to as high as 60%. To produce a cured plate containing predominantly tribasic lead sulfate, the temperature during curing must not be allowed to rise above 150°F (66°C). In practice, battery manufacturers try to keep the temperature to a maximum of 130°F (54°C).

The conversion of tribasic lead sulfate to tetrabasic lead sulfate takes place at temperatures above 160°F (71°C).

PbSO43PbOH2O + PbO → PbSO44PbOH2O

Curing Processes for Automotive and Industrial Plates

Different curing conditions are suitable for automotive and industrial plates.

Automotive plates principally require the oxidation of free lead and the formation of tribasic lead sulfate. At the start of the process, the temperature of the curing chamber must not be above 100°F (38°C). The relative humidity in the chamber must be above 90% to minimize plate-water loss. Pallets of pasted plates are loaded into the curing chamber as soon as they are full. Chamber doors must be kept closed when not loading plates, as 20 pallets may be pasted during a typical shift. They must be placed in the chamber with adequate air circulation between them. Pallets are stacked in rows, three pallets high. Air circulation must be maintained during loading.

When the chamber is full, or at shifts end, the door is closed, temperature is maintained at below 130°F (54°C) and relative humidity is at a constant 90%. It takes about 16 hours to complete the process. As free lead is oxidized, heat is evolved and chamber temperature is increased. Since plate-stack temperature is higher than the surrounding air, moisture is lost from the stack to the atmosphere. This allows greater penetration of air into the stack, accelerating the oxidation process. After about 16 hours with free-lead oxidation completed, relative humidity is reduced to allow the plates to dry. Drying time may be several hours depending on the number of plates in the stack. A properly cured plate has a free-lead content of less than 2% and a moisture content of less than 1%.

Industrial plates are considerably thicker than automotive battery plates. This reduces the rate of diffusion of oxygen into the stack. Thus, industrial plates are placed on racks rather than in stacks. In addition to oxidizing residual free lead, the objective of curing industrial-battery positive plates is to produce tetrabasic lead sulfate. Curing processes for both types of batteries are the same, except for temperature. Upon completion of the loading of industrial plates into the chamber, the temperature is increased to 170°F - 180°F (77°C - 82°C).  Relative humidity remains at 90%. Due to the greater thermal conductivity of the grid metal, it expands faster than the paste. This causes paste to loosen from the grid if the temperature is increased too quickly.

Therefore, the temperature is increased slowly to ensure that paste and gird remain at the same temperature. At completion of loading, chamber temperature is between 100°F - 120°F (38°C - 49°C). A 5°F (3°C)-per-hour increase is introduced until 170°F - 180°F (77°C - 82°C) is reached, maintaining 90% relative humidity throughout the process. At the high temperature, the conversion of tribasic lead sulfate to tetrabasic lead sulfate proceeds quickly. Maintaining the high temperature for two hours produces an acceptable amount of tetrabasic lead sulfate in the positive plates. Then, the temperature is reduced to 130°F (54°C) and relative humidity to 70%. This initiates the drying process, promoting oxygen permeation of the plates and oxidation of free lead. Under these circumstances, drying is achieved in eight hours. Cured positive plates contain less than 2% free lead, have a moisture content of less than 1% and have about equal amounts of tribasic and tetrabasic lead sulfates. Tetrabasic lead sulfate is not formed in the negative plate. Thus, they can be cured at the temperature level as are automotive battery plates. Figure 3-2 shows an example of a typical industrial late curing process.

 

Figure 3-2:  Typical industrial battery plate curing profile

 

Sometimes, after curing is completed, the plates are subjected to a second drying process to remove the last traces of moisture. The density of the paste is about 4.5 times that of water, meaning 1% of water by weight is equal to 4.5% by volume of paste. This water can inhibit formation by occupying space in the active mass.

3.3 Expanders

The term expander applies to various mixtures added to the negative paste to maintain performance of the plate during the life of the battery. Early in the development of lead-acid batteries, wood separators were widely employed. At the time, expanders were not used nor were they thought necessary. However, when wood separators began to be replaced by rubber separators, a significant loss of life was observed. Clearly, some component in the wood was contributing to the life of the battery. So wood flour was added to the negative plate to correct the problem. Subsequently, lignin, which had probably been sulfonated to some degree by the sulfuric acid electrolyte, was discovered to be the active species in the wood.

Today, most expander blends use lignosulfonates as a major component. There is also some use of synthetic compounds based on b-naphthalene/formaldehyde condensates. Barium sulfate and carbon black, too, are used in expander formulations. Thus, the modern expander usually is a mixture of barium sulfate, carbon black and an organic material. The proportions of these materials depend on the desired characteristics and the life desired from the battery.

The expander is both an anti-passivating agent and an anti-coalescing agent. It prevents the negative plate from being passivated by lead sulfate during discharge and it also prevents the lead particles from coalescing to form a hard, dense, low-porosity mass. Although the amount of expander added to negative plates is very small, it plays a major role in both the performance and the life of the battery. These materials are so effective that battery failure caused by the negative plate is virtually unknown.

Barium sulfate functions to provide sites for precipitation of lead sulfate during discharge. It is electrochemically inactive and has extremely low solubility in sulfuric acid. These properties ensure that it remains chemically unchanged in the negative plate even after prolonged cycling. The ability of barium sulfate to act as a site for lead sulfate precipitation is due to the similar crystal structure of the two materials. Strontium sulfate has also been shown to be an effective anti-passivating agent (A.K. Lorenz, Dissertation, Leningrad, Russia, 1953).

Barium, lead and strontium sulfates are isostructural (M. Miyake, H. Morikawa, I. Minato and S. Iwai, American Minerologist, Vol 63, (1978), 506-510). They belong to the orthorhombic space group and have similar R values and bond lengths as shown in Table 3-1.

 

 

Barium Sulfate

Strontium Sulfate

Lead Sulfate

R

0.043

0.053

0.067

Cation-O Bond Length

2.952

2.831

2.87

S-O Bond Length

1.478

1.474

1.490

Table 3-1: Structural characteristics of barium, strontium and lead sulfates

 

By providing a large number of sites for the precipitation of lead sulfate, the barium sulfate prevents lead sulfate deposition as a thin, impermeable passivating film. The unit cell dimensions of lead sulfate and barium sulfate are so similar that less energy is expended by lead sulfate precipitating on barium sulfate crystals than if energy of nucleation was used to form a new lead sulfate crystal. In other words, lead sulfate deposits on sites where the least expenditure of energy is possible.

Two forms of barium sulfate are used in expanders; blanc fixe and barytes. Blanc fixe is a finely divided, chemically precipitated material, while barytes is ground, purified mineral ore. Typically, blanc fixe will have a median particle size of approximately 1 μm, while that of barytes will approximate 3.5 μm. Blanc fixe is a very effective anti-passivating agent; its small particle size provides millions of sites for nucleation to take place. Barytes is much less effective than blanc fixe and is considered virtually a filler. However, speculation has been proffered that barytes acts as a material for slow release of finely divided barium sulfate. Expanders are frequently made without barytes, but barytes is never used without blanc fixe.

Lignosulfonates and synthetic organic compounds are derived from wood as a byproduct of paper manufacturing. The pulping process for making paper involves either alkaline or acidic treatment of wood chips. In the alkaline pulping process, called the Kraft process, sodium hydroxide and sodium sulfide are used to treat the wood chips. This produces a liquid known as black liquor which contains kraft lignin, sugar acids and various inorganic compounds. This is subsequently treated with acid to precipitate the lignin which is then dissolved in alkali to obtain a purified kraft lignin. At this stage, the lignin is not sulfonated. If desired, the kraft lignin can subsequently be sulfonated to the required degree.

In the acidic process, the wood chips are treated with an alkali metal sulfite (usually Ca++, NH4+, Mg++, or Na+), producing what is known as spent sulfite liquor. During this process, the lignin is partially sulfonated. The spent sulfite liquor contains lignosulfonates, polysaccharides and sugars. For the production of battery additives, the spent sulfite liquor is subjected to high-pressure and temperature catalytic oxidation to produce a partially desulfonated, oxidized lignosulfonate. The major differences between the two processes and the products derived from them are shown in Table 3-2.

 

Property

Kraft Process

Oxylignin Process

 

 

 

pH during pulping

High (alkaline)

Low (acidic)

Sulfonation during pulping

No

Yes

Molecular weight

8,000 - 12,000

10,000 - 50,000

Polydispersity

2 - 3

6 - 8

Sulfonate groups

0

0.2 - 1.2 per monomer

Organic sulfur

1% - 5%

4% - 8%

Solubility

Alkali (pH>10.5)

Acetone

Dimethyl formamide

Methyl cellusolve

Water

 

 

 

Color

Dark brown

Light brown

Functional groups

Larger quantities of phenolic

hydroxyl, carboxyl and

catechol groups. Some side

chain saturation

Smaller quantities of phenolic

hydroxyl, carboxyl and

catechol groups. Little side

chain saturation.

Table 3-2:   Comparison Between the Alkaline and Acidic Pulping Processes

 

Lignosulfonates are very strong rheological modifiers and are used in the production of ready-mixed concrete to reduce the amount of water needed to make a satisfactory consistency. This allows the concrete to dry quicker with less shrinkage and cracking. The same behavior takes place in the preparation of battery paste mixes. They are also strong anti-flocculents with a large hydrophobic organic part (R+) and a small hydrophilic inorganic fraction (SO3-).

 

RSO3Na = RSO3- + Na+

 

The hydrophobic part of the RSO3- anion is adsorbed on the surface of the lead particles, leaving the hydrophilic part facing out to the aqueous electrolyte phase. This results in an increase in the repulsion potential which prevents the particles from coalescing or sintering. This shows up in practice by a reduction in the particle size of the lead with a corresponding increase in the surface area. This improves the high-rate performance of the plates, particularly at low temperature. Examples of the effect of various lignosulfonates on the surface area of negative active material are shown in Table 3-3 (Boden et al Paper presented at the Battery Council International Convention, Nashville, TN, May 1999).

 

 

0.0%

0.25%

0.52%

0.75%

No lignosulfonate

0.2

 

 

 

Vanisperse A

0.2

0.67

0.77

0.83

Maracell XC-2

0.2

0.58

0.78

0.70

Lignotech 1380

0.2

0.56

0.76

0.67

Kraftplex

0.2

0.50

 

0.60

Indulin AT

0.2

0.30

 

0.65

Table 3-3: Effect of various lignosulfonates on the surface area (m2/g) of negative plates (concentrations in % of oxide weight)

 

The organic materials also inhibit passivation by modifying the surface coverage of active lead sites during high rates of discharge. Studies have shown that the organic becomes attached to the active lead surface, inhibiting precipitation of lead sulfate (E. Willihnganz. Transactions of the Electrochemical Society, 92, (1947), T.F. Sharpe, Electrochimica Acta, 1, 635, (1969)).

In addition to their effect on surface area, lignosulfonates also affect the electrode kinetics of lead sulfate reduction to lead. In cyclic voltammetry studies, a pronounced effect was observed on both the potential and shape of the reduction peak (Report on Advanced Lead-Acid Battery Consortium Project BE97-4085, 12/31/98).

The most pronounced effect of lignosulfonates on battery performance is the improvement in low temperature performance at high rates. Consequently, expanders for automotive batteries contain a high proportion of organic material. Many different lignosulfonates have been employed as expanders and these have widely different effects on the performance of lead-acid batteries.

Carbon is added to the expander to improve the conductivity of the active material. It assists in the initial formation and improves performance during deep discharges where the concentration of highly resistant lead sulfate is high. It is usually added to the expander formula in an amount equal to the lignosulfonate.

Expander compositions for various battery applications have specific formulations, including those for automotive, motive-power and standby-power batteries. These are characterized by different operating conditions, discharge rates and depths of discharge and require different proportions of materials as shown in Table 3-4.

 

 

Automotive

Motive Power

Telecommunications

UPS and Other

Barium Sulfate

40% - 60%

70% - 90%

80% - 92%

70% - 80%

Lignosulfonate

25% - 40%

3% - 10%

0% - 10%

10% - 20%

Carbon

10% - 20%

5% - 15%

3% - 8%

5% - 15%

Addition Rate

0.5% - 1.0%

2.0% - 2.5%

2.0% - 2.5%

2.0% - 2.5%

Table 3-4: Typical expander formulations for different battery applications

 

Occasionally, wood flour and soda ash are added in small amounts to motive-power expanders. Barytes is often a component of the total barium sulfate in industrial applications. Expander is added to automotive-battery negative plates at a rate of 0.5% - 1.0%, while 2.0% - 2.5% is generally recommended for industrial-battery applications.

The principle difference in the expanders used in automotive and industrial applications is the ratio of barium sulfate to carbon. In automotive batteries a high fraction of lignosulfonate is used, while in industrial batteries a small percentage of lignosulfonate is employed. The high percentage of lignosulfonate in automotive plates is necessary to produce the high cold-cranking rates required by these batteries. On the other hand, the large amount of barium sulfate in industrial batteries prevents passivation during deep cycling and gives excellent durability.

For telecommunications applications where uniformity of float voltages is important in long cell strings, lignosulfonate is often omitted from the expander due to the organic constituent having a strong effect on the hydrogen overpotential. This causes variations in voltage from cell to cell which can result in the string becoming unbalanced.

Both the type and amount of lignosulfonate and the amount of barium sulfate used in the plate have a marked effect on plate performance and life. Studies on automotive batteries (D.P. Boden, Journal of Power Sources, 1998) indicate that cold-cranking performance and cycle life increase as the amount of lignosulfonate is increased up to 0.5%. See Figures 3-3 and 3-4. Above this, the performance declines due to overexpansion of the plate with the consequent loss in conductivity.

 

Figure 3-3: Effect of lignosulphonate concentration on cold-cranking performance

 

Figure 3-4: Effect of lignosulphonate concentration on life cycle

 

The cold-cranking performance of automotive batteries is independent of the amount of barium sulfate in the plate as seen in Figure 3-5. This indicates that passivation is not a serious performance limiter at high rates of discharge. A more plausible explanation is that the performance is limited by mass transfer. Barium sulfate does, however, have a significant effect on cycle life as shown in Figure 3-6. The number of J240 cycles increases as the amount of barium sulfate is increased up to a concentration of 1.0%. Above this, the cycle life remains constant.

 

Figure 3-5: Effect of barium sulphate concentration on cold-cranking performance

 

Figure 3-6: Effect of barium sulphate concentration on life cycle