We are dependent on lead-acid batteries for many uses in our lives that can be subdivided into three broad categories: engine-starting, motive power and standby power.
The most common use of engine-starting batteries is in automobiles and trucks. They provide energy for starting, lighting and fuel ignition. Other uses occur in lawn mowers, snowmobiles, boats and all-terrain vehicles. The features of the battery for these applications are, for example:
discharge at very high rates and at temperatures ranging from -20°F to 200°F
a sufficient reserve capacity to operate vehicle electrical systems when charger fails and to power off-key drains
thousands of engine starts (resulting in shallow discharge/charge cycles)
low self-discharge rates so they operate after long periods of non-use
charging by an alternator
An engine-starting battery must be capable of high bursts of power. All types of polarization must be minimized to produce the highest voltage possible. Therefore, engine-starting batteries are designed with a very large area of working electrode surface. This reduces current density, which in turn reduces activation polarization during discharge. Large surface area is achieved by designing batteries with large numbers of very thin (as thin as 1 millimeter) electrodes. Resistive polarization is minimized by reducing the spacing between plates, reducing metallic conductive paths and using heavy-duty plate-connecting straps. These design features also reduce concentration polarization since the diffusion gradient from the bulk electrode to the reacting surface is reduced.
To provide reserve power in case of charger failure and to operate electrical components with the engine off, the engine-starting battery must contain sufficient active material (lead dioxide, lead and sulfuric acid) in the plates. This necessitates a design compromise requiring the use of thicker plates and a larger amount of electrolyte. The result is an increase in polarization and a reduction in power. Design invariably involves a trade-off between the highest cranking power and adequate reserve capacity.
The function of this type of battery, also called traction battery, is to propel an electric vehicle (EV). EVs are widely used in the material handling industry for supplying energy to fork lift trucks. Other uses include electric golf cars, mining vehicles, airport baggage handling tugs, sweepers/scrubbers and wheelchairs. These applications require the battery to be capable of:
moderate discharge rates (three to six hours of discharge before recharging)
high capacities up to thousands of ampere-hours
cycling at high depths of discharge (over 80% of capacity removed before recharging)
operation over a wide range of temperatures from 0°F to 100°F
five-year life (approximately 1500 cycles).
The principal requirements of traction batteries are high capacity and long life. Since capacity is the fuel that powers an EV, the higher the capacity, the more work that can be done before the need to recharge. For greater capacity, a battery is designed with thick electrodes containing large amounts of active material. To obtain increased active material, the density is increased to a higher level than that used in automotive batteries. Electrodes are considerably thicker than those in automotive batteries. Typical motive-power plates can be 6-7 millimeters thick. Thick electrodes give batteries longer life. Since the principal mechanism that causes these batteries to wear out is grid corrosion, using thick grids also extends life. Since discharge rates are moderate to low and the electrodes are made from thick lead-alloy grids, activation and resistive polarization are not major components of the voltage drop during discharge. This is primarily caused by concentration polarization due to the increased distance for ionic migration and diffusion. However, remember that during deep discharge, the amount of lead sulfate in the electrodes is increased significantly, causing an increase in resistance as discharge progresses. Eventually, active materials in the plates or sulfuric acid are consumed, causing a rapid increase in polarization and a reduction in the voltage at the end of discharge.
The battery is both the energy source and the fuel. Thus, it is important to know the depth of discharge or how much fuel remains. Since the life of a motive-power battery is proportional to depth of discharge, knowing how much capacity has been removed permits recharging before damaging the battery by over discharging.
Most of us never see a standby-power battery, though they are a major segment of the battery industry. Their use is growing at a faster rate than engine-starting and motive-power batteries. They are mostly used as components in larger systems and housed away from public view in dedicated rooms and cabinets. They function to provide energy when the main power is interrupted, i.e., during power outages. For example, we take for granted that our telephones operate during a blackout. In the United States and in other developed countries, entire telephone systems are supported by batteries that can supply power for up to several hours. Standby power permits making emergency calls in spite of power outages. The principal areas where standby-power batteries are used are:
uninterruptible power systems (UPS)
switchgear and control operation
Of these, telecommunications and uninterruptible power systems are the largest segments in the United States with 48% ($310 million) and 27% ($174 million), respectively, of the total standby-power battery sales (1998 figures).
Each of these applications has different electrical requirements which, in turn, require different battery designs. However, they have in common the following features:
wide range of discharge rates
shallow cycling except in emergency situations
narrow temperature range of operation
up to 20-year-life expectancy.
Standby batteries spend most of their life being charged at a rate that is just sufficient to maintain the battery in a state of full charge. This is known as float charging. It is also important that these batteries have low gassing rates during float charging so that water loss is minimized. A low gassing rate is achieved by controlling the polarization of the positive and negative electrodes to a value just sufficient to maintain full charge while at the same time minimizing electrolysis of water. The gassing rate is also reduced by the use of calcium, calcium/tin or low antimony-alloy grids. These alloys have low corrosion rates, thereby prolonging battery life.
The lead-calcium alloy grids in the positive plates slowly degrade by intergranular corrosion. In this process, corrosion takes place between the metallic grains and produces corrosion products that progressively push grains apart. This causes the alloy to expand and the plates to grow larger with time. To allow for this, the battery designer suspends the positive plates on a plastic bar attached to the negative plates, thereby providing space for the positive plate to expand. This design feature allows the grid to expand without placing strain on the cover seal.
Another feature of standby batteries is the use of flame-retardant vent plugs. These are necessary to prevent sparks or arcs from outside the battery communicating with the gases inside the cells and causing an explosion.
As we have already seen, conventional lead-acid batteries evolve hydrogen and oxygen when they are charged. These gases are a result of the electrolysis of water inside the battery and, therefore, water is consumed and must be replaced. Water replacement must be carried out frequently enough to avoid drying out the battery, a job that comprises a major activity of battery maintenance. Also, these gases are an explosion hazard and carry with them a fine mist of sulfuric acid that can be deposited on external conductors, resulting in corrosion. Since oxygen and hydrogen gases emitted from cells can form explosive mixtures, the battery room or enclosure has to be ventilated.
For many years, the battery industry has pursued a battery design that eliminates this maintenance and reduces the work and cost of owning batteries. The primary objective has been to develop a way to recombine the hydrogen and oxygen back into water inside the battery, eliminating the need to add water. The objective has been achieved and batteries that employ recombination principles are now widely available.
This new lead-acid battery design is the valve-regulated or recombination-type battery. Operating on the gas-recombining principle, it gets its name because it is fitted with a pressure-release valve that maintains a certain oxygen pressure inside the battery. In these batteries, oxygen generated inside the cell during charging is recombined within the cell to re-form water.
Normally, when a lead-acid battery is overcharged, oxygen is evolved at the positive plate and hydrogen is evolved at the negative plate. These gases are vented from the battery and the water that is consumed in producing them is replenished periodically from an outside source during normal maintenance. In the valve-regulated battery, a method has been found to recombine the gases inside the cell, thereby avoiding gas emission and the need to add water during the life of the battery.
Some years ago, it was discovered that if oxygen gas diffused to the negative plate, it would react with the negative sponge lead and be consumed. However, the amount of oxygen that could effectively reach the negative plate was severely restricted by the separators and the electrolyte. These formed a barrier to the diffusion of oxygen so that it was easier for the gas to escape from the cell than to migrate to the negative plate. With recently discovered and instituted design changes that promote diffusion of oxygen, virtually all of it can reach the negative plate and be recombined to water.
Oxygen will react at the negative plate in the presence of sulfuric acid as quickly as it can diffuse to the lead surface according to the following reaction:
Pb + H2SO4 + -O2 = PbSO4 + H2O
Thus, the oxygen that diffuses to the negative is converted to water. As a result of this reaction, no water is emitted from the cell and, therefore, no water needs to be added. For this reason, these batteries are sometimes referred to as maintenance-free, although other forms of routine maintenance are still required.
There are two distinct designs of recombination battery currently in use: absorbed electrolyte and gelled electrolyte.
The separator is replaced by a layer of porous glass mat. The cells are sealed with a valve to keep the cell pressurized at 2-5 pounds per square inch. The cell is filled with just enough electrolyte to wet the plates and partially wet the separator, thus creating an electrolyte-starved condition. Because the separator is not completely saturated with electrolyte, oxygen gas generated at the positive plate can diffuse through it and migrate to the negative plate. The pressure valve keeps the gas inside the cell long enough for diffusion to take place. As the oxygen is reduced at the negative plate, the negative-plate lead is oxidized to lead sulfate. This prevents the negative plate from becoming fully charged. Therefore, it does not start to evolve hydrogen. In some designs, an excess of negative active material is included to ensure the negative plate does not become fully charged. This provides additional protection against hydrogen evolution. Since the container of the cell or battery is held under pressure, it must be made of a material that will not distort.
In the gelled electrolyte design, the plates are separated by conventional separators. The cell is filled with a gel composed of sulfuric acid and silica. After the gel is added to the cell, it hardens in a manner similar to gelatin so that it is immobilized. In time, the gel gradually dries out, creating very small cracks and fissures. These act as channels for oxygen to diffuse from the positive plate to the negative electrode.
Though the way in which transport of oxygen to the negative plate is different between the absorbed and gelled electrolyte designs, the principle of operation is the same for both. With the exception of the separator and gel, both types of batteries are constructed the same way.
The recombination reaction in a valve-regulated battery is very sensitive to poisoning by low levels of impurities in the grid, active materials and electrolyte. For this reason, it is important that the alloys used to make the grids contain very low levels of metallic impurities. Virtually all valve-regulated batteries have positive grids made from either lead-calcium-tin alloys or pure lead while the negative grids are cast from lead-calcium alloy.
The plates are made in a manner similar to conventional automotive and industrial batteries. They are coated with a paste made from leady oxide, water and sulfuric acid. Like conventional batteries, the negative plates contain expander. After pasting, the plates are cured and dried using methods yet to be described. Since the amount of compression of the glass mat in an absorbed electrolyte design is very important to achieve exactly the right amount of wetting, it is important also to control the thickness of the plates very carefully.
The assembly methods of valve-regulated batteries differ substantially from those of automotive and industrial batteries. In the absorbed-electrolyte type, the plates are clad in glass felt which acts both as separator and electrolyte absorber. This glass material is highly porous and has the ability to absorb a considerable amount of sulfuric acid. The amount of acid the glass absorbs can be adjusted by altering its compression - the greater the compression, the less the electrolyte absorbed. The correct compression is important to ensure that the cell contains enough electrolyte to sustain the discharge reaction of the battery while making sure that it is not saturated with acid. Mat oversaturation can prevent the diffusion of the oxygen gas from the positive plate to the negative plate.
In the absorbed-electrolyte design, the glass-clad plates are stacked to produce the required capacity and then the element is compressed so that it can be inserted into the container. The cover is cemented or welded in place and the electrolyte is then added. The assembled cell or battery can then be formed in a manner similar to that used for motive-power or standby batteries.
In the gelled-electrolyte type, the cell element is stacked in the standard manner with separators between the positive and negative plates. It is then inserted into the container and the cover is installed. The cell then can be filled with either gelled or liquid electrolyte. If liquid electrolyte is used, the cell is formed, drained and then refilled with gel. Alternatively, the battery can be filled with gelled sulfuric acid and then formed. Cell formation is done more quickly with the liquid electrolyte process than with the gel-filled. However, it does involve the extra step and added cost.
Without the need to add water, most routine maintenance required on flooded batteries is eliminated. Periodic cleaning and servicing are greatly reduced due to the elimination of water spills on top of the battery and the associated corrosion of terminals.
No gases are evolved from the battery because they are recombined inside the container. Thus, there is no need to ventilate the battery compartment or room. These batteries are intrinsically safer than flooded types. The acidic spray emitted from vents of flooded batteries during charging that can cause corrosion of battery terminals and affect adjacent electronic circuitry does not occur.
Electrolyte immobilization prevents acid spills. These batteries can be used on their side, a big advantage in installations where space is limited.
At the time of this writing, valve-regulated batteries have not been developed for applications requiring a large number of deep cycles. With deep-discharge cycling, these batteries display a rapid decline in capacity, known as premature capacity loss (PCL). Two reasons for PCL are proffered:
Another interesting finding, though not fully understood, is that rapid recharging, maybe resulting in better preservation of the structure of the active material, considerably reduces the capacity loss.
Unfortunately, lead-antimony alloys, which are widely used in batteries designed for cycling, cannot be used in valve-regulated batteries. They release antimony ions into the electrolyte that deposit on the negative electrode and increase hydrogen evolution.
These batteries do, however, have better cycle life than conventional flooded lead-calcium batteries. Therefore, they can be used satisfactorily in limited-cycle-life applications.
Though these batteries are often used in float applications, they do not match the 20-year life of flooded types. While still in dispute, a ten-year life is likely the maximum achievable for valve-regulated batteries. A major reason for this is the difficulty in maintaining the proper degree of polarization of the electrodes during float charging. When the cell is charged, the current that would normally polarize the negative electrode and keep it charged is fully consumed in reducing the oxygen that has migrated from the positive electrode. Consequently, the negative electrode is depolarized and will be at its open circuit potential. Under this condition, it gradually self-discharges due to local action and loses capacity. However, if the float current is increased to assure that the negative remains fully charged, it will produce hydrogen gas that will cause loss of water and eventual failure by drying out. This knife edge situation makes it almost impossible to control the polarization of both electrodes to the correct degree during float charging. Impurities in the active material, grids and electrolyte can also affect the polarization of the negative electrode and either increase hydrogen evolution or self-discharge. Work is under way to develop charging strategies to overcome the problem. Catalysts are being looked at that would recombine hydrogen and oxygen, thus allowing the negative to be adequately polarized without risking dry-out.
Valve-regulated batteries are becoming more widely used for standby applications and are finding limited use in cycling applications such as motive power. The major standby uses include:
The major attraction is reduced maintenance for the user.
The most popular motive-power application, at present, is for wheelchairs. The spill-proof properties and lack of gassing are the most attractive features.