Tubular plates: tall tubular battery vs flat plate battery
1. Types of lead acid battery plates
Introduction to batteries
There are several types of electrochemical power sources (also known as galvanic cells, voltaic cells or batteries). A battery is defined as an electrochemical device which converts chemical energy into electrical energy and vice versa. The subject of battery comes under electrochemistry, which is simply defined as the subject which deals with the interconversion of chemical energy and electrical energy. In this article we will discuss more in detail about tubular plates.
These cells produce electrical energy by spontaneous oxidation-reduction reactions (redox reactions) involving the chemicals in the positive, negative electrodes and electrolyte, occurring in each electrode, called a half cell. The chemical energy in the active materials converted into electrical energy. The electrons produced in the reduction reaction goes through the external circuit connecting the two half-cells, thus producing an electric current. The oxidation reaction occurs by releasing the electrons from the anode material (mostly metals) and the reduction reaction occurs when the electrons reach the cathode (mostly oxides, chlorides, oxygen etc.) through the external circuit. The circuit is completed via the electrolyte.
Lead-acid battery system:
When the external circuit is closed, electrons begin to travel from the negative pole as a result of the reaction that converts (electrochemically oxidizes) the lead (Pb) to divalent lead ions (Pb2+). (The latter ions react with sulphate molecules to form lead sulphate (PbSO4) inside the cell). These electrons travel through the external circuit and reach the positive plate where they convert the lead dioxide into lead sulphate i.e., the lead dioxide is electrochemically reduced to lead sulphate as a result of the Pb4+ ions being converted into Pb2+ ions in PbSO4.
The cell overall reaction is written as:
PbO2 + Pb + 2PbSO4 Charge ↔ Discharge 2PbSO4 + 2H2O
We can see that the valency of lead (Pb°) increases to Pb2+, by releasing 2 electrons during discharge. This increase in valency is termed oxidation in electrochemical terminology.
In the other direction, the valency of lead in the lead dioxide (Pb has 4 valencies in lead dioxide) gets reduced to 2+
by absorbing the two electrons coming from the oxidation reaction. This decrease in valency is termed reduction in electrochemical terms.
These terms can also be described by the changes in the individual electrode potentials of the cell during discharge. The potential (voltage) of the lead electrode (anode during discharge) increases by moving to more positive values during a discharge. This increase in potential value is termed oxidation. Thus the negative plate potential of lead in the lead-acid cell changes from about -0.35 to about -0.20 volts. This is an increase in potential. Therefore this reaction is termed anodic in nature.
On the contrary, the potential of the lead dioxide electrode (cathode during discharge) decreases by moving towards the negative side, i.e., the value becomes lower and lower as the discharge proceeds. The positive plate potential of lead dioxide in the lead-acid cell changes from about 1.69 to about 1.5 volts. This is a decrease in potential. Therefore this reaction is termed cathodic in nature and we say reduction occurs on a positive plate during discharge.
These reductions in working voltages during discharge arises due to what is called polarization, caused by a combination of overvoltage, η, and internal resistance, which occurs on both electrodes. Simply said, overvoltage is the difference in the OCV and the operating voltages.
Thus, during discharge, Edisch = EOCV – ηPOS – ηNEG – IR.
But, for the charging reaction ECh = EOCV + ηPOS + ηNEG + IR.
IR refers to the internal resistance offered by materials inside the cell like electrolyte, active material etc. The IR depends on the design of the cell, namely the separator used, the pitch between the plates, the inner parameters of active material (particle size, surface area, porosity, etc), temperature and amount of PbSO4 in the active material. It can be presented as the sum of several resistances offered by the top lead, the active mass and corrosion layer, electrolyte, separator and polarization of the active materials.
The first three factors are affected by the cell design. No general statement can be made about the polarization values, but it is usually in the same magnitude as the initial resistance offered by the top lead. Longer plates have more IR. It can be determined from the slope of the initial part of the discharge curve. For the same design, a cell of higher capacity will have a lower internal resistance. The internal resistance of a 12V/28Ah VRLAB is 6 mΩ, whereas that of a lower capacity battery (12V/ 7Ah) is 20 to 23 mΩ.
At very low η values, the relationship between η and current, I, takes the form of Ohm’s law and the above-referred equations get simplified as
Edisch = EOCV – IR.
ECh = EOCV + IR.
The above discussion deals with the discharge reaction of a lead-acid cell.
The opposite phenomena occur during the charge reaction of the lead-acid cell.
In the case of primary batteries, the positive electrode is usually called cathode while the negative electrode is called the anode, and this is unambiguous since only discharge occurs.
Thus the lead electrode which acted as an anode behaves as a cathode during charging reaction and the lead dioxide electrode which acted as a cathode now behaves as an anode. To avoid ambiguity, we use simply positive and negative electrodes or plates in secondary cells.
To illustrate how this works in practice, the following figure shows some hypothetical curves for the discharge and charge of a lead-acid battery.
It is clearly seen that the practical discharge voltage lies below the open-circuit voltage of 2.05V, and the practical charge voltage lies above this value. The deviation from η is a measure of the combined influence of the internal resistance of the cell and the polarization losses. Whenever the discharge or charge current is raised, the value of η becomes greater, in accordance with the equations given above.
To summarise the reactions:
Lead, the negative active material:
During discharge: Pb → Pb2+ + 2e-
During charge: Pb2+ → Pb (i.e., PbSO4 → Pb)
Lead dioxide, the positive active material:
During discharge: Pb4+ → Pb2+ (PbO2 → PbSO4)
During charge: Pb2+ → PbO2 (i.e., PbSO4 → PbO2)
Since both the electrode materials are converted to lead sulphate, this reaction was given the name “double sulphate theory” by Gladstone and Tribe in 1882.
Classification of batteries
Depending on the nature of the electrochemical reactions occurring in these cells, they can be classified into
- Primary batteries
- Secondary (or storage battery or accumulator)
- Fuel cells
At the outset, it is better to understand the differences between these types. In the primary battery, the electrochemical reaction is irreversible, whereas, the secondary cells are known for their reaction reversibility. The fuel cell is also a primary cell, but the difference between the fuel cell and a primary cell is that the reactants are kept outside the cell container, whereas in a primary cell the reactants are there inside the cell.
- In the primary cells (e.g., silver-oxide-zinc cells used in wristwatches, MnO2- Zn cells used for flash torches and remotes for AC units, TVs, etc) fall in this category, In these cells, the reactions can proceed only in one direction and we cannot reverse the reaction by passing electricity in the opposite direction.
- On the contrary, the secondary calls are known for their reversibility of the energy-producing reactions. After discharge, if we pass direct current in the opposite direction, the original reactants are regenerated from the reaction products. Examples for this type of battery are lead-acid battery, Li-ion battery, Ni-Cd battery (actually NiOOH-Cd battery), Ni-Fe battery, Ni-MH battery, to mention the most common secondary batteries.
- To elaborate the reversibility concept, the lead dioxide (PbO2) in the positive electrode (commonly called “plates”) and lead (Pb) in the negative plate of a lead-acid cell, are both converted to lead sulphate (PbSO4) when both the materials react with the electrolyte, dilute sulphuric acid, during the energy production reaction. This is represented by electrochemists as follows:
- PbO2 + Pb + 2PbSO4 Charge ↔ Discharge 2PbSO4 + 2H2O
- A fuel cell is also a primary cell, but its reactants are fed from outside. The electrode of the fuel cell are inert in that they are not consumed during the cell reaction, but simply help in electronic conduction and have electrocatalytic effects. The latter properties enable the electro-reduction or electro-oxidation of the reactants (the active materials).
- The anode active materials used in fuel cells are usually gaseous or liquids fuels such as hydrogen, methanol, hydrocarbons, natural gas (the materials that are rich in hydrogen are called fuels) which are fed into the anode side of the fuel cell. As these materials are like the conventional fuels used in heat engines, the term ‘‘fuel cell’’ has established itself to describe such type of cells. Oxygen, mostly often air, is the predominant oxidant and is fed into the cathode.
By theory, a single H2/O2 fuel cell could produce 1.23 V at ambient conditions.
The reaction is: H2 + ½ O2 → H2O or 2H2 + O2 → 2H2O E° = 1.23 V
Practically, however, fuel cells produce useful voltage outputs that are far removed from the theoretical voltage of 1.23 V and as a result, fuel cells generally operate between 0.5 and 0.9 V. The losses or reductions in voltage from the theoretical value are referred to as ‘‘polarization,’’ which term and phenomenon is applicable to all batteries to different extents.
Lead acid battery
In the production of lead-acid battery, a variety of positive electrodes (or as commonly called, “plates”) is employed:
a. Flat plate or grid plate or pasted plate or lattice-type or Fauré plate (1.3 to 4.0 mm thickness)
b. Tubular plates (inner diameter ~ 4.9 to 7.5 mm)
c. Planté plates (6 to 10 mm)
d. Conical plates
e. Jelly roll plates (0.6 to 0.9 mm)
f. Bipolar plates
- Of these the first-mentioned flat-plate type is the most widely used; though it can supply heavy currents for a short duration (for example, starting an automobile or a DG set), it has a shorter life. Here, a lattice type of rectangular current collector is filled with a paste made from a mixture of leady-oxide, water and sulphuric acid, carefully dried and formed. Both positive and negative plates are made in the same manner, except for the difference in additives. Being thin, batteries made from such plates can supply very high currents needed for starting an automobile. The life expectancy is 4 to 5 years in such an application. Before the advent of the alternator-rectifier arrangement, the life was shorter.
- Tubular plates: The next widely used type of plate is the tubular plate which has a longer life, but cannot supply a burst of current as in the flat plate type of batteries. We discuss the tubular plates in detail below.
- For a long life with the most stringent reliability requirement in places like power stations and telephone exchanges, the type of lead-acid cell preferred is the Planté type. The starting material for the tubular plate is about 6-10 mm thick casting of high purity lead sheets with numerous thin vertical laminations. The basic surface area of the tubular plate is vastly enhanced by the lamellar construction, which results in an effective surface area which is 12 times that of its geometric area.
- Conical plate is lattice-type circular-shaped pure lead grids (cupped at a 10° angle), plates stacked horizontally one above the other and made from pure lead. This was developed by Bell Telephone Laboratories, USA.
- Jelly roll plates are thin continuous grid plates made from a low-lead tin alloy of 0.6 to 0.9 mm thickness facilitating high rates. The plates are pasted with lead oxides, separated by an absorbing glass mat, and spirally wound to form the basic cell element.
- Bipolar plates: These plates have a central conducting sheet made either from metal or conducting polymer and having positive active material on one side and negative material on the other side. Such plates are stacked in such a manner that opposite polarity active materials face each other with a separator in between them., to obtain the required voltage.
- Here separate inter-cell connection is eliminated, thereby reducing the internal resistance. It may be noted that the extreme plates in a bipolar battery are always of the mono-polar type, either positive or negative
2. Differences in the performance of various types of plates
Flat plate batteries are meant for high current, short duration discharge as in the automobile and DG set starting batteries. They usually have a life of 4 to 5 years and the end of life is mainly due to the corrosion of the positive grids, resulting in the loss of contact between the grid and active materials and subsequent shedding.
Tubular plates are robust and hence have a life of about 10 to 15 years in float operation. They are also suitable for cyclic duty and offer the highest cycle life. The active material is contained in the annular space between the spine and the oxide-holder. This restricts the stress due to the volume changes occurring when the cells are cycled.
The end of life is again due to the corrosion of the spines and loss of contact between the spines and the active material. However, the contact area between the spine and the active mass is reduced in such a construction and hence under heavy current drains, the higher current density results in local heating leading to rupture of tubes and crack in the corrosion layer.
Planté plate cells have the longest lifetime, but the capacity is poor compared with other types. But these cells offer the highest reliability and longest float lives. Their cost is also higher, but if it is estimated over the lifetime it is actually lower in comparison with other stationary type cells. The reason for longer life is that the positive plate surface is continuously regenerated with virtually no loss in capacity over its lifetime.
Conical plate cells are specially designed by Lucent Technologies (formerly AT&T Bell Laboratories) for a very long life of more than 30 years. Recent 23-year corrosion data projects a life of 68 to 69 years for such batteries.
The jelly roll design lends itself to mass production due to excellent mechanical and electrical characteristics. Jelly-roll construction (spirally-wound electrodes) in a cylindrical container can maintain higher internal pressures without deformation and can be designed to have a higher release pressure
than the prismatic cells. This is due to an outer metal container used to prevent deformation of the plastic cases at higher temperatures and internal cell pressures. The range of venting pressures may be as high as 170 kPa to 275 kPa (25 to 40 psi » 1.7 to 2.75 bar) for a metal-sheathed, spirally wound cell to of 7 kPa to 14 kPa (1 to 2 psi » 0.07 to 0.14 bar ) for a large prismatic battery.
Bipolar plate batteries
In the design of a bipolar plate, there is a central electronically conducting material (either a metal sheet or a conducting polymer sheet) on one side of which is positive active material and the other, a negative active material. Here separate inter-cell connection is eliminated, thereby reducing the internal resistance. It is to be noted that the extreme plates in a bipolar end cells are always of the mono-polar type, either positive or negative.
These batteries have
- Higher specific energy and higher energy density (i.e., 40% less volume or 60% the size of a regular lead-acid battery, 30% less weight or 70% the mass of regular lead-acid batteries.
- Double the cycle life
- Half as much lead is needed and other materials are also reduced.
3. Applications of tubular plate batteries
Tubular plate batteries are used mainly where there is a requirement of a long life with higher capacity. They are mainly used in standby applications in telephone exchanges and large factories for material handling trucks, tractors, mining vehicles, and, to some extent, golf carts.
Nowadays, these batteries are ubiquitously found in every household for inverter-UPS applications.
Extra tall type plates (as tall as 1 metre and more) are employed in submarine batteries to provide power when the submarine is submerged. It provides silent power. The capacities vary from 5,000 to 22,000 Ah. The submarine cells have air-pumps inserted into them to nullify the acid stratification of electrolyte for 1 to 1.4 m tall cells.
Gelled electrolyte tubular plate valve-regulated lead-acid batteries are extensively used in non-renewable energy systems like solar applications.
Thin tubular plate EV batteries for vans and buses find applications in EV field and are able to deliver 800 to 1500 cycles depending on the spine thickness and specific energy.
The following table illustrates the relationship among the spine thickness, plate pitch, electrolyte density, specific energy and number of life cycles.
|Tube Diameter mm -->||7.5||6.1||4.9|
|Electrolyte Density (Kg/Litre)||1.280||1.300||1.320|
|Number of spines||19||24||30|
|Tubular plate pitch||15.9||13.5||11.4|
|Specific energy (Wh per kg) at 5 hour rate||28||36||40|
Reference: K. D. Merz, J. Power Sources, 73 (1998) 146-151.
4. Manufacture of tubular bags, tubular plates and tubular plate batteries:
The early tubular plate was constructed with individual rings by Phillipart and with tubular bags by Woodward were reported in 1890-1900 and the use of slotted rubber tubes (Exide Ironclad) was developed by Smith in 1910.
The assembly of individual tubes on to the spines was practised earlier and this was a slower operation than inserting a complete grid into a multi-tube design. Moreover, the physical bonding between the individual tubes of the multi-tube gives a greater rigidity during the unit operation of filling. The bowing of the spines due to lateral movement is eliminated. These are the reasons why battery manufacturers prefer to use PT Bags multi-tube gauntlets.
Tube preparation. Nowadays multi-tubes or PT Bags (gauntlets) are produced from chemically resistant glass or organic fibres (polyester, polypropylene, acrylonitrile copolymers, etc.) by weaving, braiding or felting methods.
In the early days of multi-tubes, horizontally woven cloth in a yarn of the copolymer of vinyl chloride and vinyl acetate was used. Two layers of the cloth were passed on either side of a row of cylindrical formers (mandrel) and the seam between adjacent formers was heat welded.
But the vinyl acetate degenerated to release acetic acid which, in turn, resulted in spine corrosion and premature battery failure. Furthermore, heat sealing had to be controlled and dimensioned. If the sealing pressure was exceeded a limit, the seams were weak and soon the layers separated in service. On the contrary, if the sealing pressure was too heavy, the sealed was good but the actual seam was thin and soon came apart in service.
Whilst this did not cause a serious problem in service, there was the tendency for the seam to separate during the initial operations of handling and filling and the centre of the tubular plate tended to bow, which created problems in the following unit operations, e.g., sometimes there was difficulty in inserting the plate into the cell container due to the oversized plates.
Various methods were tried to replace the heat sealing, like composite weaving technique in which the tubes were woven in one operation with the filaments crisscrossing between tubes to form an integral seam. Modem multi-tubes use heat sealing or stitching with polyester filaments woven into cloths or nonwoven polyester cloths.
The attraction of the nonwoven cloths lies in the fact that the manufacturing cost is lower because of lower basic material cost through the elimination of the weaving process. However, to attain the same order of burst strength, the nonwoven tube has to be thicker than its woven counterpart. This reduces both the working volume of electrolyte (due to the greater volume nonwoven tube material). The volume of active material within the tube is also reduced, which, in turn, reduces marginally the capacity of the cell.
Excellent tubular plates can be made with either individual tubes or multi-tubes provided
the yarn used in the making of the tubes is one which does not readily denature in service. Both specially formulated glass and polyester filaments meet this requirement.
Tubular plate batteries are either stationary in application or in rolling stock, usually float-charged at a voltage of 2.2 to 2.30 volts per cell, depending on the specific gravity of the electrolyte. Examples are the common inverter/UPS batteries, telephone batteries and train-lighting and air-conditioning cells (TL & AC cells).
In a tubular plate, a series of spines of suitable thickness cast from a lead alloy is connected to a top bus bar, either manually or using a pressure die-casting machine. The spines are inserted into tubular bags and the space between the spines and PT bag (also called oxide-holder) is filled with either dry oxide or wet thixotropic paste. The spines are kept in centre position by star-like protrusion provided in the spines. The PT bags are made invariably from woven or felted polyester fibres. The tubular plates so prepared are subsequently pickled, cured/dried and either tank-formed or jar-formed with suitable electrolyte density.
The filling oxide can have any composition: only grey oxide, grey oxide and red lead (also called “minium”) in varying proportions.
The benefit of having red lead in the positive mix is that the formation time is reduced proportionately to the percentage of red lead it contains. This is because the red lead already contains about one third lead dioxide, the remainder being lead monoxide. That is, the red lead Pb3O4 = 2PbO + PbO2.
Alternately, the filled tubular plates can be directly assembled, after removing the loose oxide particles adhering to the tubes outside, into cells and batteries and jar-formed.
The negative plate is made as usual by following the flat plate manufacturing practice. The expanders are the same, but, the amount of “blanc fixe” is more compared with an automotive paste. The tubular plates are cured in curing ovens for about 2 to 3 days, after being passed through a drying tunnel heated by electricity or gas to remove the superficial moisture, so that the plates do not stick with each other during the subsequent handling processes.
The difference in the initial filling specific gravity of the acid for pickled and unpickled pales arises from the fact that the former contains more acid and so a lower specific gravity is chosen for pickled tubular plate batteries, usually about 20 points lower. The finishing specific gravity of the electrolyte is 1.240 ± 0.010 at 27°C.
The higher the specific gravity of the electrolyte, the more will be the capacity obtainable fro0m these batteries, but the life will be adversely affected.
Or, the tubular plates can be tank-formed, dried and assembled and charged as usual.
5. Different types of Tubular plate
Most of the battery manufacturers employ cylindrical tubes for making tubular plates and batteries. Even in this the diameter of tubes and consequently, that of the spines may vary from about 8 mm to 4.5 mm.
However, the tubes may also be oval or flat or square or rectangular types. The basic structure is the same as the forerunner cylindrical tubular plates (as shown above).
7. Advantages of using Tubular plates
Tubular plates are very much noted for their long life because of the absence of shedding of active material. The active material is held by the tubular bag and hence a lower packing density can be used for maximising the coefficient of use. The higher porosity thus resulting can also help in using more active material in the energy production process. The thicker the spine, the more will be the life cycles that can be obtained from such tubular plates.
The number of life cycles is anywhere between 1000 to 2000 cycles depending on the thickness of the plates. The thicker the tubular plate, the more will be the number of cycles they give. It is said that the tubular plates can offer twice the number of life cycles when compared with a flat plate of the same thickness.
8. How the battery life is improved by using Tubular plates?
As discussed above, the life of a tubular plate battery is higher than the flat plate batteries. The following sentences describe the reasons for the longer life expectancy of tubular plate batteries. Most importantly, the active material is rigidly held by the oxide holder tubes, thus preventing the shedding of the material, which is the main reason for the failure of batteries. Also, in the course of time, the spines get a protective cover of lead dioxide which helps in reducing the corrosion rate of the spines. Corrosion is simply, the conversion of the lead alloy spine into lead dioxide.
Thermodynamically lead and lead alloys are unstable under a high anodic potential of more than 1.7 to 2.0 volts and under the corrosive atmosphere of sulphuric acid tends to be corroded and converted to PbO2.
Whenever the cell is on charge at voltages far removed from the open-circuit voltage (OCV) on the higher side, oxygen is evolved as a result of electrolytic dissociation of water and the oxygen is evolved on the surface of the positive tubular plates and has to diffuse to the spine for corroding it. Since there is a thick layer of positive active material (PAM) surrounding the spines, the oxygen has to travel from the surface by a long distance and so the corrosion rate tends to be reduced. This helps in prolonging the life of tubular plate cells.
9. What battery applications should ideally use tubular battery plates?
Tubular plates are employed mainly for high-capacity long-cycle-life batteries such as in industrial in-house transport vehicles (forklifts, electric cars, etc.). It s also used for energy storage application like Battery Energy Storage System (BESS), where the capacity of the cells may be as high as 11000 Ah and 200 to 500 kWh and up to a 20 MWh.
Typical applications for BESS are for Peak shaving, Frequency control, Spinning reserve, load levelling, Emergency power, etc
Nowadays, each and every household in some countries have at least one tubular plate battery for inverter-UPS applications. Not to mention some commercial establishments, for example, browsing centres, where a continuous supply of energy is needed.
Recently, gelled tubular plate valve-regulated lead-acid batteries are extensively used in non-renewable energy systems like solar applications. Here the gelled type is the best suited.
EVs requiring 800 cycles with 40 Wh/kg specific energy can best use the thin tubular EV batteries. The capacity range available is 200Ah to 1000Ah at 5 h rate.
10. Important technical features of a tubular plate battery
The most important technical feature of the tubular plate battery is its ability to retain the active material throughout its life expectancy without the shedding process happening in the normal course and thus laying the foundation for long life.
The batteries employing such plates have a long life of 15-20 years in stationary applications under float charge conditions, such as telephone exchanges, energy storage. For cyclic operations (such as traction batteries), the batteries can deliver anywhere from 800 to 1500 cycles depending on the per-cycle energy output. The lower the per-cycle energy output, the higher will be the lifetime.
The tubular plates are best suited for the solar applications in gelled electrolyte valve-regulated version with no stratification problem in the electrolyte. Since it requires no periodic topping up with approved water and since no obnoxious gases are emanating from these cells, they are eminently suited for solar applications.
Of the electrochemical power sources used nowadays, the lead-acid battery outnumbers all other systems considered individually. In the lead-acid battery, the ubiquitously present automotive batteries lead the team. Next comes the tubular plate industrial battery. The automotive batteries have capacities in the range of 33 Ah to 180 Ah, all in monoblocs containers, but the other type has a capacity of 45 Ah to thousands of Ah.
Small capacity tubular plate batteries (up to 200 Ah) are assembled in monoblocs and large capacity 2v cells in single containers and connected in series and parallel arrangements. Large capacity tubular plate batteries are used as stationary power sources in telephone exchange, energy storage establishments etc. Traction batteries have several applications like material handling trucks, forklifts trucks, Golf carts etc.