ACCUMULATOR (STORAGE CELL). In the case of a voltaic electrical battery or primary cell, the progress of a chemical reaction or series of reactions is utilized for the p..oduc don of an electric current, and on completion of the chemical changes, the efficiency of the battery may be restored by renewing the electrodes and electrolyte. With an accumulator or secondary cell, on the other hand, the change of chemical into electrical en ergy is a reversible process and, by applying an electric current to the cell, the chemical substances involved in the reaction are re stored to their original condition. Theoretically any reversible cell can be used as an accumulator of electrical energy. Cells of practi cal importance to which this article is limited are the lead-acid cell and the nickel-iron alkali cell. The former of these is based on the changes which metallic lead and its compounds undergo in dilute sulphuric acid by the action of the current. The accumulator devised by Plante in 1859, con sisted of two sheets of lead sepa rated by strips of rubber and rolled in the form of a spiral.
After repeatedly charging and discharging with occasional re versals of polarity the capacity of the cell is gradually built up, one plate acquiring a coating of spongy lead and the other one of lead peroxide (fig. I).
In 1881 Faure' devised a process for pasting on the plates com pounds of lead which could be more easily formed into the active materials of the cells. A marked increase in the capacity over the Plante cell was thus obtained, but the adherence of the active material to the plates proved to be unsatisfactory. The numerous types of cells now in use differ principally in the manner in which these active masses of spongy lead and lead peroxide are pro duced and retained in the electrodes. Volckmar in 1881 introduced the use of lead plates provided with perforations which were filled with a mass of pulverized lead. Swan' devised a grid of cellular structure to act as support for the active material. The Correns grid patented in 1888 consists of a double lattice or mesh work of intersecting ribs of triangular cross section with the apexes pointing inwards so as to form rectangular spaces into which the paste is pressed.
The Plante type of plate which is largely used for positives may consist of a lead plate provided with deep grooves or thin vertical laminations strengthened at intervals by horizontal binding ribs. In a further modification, the Rosette positive plate (fig. 2), the plate is provided with circular perforations into which are pressed rosettes of corrugated pure lead tape. These rosettes provide the active material, and during the process of formation they expand in the holes in the grid.
A form of plate employed extensively for the negatives of sta tionary batteries is the box type (fig. 3).
This consists of a hard lead alloy frame cast in two halves, each having an outer face of finely perforated sheet lead and a grid cast on the inner face. The active material is inserted into the box or cages formed by placing the two halves of the grid against each other, after which the latter are riveted together. This device enables the use of spongy lead in a loose form which would not be retained by an open grid. This type of plate is not, however, suitable for light portable cells on account of the extra weight and thickness of the plates.
The Plante type of plate is used almost exclusively for the posi tives of modern cells for general purposes where weight is not the main consideration but for negatives this type of plate has been practically entirely superseded by other designs which employ the Faure system of the pasted plate. The life of a pasted positive is short compared with a Plante plate but for most purposes this disadvantage is compensated by its greater capacity for a given weight, and the pasted type of positive is in general use for every kind of portable cell, such as those employed for car lighting and starting, electric vehicles and tram lighting.
The accumulator or battery is completed by assembling in juxtaposition a group of the positive plates with a suitable par allel spacing. The plates are secured by burning the projecting lugs on the top surface to connecting bars of lead or lead-antimony alloy. A group of negative plates are arranged in the same man ner, with the projecting lugs on the side opposite to the lugs of the positives. The two groups are interleaved, the positive and negative groups alternating. Actual contact between plates of op posite polarity is prevented by the insertion of a suitable type of separator.
The types of separators used include grooved boards of wood, glass, rubber or celluloid which secure the plates along their edges or else thin perforated diaphragms of hard rubber which are connected between the plates and prevent short circuiting through buckling of the plates or the formation of metallic con nection through scaling or "sprouting" from the electrodes. An improved design of separator is described by M. Wildermani. In this method use is made of a porous form of ebonite which is moulded from powder and mounted in a frame of solid ebonite. By means of electro-osmotic action, liquid is drawn through the pores of the diaphragm during the charge or discharge of the cell and the circulation of electrolyte on the surfaces of the plate is thus promoted. The increase in the diffusion of free acid brought about by this automatic mixing and circulation of the electrolyte between the electrodes improves the performance of the cell by raising the capacity at a given discharge rate. The frame of the diaphragm which fits closely in between the plates prevents at the same time their buckling and bending. The assembled plates are then mounted in the containing vessel composed of glass, cellu loid, hard rubber or lead-lined wood and an electrolyte of a solution of sulphuric acid in water added. The complete cell is always arranged so that the outside or end plates are negative. The cells are connected in series for which purpose the positive bar of each cell is joined by burning or bolting to the negative bar of the next cell in the series.
The electrolyte employed for lead batteries consists of a dilute solution of sulphuric acid. For stationary batteries of low discharge rate, a solution of density 1.20 to 1.23 is employed, cor responding to 28-32% by weight of acid in solution while for portable batteries and those of high discharge rate such as em ployed in starting, and truck and tractor batteries, a solution of density 1.27 to 1.3o is employed corresponding to 36-4o% by weight of acid in solution. In hot climates acid of lower concentra tion is required on account of the increased rate of chemical action.
Chemical Characteristics of the Lead Accumulator._A feature of the lead accumulator is that the reacting lead com 'Chem. Ind. 1927, xlvi. 5x.
pounds and products are highly insoluble in the electrolyte ; the only change which is produced in the electrolyte during charging and discharging is consequently a variation in the concentration of sulphuric acid. This change in concentration is brought about both through the formation of lead sulphate and through the production of water by the reaction attending the discharge, and conversely, the production of sulphuric acid and loss of water from the electrolyte.
The mechanism of the action of the lead accumulator during the charging is most satisfactorily explained by the "double sul phate" theory first put forward by Gladstone and Tribe in 1882. This theory accounts most directly for the observed formation of lead sulphate at the electrodes and the fall in concentration of sulphuric acid during discharge. The total chemical change occur ring during the operation of the cell is accordingly expressed by the equation: the reaction proceeding from left to right during discharge and conversely from right to left during charge. During the discharge, both the lead peroxide of the positive and the lead of the nega tive plate are thus transformed into lead sulphate, and sulphuric acid is removed from, and water added to, the electrolyte. During the charge, lead sulphate is oxidized at the anode to lead peroxide and reduced to lead at the cathode, in both cases with liberation of sulphuric acid to the electrolyte while loss of water occurs by electrolytic decomposition. A cell is fully charged when the plates regain their original condition and the passage of the current pro duces no further change in the active materials which consist of lead peroxide and spongy lead respectively. When current is passed beyond this stage "gassing" results at both plates due to the liberation of free hydrogen which occurs at the cathode and oxygen at the anode. The free "gassing" at both plates and the attainment of a maximum of specific gravity of the electrolyte are indications of the completion of charging.
At the positive pole, according to a theory of Le Blanc, lead peroxide passes into solution in the form of hydrate Pb(OH)4 which dissociates to give tetravalent lead ions, and OH'. The concentration of these ions is determined by their solubility product in accordance with the relation C is the concentration and k is a constant. The Pb— ions may also be regarded as entering into equilibrium with the SO," ions to form Pb This compound can only be present at a very low concentration and exerts a tendency to lose two positive charges and change to bivalent lead, or +2 .3). The positive charge is delivered to the outside circuit and the bi-valent lead then reacts to form lead sulphate as at the negative pole and the lead peroxide of the positive is thus gradually replaced by solid lead sulphate. A further chemical reaction which accom panies the above changes during the discharge of the cell is the combination of H' ions formerly associated with 504" and the OH' ions from the lead tetrahydrate, to form water. The complete reactions which take place during the discharge can be expressed by the following scheme :— At the negative pole lead sulphate dissolves to a certain extent and dissociates to and ions and on the application of an external potential more negative than the cell the charges on the Pb" ions are neutralized and the atoms are deposited on the elec trode as solid lead, while the accompanying SO," ions migrate towards the positive pole. At the positive pole the bi-valent lead ions acquire a further charge and are changed into tetravalent ions which then react with water in accordance with the following reactions:— The tetravalent lead may alternatively be regarded as first forming lead bisulphate with the free ions. The lead bisul phate reacts as follows :— The total change in the cell during discharge may accordingly be represented by the scheme:— and conversely, during the charge the reaction is represented by the table : Liebenow (Zeit. Elektroch, 420, 653 (1895) developed a somewhat different theory to explain the reactions taking place. in the lead accumulator. This is based on the assumption that a solution of lead sulphate contains, in addition to and ions, a certain quantity of ions, formed by the union of with 0" ions of the water, or else by hydrolysis, thus At the positive electrode, lead peroxide passes into solution as imparting two positive charges to the electrode. This then reacts with the H' ions of the acid as follows:— The lead ions combine with the 504" ions of the acid to form the solid sulphate At the negative electrode, the change consists in the passing of lead into the ionic condition and then combining with a SO," ion to form solid sulphate as represented by the Le Blanc theory.
During charging the reverse action occurs, i.e., at the positive pole, ions and at the negative ions are precipitated from solution. The ions in solution are furnished by the accumulated at the electrodes.
Influence of Impurities in the Electrolyte on the Opera tion of the Cell—On account of the deleterious action of most impurities, distilled water only should be used in the preparation of the electrolyte. Rain water, if uncontaminated, may be em ployed The influence of different impurities on the action of the cell has been investigated by G. W. Vinal and F. W. Altrup' who ascertained, by periodic weighings of the plates, the amount of formation of lead sulphate which results from the discharge of either the positive or negative plates in sulphuric acid. A certain amount of lead sulphate is formed as the result of local action when the plates are immersed in even the purest acid solutions obtainable. Detrimental impurities may act in the following ways : (I) by corroding the plate, (2) by accelerating the formation of lead sulphate or (3) by being deposited in the pores of the plates. It was found that electrolyte, containing only one part of plati num in ten million parts of solution, increased the local action at the negative plates by 50%. A metal of this type is mainly harm ful in destroying the "overvoltage" of the cell on which its stability depends. The potential difference between the charged accumu lator plates is greater than the value normally required to decom pose the electrolyte with the liberation of hydrogen and oxygen. The fact that this reaction does not occur and lead to the dis charge of the cell on standing is due to the high overvoltage or passivity exerted by the electrodes to the evolution of these gases. In the presence of a highly electro-negative metal this overvoltage effect is destroyed.
Copper, like platinum, deposits on the negative plate, but pro duces less effect. Iron in the ferric condition is reduced to the ferrous condition at the negative plate and then is oxidized to the ferric condition at the positive plate, and also to some extent by the air. The action of iron on the negative plates is much more pronounced than on the positives and the local action produced is in excess of the amount which would be calculated from the reduc tion of the ferric sulphate. Manganese deposits as peroxide upon the positive plates, and produces serious effects. It covers the active material of the plate, closes the pores and causes a large amount of charging current to be wasted as gas.