ACCUMULATOR - ELECTRICAL CHARACTERISTICS Accumulators in Repose.-Accumulators contain only three active substances—spongy lead on the negative plate, spongy lead peroxide on the positive, and dilute sulphuric acid between them. Sulphate of lead is formed on both plates during discharge and brought back to lead and lead peroxide again during charge and there is a consequent change in the strength of acid during every cycle. The chief properties of these substances are shown in the Table.
'Jour. Amer. Instit. Elect. Eng. 1924, xliii. 709.
The curve in fig. 4 shows the relative conductivity of all the strengths of sulphuric acid solutions, and by its aid and the figures in the preceding table, the specific resistance of any given strength can be determined.
The lead accumulator is subject to three kinds of local action. First and chiefly, local action on the positive plate because of the contact between lead peroxide and the lead grid which supports it. In carelessly made or roughly handled cells this may be a very serious matter. It would be so in all circumstances if the lead sulphate formed on the exposed lead grid did not act as a covering for it. It explains why Plante found "repose" a useful help in "forming" and also why positive plates slowly disintegrate ; the lead support is gradually eaten through. Secondly, local action on the negative plate when a more electro-negative metal settles on the lead. This often arises when the original paste or acid contains metallic impurities. Similar impurity is also introduced by scrap ing copper wire, etc., near a battery. Thirdly, local action due to the acid varying in strength in different parts of a plate. This may arise on either plate and is set up because two specimens of either the same lead or the same peroxide give an e.m.f. when placed in acids of different strengths. J. H. Gladstone and W. Hibbert found that the e.m.f. depends on the difference of strength (Jour. Inst. Elect. Eng. 1892). The observations with very strong acid were difficult to obtain. C. Hein (Elek. Zeit 1889), F. Strienz (Ann.
Phys. Chem. xlvi. 449) and F. Dolezalek (Theory of Lead Accu mulators, p. 55) have also given tables.
It is only necessary to add to these results the facts illustrated by the following diffusion curves, in order to obtain a complete explanation of the behaviour of an accumulator in work. Fig. 6 shows the rate of diffusion from plates soaked in acid of density 1.'75 and 'then placed in distilled water as described in a paper by R. Duncan and H. .Weigand (Elec. World N.Y. 1889), who were the first to show the im portance of diffusion. About one half the acid diffused out in 3o minutes, a good illustra tion of the slowness of this process. The rate of diffusion is much the same for both posi tive and negative plates, but slower for discharged plates than for charged ones.
The influence of diffusion on the electromotive force is illus trated by fig. 7. A cell was pre pared with 20% acid. It also held a porous pot containing stronger acid, and into this the positive plate was suddenly transferred from the general body of liquid. The e.m.f. rose by diffusion of stronger acid into the pores. Curve I. in fig. 7 shows the rate of rise when the porous pot con tained 34% acid, curve II. was obtained with the stronger (58%) acid. Of these two curves the first is more useful because its conditions are nearer those which occur in practice.
At the end of a discharge it is a common occurrence for the plates to be standing in 25% acid, while inside the pores the acid may not exceed 8% or o%. If the discharge is stopped, we have conditions somewhat like fig. 7 and the e.m.f. begins to rise and increases by about o•o8 volt in one minute.
In examining the effect of repose on a charged cell, Gladstone and Tribe's experiments show that peroxide of lead lying on its lead support suffers from local action, which reduces one molecule of Pb0, to sulphate at the same time that an atom of the grid below it is also changed to sulphate. There is thus not only a loss of the available peroxide but a corrosion of the grid or plate. It is through this action that the supports gradually give way. On the negative plate an action arises between the finely divided lead and the sulphuric acid, with the result that hydrogen is set free :— This involves a diminution of available spongy lead, or loss of capacity, occasionally with serious consequences. In the discharge it becomes sulphated in excess since the better positive maintains the e.m.f. too long. In the succeeding charge, the positive is fully charged before the negative and the differences between them tend to increase in each cycle.
Fig. 8 shows a typical discharge curve. Noteworthy points are: (I) at the beginning and at the end there is a rapid fall in P D. with an intermediate period of fairly uniform value, (2) when the P.D. reaches 1.6 volt the fall is so rapid that there is no advantage in continuing the action. When the P.D. had fallen to 1.6 volt the cell was automatically switched into a charging circuit, and with a current of 9 amperes yielded the curve in fig. 9. Here again there is a rapid variation in P.D. (in these cases a rise) at the beginning and end of the operation. The cells were now carried through the same cycle several times, giving almost identical values for each cycle. After some days, however, they became more and more difficult to charge, and the return on discharge was proportionately less. It became impossible to charge up to a P.D. of 2.4 volts, and finally the capacity fell away to half its first value. Examination showed that the plates were badly scaled, and that some of the scales had partially connected the plates. These scales were re moved and the experiments resumed, limiting the fall of P.D. to 1.8 volt. The difficulties then disappeared, showing that discharge to 1.6 caused injury that did not arise at a limit of i.8. Before describing the new results it will be useful to examine these two cases in the light of the theory of e.m.f. already given. (a) At the moment when previous charging ceases the pores of the positive plate contain strong acid, brought there by the charging current. There is consequently a high e.m.f. But the strong acid begins to diffuse away at once and the e.m.f. falls rapidly. Even if the cell were not discharged this fall would occur, and if it were allowed to rest for 3o minutes or so the discharge would have begun with the thin line (fig. 8). (b) The pores being clogged by sulphate, the peroxide cannot acquire acid by diffusion, and when 5% is reached the fall in e.m.f. is disproportionately large (see fig. 5). If discharge is stopped, there is an almost instantaneous diffusion inwards and a rapid rise in e.m.f.
In fig. 10 diagrams are given of two types of chloride cells showing the curves of voltages during charges following 5-hour discharges, and curves for discharges at 1, 3, 5 and i o hour rates.
Each pair of plates is separated either by glass tubes which are held in position by guiding pieces cast on to the lugs of the nega tive plates, or by wood board separators which have previously been specially treated to remove deleterious matter. In cells with lead-lined wood boxes, glass tubes are also inserted between the end negative plates and the lead lining of the box to prevent the plates coming into direct contact with the lead lining.