showing that discharge to 1'6 volt caused injury that did not
Capacity and Efficiency under Various Conditions
of Working.
arise at a limit of 1 '8. Before describing the new results it will he useful to examine these two cases in the light of the theory of Charge. Efficiency. e.m.f. already given. Experiment. Ampere Watt Ampere Watt (a) Fall in e.m.f. at beginning of discharge.—At the moment Hours. Hours. Hours. Hours. Quantity. Energy. when previous charging ceases the pores of the positive plate contain strong acid, brought there by the charging current. There Normal cycle . 102 97-2 87-4 201-7 104-5 230-7
is consequently a high e.m.f. But the strong acid begins to Restoration diffuse away at once and the e.m.f. falls rapidly. Even if the 103-8 228-2 96-8 85-8 after 1st rest. 100 190 cell were not discharged this fall would occur, and if it were allowed Ditto, after 2nd to rest for thirty minutes or so the discharge would have begun 82-8 rest 176-7 96-8 213-2 9491 with the dotted line (Fig. 14). (&) Final rapid fall. The pores Ditto, after 3rd being clogged by sulphate the plugs cannot get acid by diffusion, 9584-7 rest 82-6 161-3 86-2 190-5 and when 5 per cent, is reached the fall in e.m.f. is disproportion- Discharge') 65-5 581 86-2 190-5 110-5 ately large (see Fig. 11). If discharge be stopped,_ there is an 56-5 immediately j79-6 69-6 almost instantaneous diffusion inwards and a rapid rise in e.m.f. after rest J 56-5 110-5 71 "1 ii 158-3 (c) The rise in e.m.f. at beginning and end of the charging is due to Restor a tio n acid in the pores being strengthened, partly by diffusion, partly by 85 156-9 83-8 1 184-6 95-5 after 8 cycles. 80 formation of sulphuric acid from sulphate, and partly by electrolytic carrying of strong acid to the positive plate. The injurious results at 1-6 volt arise because then the pores contain water. The The table shows that the efficiency in a normal cycle may be as high as 87*4 per cent. ; that during a rest of sixteen days the charged accumulator is so affected that about 30 per cent, of its charge is not available, and in subsequent cycles it shows a diminished capacity and efficiency; and that by repeated charges and discharges the capacity may be partially restored and the efficiency more completely so. These changes might be due to—(a) leakage or shertcircuit, (b) some of the active material having fallen to the bottom of the cell, or (c) some change in the active materials.
(a) is excluded by the fact that the subsequent charge is smaller, and (b) by the continued increase of capacity during the cycles that follow the rest. Hence the third hypothesis is the one which must be relied upon. The change in the active materials has already been given. The formation of lead sulphate (on both plates) explains the loss of energy shown in Fig. 24, while the fact that it is probably formed, not in the path of the regular currents, but on the wall of’ the grid (remote from the ordinary action), gives a probable explanation of the subsequent slow recovery. The action of the acid on the lead during rest must not be overlooked. We have seen that capacity diminishes as the discharge rate increases ; that is, the available output increases as the current diminishes. Crompton’s diagram illustrating this fact is given in Fig. 28. At the higher rates the consumption of acid is too rapid, diffusion cannot maintain its strength in the pores, and the fall comes so much earlier. The resistance varies with the condition of the cell, as shown by the curves in Fig. 29. It may be unduly increased by long or narrow lugs, and especially by dirty joints between the lugs. It is interesting to note that it increases at the end of both charge and discharge, and much more for the first than the second. Now the composition of the active materials near the end of charge is almost exactly the same as at the beginning of discharge, and at first sight there seems nothing to account for the great fall in resistance from 0-0115 to 0'004 ohm; that is, to about one-third the value. There is, however, one difference between charging and discharging—namely, that due to the strong acid near the positive, with a corresponding weaker acid near the negative electrode. The curve of conductivity for sulphuric acid shows
chemistry is altered, oxide or hydrate is formed, which will partially dissolve, to be changed to sulphate when the sulphuric acid subsequently diffuses in. But formed in this way it will not appear mixed with the active masses in the electrolytic paths, but more or less alone in the pores. In this position it will more or less block the passage and isolate some of the peroxide. Further, when forming in the narrow passage its disruptive action will tend to force off the outer layers. It is evident that limitation of p. d. to 1'8 volt ought to prevent these injuries, because it prevents exhaustion of acid in the plugs. The other curves will now be easy to follow. Figs. 16 and - 17 show a normal charge and discharge between the limits of 2 4 and 1'8 volt. After the next charging the cells were allowed to rest for ten days. On discharge they showed a smaller capacity, and this was the case for several subsequent charges and discharges, though repeated chargings at last brought them back to something like the previous values. Compare Figs. 18 and 19. Two other similar series were tried ; that is, rest followed by repeated and continuous work to bring the cell back to its first state (see Figs. 20, 21, 22, 23). Then another charge was followed by a rest of sixteen days. The discharge which immediately followed the rest is shown in Fig. 24, while Fig. 25 shows the next charge—a great falling - off. The cells were now taken through eight cycles of 1 This discharge is here compared with the charge that preceded the charge and discharge, and were then so far restored as to give
Figs. 26 and 27. Integrating the curves, the following numbers rest; in the next line the same discharge is compared with the charge following the rest. were obtained :—
