Walter Hibbert

“ At the end of a discharge it is a common thing for the plates to be standing in 25% acid, while inside the pores the acid may not exceed 8% or 10%. If the discharge be stopped, we have conditions somewhat like fig. 12, and the e.m.f. begins to rise. In one minute it has gone up by about 0·08 volt, &c.
Charge and Discharge.—The most important practical questions concerning an accumulator are:—its maximum rate of working; its capacity at various discharge rates; its efficiency; and its length of life. ”

“ The acid is generally a little stronger in order to get a higher electromotive force (e.m.f.) . To prevent the active material from being shaken out of the grids, corrugated and perforated ebonite separators are placed between the plates. The “chloride” traction cell uses a special variety of wood separator: the “exide” type of plate is used for both positive and negative. Cells are now made to run 3000 or more miles before becoming useless. The specific output can be made as high as 10 or 11 watt-hours per pound of cell, but this involves a chance of shorter life. ”

“ It is desirable that they be disturbed as little as possible till their increase seriously encroaches on the free space. It sometimes happens that brass nuts or bolts, &c., are dropped into a cell; these should be removed at once, as their partial solution would greatly endanger the negative plates. The level of the liquid must be kept above the top of the plates. Experience shows the advisability of using distilled water for this purpose. It may sometimes be necessary to replenish the solution with some dilute acid, but strong acid must never be added. ”

“ The chief faults are buckling, growth, sulphating and disintegration. Buckling of the plates generally follows excessive discharge, caused by abnormal load or by accidental short-circuiting. At such times asymmetry in the cell is apt to make some part of the plate take much more than its share of the current. ”

“ It is worth observing that the reversal of relationship between the observed and calculated curves, which takes place at 5% or 6%, suggests that the chemistry must be on the point of altering as the acid gets weak, a conclusion which has been already arrived at on purely chemical grounds. The thermodynamical relations are thus seen to confirm very strongly the chemical and physical analyses. ”

“ Planté carefully studied the changes which occur in the formation, charge and discharge of the cell. In forming, he placed two sheets of lead in sulphuric acid, separating them by narrow strips of caoutchouc (fig. 1) . When a charging current is sent through the cell, the hydrogen liberated at one plate escapes, a small quantity possibly being spent in Planté’s cell.reducing the surface film of oxide generally found on lead. Some of the oxygen is always fixed on the other (positive) plate, forming a surface film of peroxide. ”

“ Disintegration is due to local action, and there is no ultimate remedy. The end can be deferred by care in working, and by avoiding strains and excessive discharge as much as possible.
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. ”

“ But any additional excitation leads to strong currents one way or the other. Excitation C1 rises with the load on the line, and gives an e.m.f. helping the battery to discharge most when the load is greatest. C2 is dependent on the bus-bar voltage, and is greatest when the generator load is small: it opposes C1 and therefore excites the booster to charge the battery. The exact generator load at which the booster shall reverse its e.m.f. from a charging to a discharging value is adjusted by the resistance R2 in series with C2. A similar resistance R3 allows the excitation of C3 to be adjusted. ”

“ It is necessary to stir the whole as the mixing proceeds and to arrange that the density is about 1190, or according to the recommendation of the maker. About five volumes of water ought to be taken to one volume of acid. After mixing, allow to cool for two or three hours. The strong acid ought to be free from arsenic, copper and other similar impurities. The water ought to be as pure as can be obtained, distilled water being best; rain water is also good. If potable water be employed, it will generally be improved by boiling, which removes some of the lime held in solution. ”

“ In a later pattern a kind of discontinuous longitudinal rib is put in the ribbon, and increases the capacity and life by strengthening the mass without interfering with the diffusion of acid. The negative plate was formerly obtained by reducing pastilles of lead chloride, but by a later mode of construction it is made by casting a grid with thin vertical ribs, connected horizontally by small bars of triangular section. The bars on the two faces are “staggered,” that is, those on one face are not opposite those on the other. The grid is pasted with a lead oxide paste and afterwards reduced ”

“ Accumulators in Central Stations.—As the efficiency of accumulators is not generally higher than 75%, and machines must be used to charge them, it is not directly economical to use cells alone for public supply. Yet they play an important and an increasing part in public work, because they help to maintain a constant voltage on the mains, and can be used to distribute the load on the running machinery over a much greater fraction of the day. ”

“ Except in the case of traction cells, therefore, the makers have not striven to reduce weight to its lowest values. Table I. shows roughly the weight of given types of cells for a given output in ampere hours.
Influence of Temperature on Capacity.—These figures are true only at ordinary temperatures. In winter the capacity is diminished, in summer it is increased. The differences are due partly to change of liquid resistance but more especially to the difference in the rate at which acid can diffuse into or out of the pores: obviously this is greater at higher temperatures. ”

“ The capacity ranges from 20 to 100 ampere-hours, and the current for a single cylinder engine will average one to one and a half amperes during the running intervals.
General Features.—The tendency in stationary cells is to allow plenty of space below the plates, so that any active material which falls from the plates may collect there without risk of short-circuit, &c. More space is allowed between the plates, which means that (a) there is more acid within reach, and (b) a slight buckling is not so dangerous, and indeed is not so likely to occur. ”

“ The whole then looks like a thin earthenware box with the lug of the electrode projecting from one end. The negatives consist of sheet lead covered by active material. Hatch cell. On assembling the plates, each negative is held between two positive “boxes,” the outsides of which have projecting vertical ribs. These press against the active material on the negative plates, and help to keep it in position. At the same time, the clearance between the ribs allows room for acid to circulate freely between the negative plate and the outer face of the positive envelope. ”

“ We have seen that capacity diminishes as the discharge rate increases; that is, the available output increases as the current diminishes. R. E. B. Crompton’s diagram illustrating this fact is given in fig. 16. 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.
Fig. 17.
The resistance varies with the condition of the cell, as shown by the curves in fig. 17. It may be unduly increased by long or narrow lugs, and especially by dirty joints between the lugs. ”

“ Used in parallel with the dynamo, they quickly yield current when the load increases, and immediately begin to charge when the load diminishes, thus largely reducing the fluctuating stress on dynamo and engine for sudden variations in load. ”

“ Never discharge below a potential difference of 1·85 (or in rapid discharge, 1·8) volt. (2) Never leave the cells discharged, if it be avoidable. (3) Give the cells a special full charging once a month. (4) Make a periodic examination of each cell, determining its e.m.f., density of acid, the condition of its plates and freedom from growth. Any incipient growth, however small, must be carefully watched. (5) If any cell shows signs of weakness, keep it off discharge till it has been brought back to full condition. ”

“ The acid must not be put into the cells till everything is ready for charging. (5) A shunt-wound or separately-excited dynamo being ready and running so as to give at will 2·6 or 2·7 volts per cell, the acid is run into the cells. As soon as this is done, the dynamo must be switched on and charging commenced. The positive terminal of the dynamo must be joined to the positive terminal of the battery. If necessary, the + end of the machine must be found by a trial cell made of two plain lead sheets in dilute acid. ”

“ With two lead plates, a maximum of about quarter volt was obtained, the lead in the weaker acid being positive. With two peroxide plates the maximum voltage was about 0·64, the plate in stronger acid being positive to that in weaker. The electromotive force of a cell depends chiefly on the strength of the acid, as may be seen from fig. ”

“ With stronger acid, a true cycle could not be obtained. Taking Streintz’s value, 335 for 25% acid, the second term of the equation is Td E/d T=290 × ·000335=0·0971 volt. The first term gives 88800 calories=1·9304 volt. Adding the second term, 1·9304 + 0·0971=2·2075 volts. The observed value is 2·030 volts (see fig. 10) , a remarkably good agreement. This calculation and the general relation shown in fig. 10 render it highly probable that, if the temperature coefficient were known for all strengths of acid, the result would be equally good. ”

“ Alkaline Accumulators.—Owing to the high electro-chemical equivalent of lead, a great saving in weight would be secured by using almost any other metal. Unfortunately no other metal and its compounds can resist the acid. Hence inventors have been incited to try alkaline liquids as electrolytes. Many attempts have been made to construct accumulators in this way, though with only moderate success. The Lalande-Chaperon, Desmazures, Waddell-Entz and Edison are the chief cells. T. A. Edison’s cell has been most developed, and is intended for
Fig. 24.—Edison Accumulator.traction work. ”

“ Fig. 13 shows a typical discharge curve. Noteworthy points are:— (1) 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. ”

“ References.—G. Planté, Recherches sur l’électricité (Paris, 1879) ; Gladstone and Tribe, Chemistry of Secondary Batteries (London, 1884) ; Reynier, L’Accumulateur voltaïque (Paris, 1888) ; Heim, Die Akkumulatoren (Berlin, 1889) ; Hoppe, Die Akk. fur Elektricität (Berlin, 1892) ”