CHAPTER XI.

STORAGE OR SECONDARY CELL.

The development of the storage or secondary cell has been one of the most important electrical advances of the century. For purposes of experiment or work, where a large or steady current is required from compact and readily tended apparatus, the storage cell proves its utility. The simplest form was that used by the early experimenters, and as it is easy to make, a form of it may very well be described.

From a sheet of lead ⅛ inch thick two or more pieces are cut of the requisite size, say, 5 inches square. In making these plates, they should be cut so as to leave a strip 1 inch wide and 3 inches long, projecting from one corner, _A_ (Fig. 62), for the purpose of connection. This is for the reason that the fumes of the sulphuric acid solution would quickly corrode any wires or screws in the plates, and also to give a better connection. The number of plates cut must be an odd one, as it is general to make the two outside plates of the same polarity—viz., negative. These plates are then scored with a steel point across and across on both sides to perhaps a depth of 1∕64 of an inch. This scoring is not absolutely necessary; it somewhat hastens the formation of the plates. The plates are then laid face to face, being separated by pieces of wood, rubber, or, still better, by a piece of grooved wood, Fig. 63 having a thin piece of asbestos on each side. These grooves are to carry off the gas, and should run up and down the board, as in the figure. The wood is ⅛ of an inch thick or thereabouts, and preferably perforated with holes ¼ of an inch or larger. When laid together, a few strong rubber bands hold the plates from coming apart. To prevent lateral motion, a few rubber pins may be thrust through the plates. The alternate strips are to be connected together in two series, as in a condenser, and the complete series placed in a jar containing a mixture of seven parts of water to one of sulphuric acid. The terminal of the strips connected to the smallest number of plates is to be marked _P_ or +, for positive.

This terminal is now to be connected to a charging current (not over 1 ampere), as described in the directions for charging batteries, for eight hours, and then discharged at a rate not over 1 ampere for six hours. Then the connections are to be reversed and the cell charged backward, as it were, and discharged. This has to be repeated for a long period, perhaps a month, before the cell is in good condition; on the final charge it is to be connected positive to positive of charging source. This operation is called "forming," and the result is to change the metallic lead of the positive plate into red-brown peroxide of lead, and the lead negative plates into spongy lead.

In modern commercial cells this operation is no longer pursued, the plates are variously constructed of lead frameworks holding plugs of litharge or lead oxide, which is "formed" with great facility. For many purposes other than operating Ruhmkorff coils, a few simple cells made, as described, are handy to have and easy to make. In sealing the cells up for portability, care must always be taken to leave a small hole in the cover for the escape of the sulphurous acid gas.

CHARGING STORAGE BATTERIES.

Although the charging of a storage or secondary battery is by no means a difficult operation, yet it requires care, and one unaccustomed to the work will meet many slight difficulties which may seriously affect the results. Pre-eminently the best charging source is a direct current, constant potential electric-light circuit. The amount of current required varies according to the type and make of the cell, but we will select one of a capacity of 50 ampere hours for illustration.

By 50 ampere hours is meant a delivery of 1 ampere per hour for fifty hours, or a rate of discharge equal to the above, as 2 amperes per hour for twenty-five hours. In practice a secondary cell will not be found to act exactly as above, the total amount of current decreasing as the discharge is greater. Each cell is constructed to discharge at a certain rate, above which it is not safe to go. Five amperes per hour is a suitable rate for a fifty-hour cell, and should not be greatly exceeded. The Chloride type, however, is one which can be discharged at a higher rate than normal without any serious results, the latter being generally a bulging or "buckling," as it is called, of the plates whereby they short circuit or fall apart. The voltage of the charging source should be at least 10 per cent over that of the battery when fully charged. The voltage of a cell of storage battery varies from about 2.3 at commencement of discharge to 1.7, at which latter voltage discharge must be stopped and charging recommenced.

Fig. 64 shows the connections to charge a storage battery from an electric-light circuit. The latter must be direct current and of low tension. The circuit from the negative lead runs to the rheostat handle _R_, thence through as many coils as are in circuit (varied by moving the handle over the contact pieces in connection with the resistance coils). The positive of the cell is connected to the positive main.

In connecting storage cells to the mains the utmost care must be taken that the terminals are correctly attached. It happens in isolated plants that some change is made in the wiring or the switchboard, which reverses the current without warning being given to the battery charger. It is the safest way to test the polarity of the terminals of _both_ battery and mains each time charging is commenced. For polarity tests see Chapter I. It is immaterial on which side of the battery the rheostat or similar device is placed.

Fig. 65 shows the employment of lamps instead of the rheostat. The lamps _L L_ regulate the current flow by the manner in which the circuit is arranged. If only one lamp be turned on, the current necessary for only one lamp circulates through the battery. Each additional lighted lamp adds to the current by decreasing the resistance of the circuit. _S_ is a switch which must always be left open when the dynamos are to be stopped.

CHARGING FROM PRIMARY BATTERY.

In many instances an electric-light circuit is not available for charging purposes, in which event recourse must be made to a primary battery. The one most suited for the work is the modified Daniell, or copper and zinc combination in solutions of sulphate of copper (bluestone) and sulphate of zinc respectively.

There are many good forms of this cell on the market, chief of which are the simple gravity, the Gethins, and the Hussey, which have been previously described. An example will now be described of the operations necessary with the gravity cell, charging one 50-ampere hour storage cell. At least six cells of gravity will be required, as the voltage of each cell is never over 1 volt, and is dependent on the resistance in the external circuit falling as the resistance is lowered. Place the six clean glass jars on a firm foundation, where there is no liability of shaking and no dust likely to settle. Unfold the copper strips into the form of a star, bending the corners for half an inch so as to give an anchorage in the bluestone. Place them into the bottom of the jars and pour in water enough to cover them at least 3 inches below the surface. Now carefully drop in 4 pounds of clean bluestone, which will fill in the angles between the copper wings, at the same time holding the element down to the bottom of the jar. Hang the zincs from the top edge of the jar, and fill up with water to 1 inch from the top. The addition of 5 ounces of sulphate of zinc per cell will render the cells immediately available, and for the further hastening of the chemical action, the copper wire from each copper may be inserted in the binding post-hole of the zinc belonging to its own cell and screwed tight for a few hours; or the cells may be connected together in series, and the wire from the last copper be screwed to the zinc of the first, thus putting the whole series on short circuit. The only advantage of the first method being a saving of time when a number of cells is being set up. This saving of time is often of consequence, as the longer the newly set-up cell is on open circuit, the more copper will be deposited on the zinc, which is highly undesirable. This is shown by the blackening of the zinc as soon as it is put in the solution, which blackening it is hard to prevent entirely. When the cell is working satisfactorily it will show a clearly defined line between the colorless solution above and the deep blue solution beneath.

Gravity cells should never be moved. If no sulphate of zinc is available, half a teaspoonful of sulphuric acid may be poured in over the zinc, which will tend to form the sulphate of zinc. Without any of these helps the cell will take at least twenty-four hours on a short circuit before it will give its normal current. This current should be from 4∕10 to 5∕10 of an ampere. Five cells set up by the writer varied after the addition of the zinc sulphate from 200 milli-amperes (thousandths of an ampere) to 300 milli-amperes, although they were apparently all set up alike; but after twelve hours' short circuiting they all gave a fairly uniform current of from 470 to 500 milli-amperes.

From time to time on storage battery work, say, every week, the specific gravity of the top solution must be tested with a hydrometer (see Fig. 66), which should be put into the solution and allowed to come to rest. The indicated number at the level of the liquid should be 25°. If the number is higher some solution should be drawn off and clear water added, until the hydrometer settles down to 25° or thereabouts. The inside of the glass jar for 1 inch from the top may be greased to prevent the salts of zinc creeping over the edge, or half an inch of heavy paraffin oil be poured on the top to prevent evaporation and creeping. When the zinc gets very much coated with the dark deposit it must be taken out and scraped and washed. When the bluestone needs replenishing, drop in carefully and be sure none lodges on the zinc element.

SETTING UP THE STORAGE CELL.

Each manufacturer of storage cells issues specific directions for the charging of his own make, but generally the method is as follows: The acid solution is prepared by mixing one volume of sulphuric acid to from four to seven volumes of water, according to the make of the cell. The sulphuric acid should have a specific gravity of 1.82 and be chemically pure. _The acid must always be poured into the water, and slowly, stirring all the time, then set aside for the mixture to cool._ It is best to mix the solution in a separate earthenware vessel, and when two or more cells are to be set up, to mix all the solution at one time, to ensure the same strength, unless a hydrometer is used to determine this.

A good method to ascertain the exact quantity of solution required is to place the elements in the jar and cover 1 inch deep at least with water, then remove the elements and pour off the volume of water corresponding to the proportion of acid to be added, and lastly pouring the remaining water into the mixing vessel, prepare the solution, or electrolyte, as it is called. New elements should be wetted with pure water before being immersed in the solution. An ordinary charge of the electrolyte requires from six to ten hours to cool thoroughly, as considerable heat is evolved in the mixing.

Having now prepared the storage battery solution and set up the primary cells, the charging can be proceeded with. The current must be turned on the storage cell immediately the elements are placed in the acid. Connect the wire from the zinc of the primary battery to the negative of the storage cell and the copper wire to the positive. As the current from a gravity cell is but small, it will take quite a time to charge a storage cell of 50 ampere hours' capacity fully; it is a good scheme to get the cell charged up from a dynamo source, and use the gravity cells to keep it charged; but this cannot always be done, and the gravity battery will do the work in time. As the best storage cells render but 90 per cent of the current put into them, they must be charged over the number of hours for which they are required to deliver current.

When the cell is fully charged the solution will become milky and give off gas freely. This gas in large quantities is detrimental to health, and on no account should a storage cell be _charged_ in a sleeping apartment. It affects the throat and lungs, and renders them susceptible to take cold under suitable circumstances. The average voltage of storage cells, when tested with the charging current on, is 2.4 volts, and the lowest they should be allowed to reach is 1.9 volts, unless otherwise specified by the manufacturers.

Cells in poor condition are liable to form a _white_ deposit of sulphate of lead, this fault being known as "sulphating." This trouble requires much careful nursing, and the cells must be charged for a long time at a very low rate until the plates of the positive element regain their normal gray color. Chips of straw or excelsior, etc., falling in between the plates will carbonize and cause trouble.

Most portable cells are sealed, but all cells can be easily sealed with paraffin wax for amateur use. Cover the elements fully ½ inch above the normal height of the electrolyte with water before pouring in the electrolyte. Melt some paraffin in an earthenware jar and pour it on top of the water, about the middle of the surface, when it will spread, and care having been taken to have the jar sides dry, will cake solid and form a good seal. Then bore a hole with a brace and bit or some such tool through the wax and pour out the water. The cell can then be set up as usual, the hole being only partly closed to allow of the escape of the generated gas. A glass or rubber tube can be sealed into the hole in the wax, and makes a more finished job.

While on the subject of primary batteries for charging storage cells, a few remarks on their electromotive force may not be amiss. Although the specifications issued by the manufacturers specify an excess charging voltage of 10 per cent over the total voltage of the storage cells, this does not apply to primary cells in its entirety. The voltage of five gravity cells in series would aggregate 5 volts, and the voltage of one storage cell but 2 volts, but there would not be 5 volts available to force the charging current through the latter. In the first place there is the counter electromotive force of the storage cell working against the gravity battery. Simple subtraction would show only 3 volts excess in favor of the primary electromotive force; but the working voltage of a galvanic cell varies according to external resistance of the cell and the external resistance of the circuit. When the internal resistance is high, as in the gravity cell, and the circuit resistance is low, in this case being the storage cell, the available electromotive force of the primary is low also.

In many cases it is desired to operate a Ruhmkorff coil from an electric-light main direct. This can readily be done if the circuit be of the constant potential class—that is, one constructed to furnish current for incandescent lamps in multiple. With the direct current, such as the Edison, all that is necessary is either to interpose a rheostat, as in Fig. 64, or to use the lamps, as in Fig. 65. The manner of connecting up is the same as if the storage cell B be replaced by the coil. Using the formula _C_ = _E_∕_R_, for example, if the circuit be at 110 volts and the coil require 10 amperes, a resistance of 11 ohms will be required. Or using the lamps in the diagram, Fig. 65, about 20 lamps are to be put in circuit. If the current be an alternating one, the contact-breaker will have to be screwed down or short circuited.

THE "U. S." STORAGE CELL.

This cell is of the lead-zinc type, being the practical form of the Reynier cell. It is to be recommended for working Ruhmkorff coils, its output weight for weight being far in excess of the lead-lead types. This cell is readily portable and easy of operation, the zinc electrode being the only one needing renewal, and that at very infrequent intervals.

The lead electrode consists of plates of peroxide clamped together, and presents quite a large surface. The zinc in most types is of the circular sheet form, and encloses the lead block, being kept amalgamated by mercury lying in the bottom of the cell. The E. M. F. on open circuit is about 2.5 volts, which is higher than any lead-lead combination. On closed-circuit work this drops to from 2.35 volts downwards. During action, when a large amount of current is being drawn from the cell, a white sulphate appears, but this disappears upon the cell being recharged or even left to rest. Bubbles of gas, which sometimes form under the peroxide block, should be removed by gently tilting the cell or hitting the table or shelf upon which it stands a smart blow. The large type No. 3 is suitable for X-ray work, and a still larger cell is made, which is preferable for heavy or continuous discharges of current.

HARRISON CELL.

The No. 1 cell recently put upon the market has given excellent results for open circuit work. It consists of a negative element with lead peroxide as a depolarizer. The positive element is self-amalgamating zinc, which is free from local action. The electrolyte is dilute pure sulphuric acid. The potential is high, being 2.5 volts, and the internal resistance is 0.14 ohm. This cell belongs to a group which is midway between primary and storage, or secondary cells. Its construction is similar to the lead-zinc secondary cell, in place of which it may be used, it being easy to recharge an exhausted cell by passing a weak current through it in reverse direction, thus recharging the peroxide of lead grid and renewing the zinc and electrolyte.

The large size, or type No. 3, which the manufacturers are producing, differs from the No. 1 cell in that it has a larger negative element, or grid, and has two zincs, instead of one; consequently, it has a lower internal resistance—0.07 ohm—and a higher discharge rate with a capacity of 150 ampere hours. The potential is 2.5 volts. It is suitable for coil work or for sparking gas engines, and for ease of manipulation and convenience is to be highly recommended.

The elements are shown in Fig. 67, lead grid _L_, which is filled in with paste of peroxide of lead, and which neither buckles nor disintegrates. The zinc _Z_, however, possesses a novel feature. A cavity is cast in the zinc element and filled with an amalgam of mercury, the copper electrode passing through this amalgam into the solid zinc, as shown in the cut. As the action of the battery proceeds, this amalgam forces its way into the pores of the element and keeps up so good an amalgamation of both copper rod and zinc that zincs can be used up to a point when the rising internal resistance makes it economy to throw them away, and absolutely no perceptible local action takes place in the cell upon continued open circuit. A preparation is furnished if desired, which forms a jelly of the electrolyte, making the cell readily portable. Like all of these combinations, its electromotive force exceeds two volts, and its internal resistance is low enough to advise its employment in coil work.

When a storage battery is to remain unused for a long time it must first be fully charged, and then every week or so the charging current passed through it until it bubbles. Where it is to be laid away for a long period of time, and weekly charging is not feasible, the following operations are necessary: First, fully charge battery, remove electrolyte, and replace by water immediately. Discharge at normal rate until voltage runs down to 1.7 per cell. Gradually decrease resistance until battery is almost on short circuit. Let it stand for a day, then pour off the water, and keep elements in a dry, clean place.