The Thomas A. Edison Papers Digital Edition

Transcription

I have to report on the Edison cells which I have been testing.
Four cells were submitted to me, three small and one large. The small cells have been charged and discharged for four months, the larger one for one month. Careful measurements have been made of the behavior of the cells, using instruments of great precision. The value of the currents was obtained by reference to a standard resistance of 0.01 chm verified by the Board of Trade; the voltmeter which indicated the pressure was carefully calibrated against Latimer Clark cells. The results have been checked by repetition and by various indirect methods, and their accuracy very closely substantiated.
From the extreme regularity of these results, and from the proportionality of these yielded by the small and the large cells respectively, I feel sure that the work described below can be repeated on other cells. This regularity arises, no doubt, from the special mechanical methods adopted constructing all parts of the cell, but this in itself tells in favour of the cell.
The standard automobile cell is of rectangular shape. It stands 13 inches high (overall) and measures 5.1 by 3.5 inches, horizontally. It weighs 17.8 pounds.
It contains 14 positive and 14 negative plates. Each plate is made of sheet steel, nickel-plated, punched with 18 rectangular holes. In each of these holes is inserted a flat pouch or pocket containing the active material. The walls pf these pockets are perforated by exceedingly fine short slots, through which the liquid can penetrate. Thus the current can easily pass to and from the active material contained in the pocket.
Both positive and negative plates are alike, except in respect of active material. The pockets on the negative plate contain finely divided iron, those on the positive contain peroxide of nickel.
The liquid is a 20% solution of potash. This suffers no change during the action of the cell, except the loss of a small quantity of water which is decomposed whilst the cell is being charged. As an immediate consequence, a small quantity of liquid suffices. It is wanted simply to play the part of an electrolytic conductor, and in no way to provide active material in the ordinary sense. The plates may therefore be fixed very near each other, for the narrow intervening space allows a perfectly adequate supply of potash solution. 
The proximity of the plates does not apparently involve danger of short circuiting. The plates are thin, but being made of steel, their rigidity is exceptionally good. Mechanical stability is further assured by vulcanized rubber separators, the while forming a compact mass, calculated to resist all the ordinary mechanical shocks it is likely to undergo. 
The only special mechanical difficulty which occurred to me was the chance that the gases evolved during a "charge" might eject some of the active material from the pockets. I have therefore watched the pockets carefully, especially during very heavy charges, without finding evidence of loss.
Excellence of mechanical design appears also in the external arrangements. The cell is sealed in its steel case. Two stout connecting pins (from the positive and negative plates respectively) come through liquid-tight bushes of vulcanized rubber. These pins are made slightly conical, as are also the connectors which fit on them, and the mechanical finish and easy grip of this terminal add to the value of the battery, A further advantage lies in he fact that these connecting pins have a much higher specific conductivity that those of the ordinary type of accumulators.
On the top of the case there are also:
(a)  a spring stopper with rubber flange, covering the hole by which the electrolyte is introduced, or distilled water added from time to time.
(b) a vent hole guarded by a gravity valve. This provides for the escape of the gas evolved during charge. The hole and valve are covered by a gauze nipple, which prevents escape of spray while allowing a gas to pass. Acting on the principle of Sir Humphrey Davy's safety lamp, the same gauze further prevents any chance of explosion should a  flame be brought near to the exit.
The excellence of all these features in the design added to the nature of the materials used in construction leads me to conclude that the cell is structurally of a very stable character. 
I come how to the electrical qualities of the cell. These are quite as good as the mechanical.
The electromotive force is 1.33 (W.H.) volts.
The internal resistance is 0.0013 ohm.
The out-put at 60 amperes is 210 watt hours, or at the rate of 11.8 watt hours per pound of cell. When the cell is examined as to discharging vale, its excellence becomes most pronounced. At high rates of discharge, rising to many times the normal, it suffers no appreciable polarization, and therefore recovers its normal voltage almost instantaneously when the current returns to an ordinary value.
These and other points are illustrated by the accompanying curves. Sheet 1 shows the pressure during discharge. Each line corresponds to a constant current, the discharge rates varying from 30 to 200 amperes. This last is such a high rate for a cell of this size that I hesitated about trying it, but the preliminary work indicated that my distrust had no justification. The experiment proved that the cell could stand it without injury. It took the succeeding charge in an excellent way, and yielded the 30 ampere curve immediately afterwards.
A surprising result of this set of experiments is the large relative output at the high discharge rates. At 120 amperes, the output in 91% of the maximum. Even at 200amperes, the quantity (ampere-hours) is 82% of the maximum.
The following table exhibits the actual output at varying discharge rats.
Discharge rate in amperes            Times of discharge           Output in ampere hours        %  of the 30 ampere output
30                                                  5 hrs. 46 mins                    175                                          100
60                                                  2 hrs. 51 mins                     171                                          99
90                                                   1 hr. 51 mins                       167                                         96.5
120                                                  1 hr. 21 mins                      162                                         93.5
150                                                 1 hr. 2 mins                          154.5                                      89
200                                                      42 minutes                       142                                       52
The figures in the last column are much better than those yielded by any other cell at present known.
On Sheet 2 are given the results of one out of many experiments, intended to test the flexibility of the cell.
The standard cell was discharged, starting at 60 amperes. After a time (5 minutes) the current was suddenly increased to 230 amperes. A little later, the current was reduced again to 60, and so on as shown by the curve. Evidently the cell can yield this enormous current for short intervals and recover almost instantaneously. It does not appear to be injured in any way. Subsequent charging and discharging seemed to be quite normal.
On Sheet 3 are shown curves before and after a 48 hours short circuit of a small cell. This trying experience seems to have left the cell intact. Curves B and C on this sheet show how quickly the cell picks up its normal state, even after such extreme violence.
An important point in traction cells is the rate at which they can be charged. I have made experiments on this point and find that the Edison cell will absorb 70 to 75% of its full charge in one hour.
Curve IV shows the result of one of these experiments. The cell charged for one hour only, at the excess rate of 175 amperes. It was then discharged and gave date for Curve IV. This shows that it had absorbed and could deliver 124 ampere hours out of the 175 put into it at this very great rate.
I have evidence to show that where the charging current can be further increased the one hours absorption would be still greater.
The question of life has hitherto been the weakest point of automobile cells, and the tests on this point ought to be as searching as on those I have mentioned. The final test can be made on the road only, and as yet I have not had an opportunity of observing such a test. But the laboratory work has given me substantial grounds for anticipating  a much longer life than usual.
These are as follows.
(1) After three work, with very many charges and discharges the capacity of the cells remain the same as at
(*Note by W. Hibbert. The fall of potential at this high current rate is easily shown to be equal to current x internal resistance. It therefore allows no room for polarization, a most surprising result) Signed W. Herbert the beginning. It has neither increased or diminished. A change in either way would suggest some source of instability, but so far I have not detected any difference. This is a good preliminary ground for anticipating long life.
(2) Examination of the plates after three months work, does not indicate any signs of corrosion. The plates are as even in surface and as rigid in strength as at the beginning.
(3) The standard cell on which I have been mainly working has already been subject to the vicissitudes of travel. It was sent to me from America via Paris, and has therefore undergone two tran-shipments, together with many loadings and unloadings, and journeys by rail and road. This long and varied journey by rail and road. This long and varied journeys by rail and road. This long and varied journey was made in its ordinary working conditions, the plates and fittings in the case fixed as in use. The only difference was one that might be expected to injure the cell-it traveled without the liquid, so that the plates were more or less exposed to air.
On its arrival, having in it no liquid except that which had trickled off the plates to the bottom, it was joined to a voltmeter and gave 1.33 volts.
The full quantity of electrolyte was now put in and the cell given a first charge and discharge. The output following this first charge was 174 ampere-hours, a figure identical with that obtained after further treatments.
In other words, the long journey and the simultaneous long exposure to air had not injured the cell one whit.
A further advantage possessed by this Edison cell is the fact that it does not appear to suffer from repeated discharges down to a very low pressure. The small cells have not been discharged to 0.3 volt very frequently and are not injured by it as far as I can discover. Closely connected with this is the fact that it is not necessary or even urgent to charge a cell soon after a discovery. It is not injured if it remains discharged. Two of the cells have been left discharged for very appreciable periods of time without suffering in any detectable way.
Experiments are proceeding to test how far the cell can retain a charge. Up to date, the evidence seems very favourable. 
In comparing this cell with the other traction cells, some difficulty arises from the varied aims of those who make them. Some of the lead cells made for traction are made with a complete disregard of life. By increasing the strength of acid used, by reducing the strength of the supports, and by thinning down all connections, it is possible to obtain from lead cells an output nearly equal to that found by me in the case of the Edison cells. Compared with these short lived cells, the Edison holds its own more than its ow, with respect to output. In life, it would show very great advantage. 
If for comparison, we take a traction cell made by responsible companies who appreciate all the points requiring attention, I find this Edison  cell has an output per pound ranging from 25 to 50% greater, and a probability of much greater life.
My conclusion therefore is that the Edison cell shows:
(a) Greater output per unit of weight.
(b) Greater flexibility of working.
(c) Greater working range of current.
(d) Greater ease and safety of handling
(e) Greater stability, both mechanical and electrical (f) Greater ability to retain its charge. 7 cup machines                     2800
7 machines cup material        2100
25 Hydraulic Presses               12500
3 Stamp Presses for grid          6000
65 Briquetting Machines             6500
7 Machines for putting in Briquettes  1000
50 Machines for assembly cups in grid   5000
7 machines for making [illegible]            2000
one machine for smelting copper          1500
[illegible]  rod                                        60
Total                                                      $40000
estimate                                                $100,000
Chemical works                                      20000