Voltage—uP.D?— Fig. 31 shows a typical voltage curve of an Edison cell during its "normalx' or five-hour rate of discharge, and during the corresponding charge. At lower rates of discharge the voltage is higher, while at higher rates it becomes lower. Fig. 32 sum marizes a number of discharge curves by giving the initial, the mean and the final voltage at rates up to six times the normal. It is notice able that the voltage falls off very rapidly with increasing discharge rates and that the maxi mum current obtainable is only about 14 times the normal, while the maximum watt output is reached at about seven times the normal rate.
It is of interest to notice that the mean volt age of the Edison cell is about 60 per cent that of the lead type and that the percentage drop during discharge is about triple with the Edison. It is thus necessary to employ at least 65 per cent more cells of Edison type for a given discharge voltage; and still more than this if the discharge rate be high.
Efficiency.— Comparing the mean values of the two curves of Fig. 31 we arrive at the value— 72 per cent — as the mean volt effi ciency; the corresponding ampere hour efficiency is approximately 88 per cent, while the watt hour, or energy efficiency, the product of these two, is 63 per cent.
In actual practice, the charging is frequently done from a fixed source of voltage equal to or slightly exceeding the maximum, 1.75 in the present case; hence under these conditions, the voltage efficiency is but 68 per cent.
Also, in practice the charge required is greater than Irr-Ivn in Fib. 30, so that show that at ordinary temperatures and mod erate discharge rates the Edison battery may give excellent service At high rates, or low temperatures, however, its performance is so limited that it is now seldom used where such conditions prevail. It is these conditions which have excluded it entirely from the field of auto crlf on the other hard, in other actually the ampere efficiency is about 80 per cent. The commercial watt hour efficiency of the Edison battery, where worked to its full capacity and charged from a source of constant voltage, is thus found in actual practice to average hardly above 55 per cent. Where charged from a source of variable voltage, and where worked to partial capacity only, the effi service, as for instance, train lighting and mine lamps, it has proved very satisfactory.
Characteristics of the Lead Capacity.—The capacity of a given lead cell is mainly dependent upon two variable factors: (1) the rate at which it is discharged, and (2) its temperature.
1. The effect of varied discharge rates is ciency may be much higher, depending on actual conditions.
Capacity and Weight.— The energy capacity of the Edison cell at normal rate and tempera ture is about 14 watt hours per pound. With higher rates, or lower temperatures, this value falls very markedly.
Summary.— The characteristics noted above shown by the curved line in Fig. 33, from which it will be seen that the capacity varies inversely with the rate, though not in direct ratio. While the abscissa of this figure show the discharge rate in terms of the normal, vertical lines at intervals give the rate in terms of the duration of discharge.
The shape of this curve is very characteris tic of all lead batteries, though differences in design modify it appreciably. Thinner plates tend to give a flatter curve, thicker ones a more sloping one.
A very important corollary of the variation of capacity with rate exists in the fact that a lead cell which has been completely dis charged at a high rate, if allowed to stand for some hours, will largely recover, so as to give a considerable further discharge. In the case of a continuous discharge of diminishing rate, the ultimate capacity approaches that which would have obtained had the final rate been maintained throughout. In the operation of an electric vehicle the rates on starting, up grades, etc., exceed the normal rate by five to one or more; yet owing to the periods of rest, or low rate, the capacity attainable is practically identical with that of a continuous normal rate discharge. The normal rate for batteries of this kind is usually that corre proaches a slanting straight line. Referring again to Fig. 33, the three upper lines, with the scale of ordinates at the right hand side, summarize the effect upon voltage of various discharge rates up to 10 times the normal. It is of interest to note that at 10 times normal the mean voltage has lost but 20 per cent; that the maximum watt output occurs at about 25 times normal; and that short circuit gives about 50 times normal discharge current. Com parison between these curves and the corre sponding ones for the Edison battery, Fig. 32, is very significant. The Edison battery is inferior (a) in that it has a much greater per centage drop in voltage during discharge at any given rate, and (b) in that the lead battery can discharge at about three times as high a rate as the Edison.