2. Acid Change ("Gravity").-- It has been pointed out in discussing electric-chemical equations, that the amount of free sulphuric sponding to a continuous discharge of four or five hours.
2. Temperature at time of discharge exer cises a direct influence upon capacity to the extent of about per cent per 10 degrees F. It thus comes about that at 0° the lead battery has about 54 per cent normal capacity, and that it is perfectly workable at temperatures much lower even than this, especially where the discharge rate is lower than normal.
Discharge Phenomena.— 1. Voltage (or PD).— Of equal importance with the capacity of a battery is its voltage characteristic, a qpical curve of which is shown in Fig. 34. Starting off at approximately two volts, there is a gradual falling off, till the end approaches, when the voltage rapidly drops below a use ful value. The curve shown is for the normal rate. but is fairly indicative of the general behavior of a lead cell on discharge. With higher rates, however, the curve is lower throughout its length, and more nearly ap acid varies as discharge proceeds, and the third curve of Fig. 34 shows for a particular case what this change amounts to. Barring the fact of a lag at the start, the change of acid, measured by hydrometer, varies directly with the ampere hours drawn from the cell; but the amount of change depends so entirely upon the relative volume of acid contained in a given cell, that the numerical values of this curve in Fig. 34 have no general significance.
3. Temperature Change.—The lead cell to a slight extent is a thermoelectric accumu lator, inasmuch as a slight disappearance of heat accompanies the discharge. This phe nomena is graphically shown in the lower curve of Fig. 34, where it is seen that the tempera ture dropped 9° F. during discharge. At higher rates, the heat generated by internal electric friction overbalances that absorbed, and at the one hour rate the temperature rises about to the same amount as it dropped at normal rate. The absorption of heat on dis charge, while of much theoretic interest, is of little practical value.
Internal Resistance.— One of the most valuable attributes of the lead battery is its high conductivity, which enables it to yield up its stored energy at extremely high rates. It is impossible to state the resistance definitely owing to variation of design; but by way of illustration it may he said that a cell of the type from which Fig. 34 was taken, having a
normal rate of 35 amperes, has an internal re sistance of about .0014 ohms at beginning, and .0028 ohms at end of discharge. Since it is mainly through its influence upon voltage that internal resistance is of interest, the data fur nished by the curves of Fig. 33 give the prac tical information required better than an at tempted formula for calculating resistance.
Efficiency.— During charge, the P. D. of a lead cell starting at about two volts rises gradu ditions, the volt efficiency is about 75 per cent, the watt hour efficiency about 65 per cent. These conditions are the most prevalent, except when a battery is charged directly from a generator, whose voltage is made to vary ac cording to the charging curve.
Capacity Weight Ratios.— The capacity per unit weight of lead storage batteries varies all the way between 1.4 ampere hour per pound of cell in the heavier stationary types, such as Fig. 25, to about 5.5 in the lightest thin plate types for portable service, Fig. 26. These figures, as a basis of comparison, refer in all cases to a discharge rate approximating the five-hour. To find the corresponding values for other rates, reference should be had to the capacity curve of Fig. 33, bearing in mind that 100 per cent in this figure corresponds to an actual capacity of 4.6 ampere hours per pound.
Since the mean discharge voltage under ally and finally becomes constant at a rather indefinite value, from 2.5 to 2.6 volts, follow ing the general trend of the upper curve of Fig. 34. The mean height of this curve is 2.3 volts; that of the discharge curve 1.95 volts. Hence the volt efficiency is 85 per cent.
In commercial operation, it is found neces sary that the charge exceed the discharge by about 15 per cent, so that the ampere hour efficiency is about 87 per cent, the watt hour efficiency about 75 per cent. When worked to less than 100 per cent capacity, both voltage and current efficiency are higher; so that in such cases it may reach or even exceed 90 per cent, as in regulating service, where charge and dis charge succeed each other rapidly and for a few minutes duration only.