Electrical Characteristics of the Lead-acid Cell
The three important Electrical Characteristics of the Lead-acid Cell, of interests to an engineer, are
(1) voltage (2) capacity and (3) efficiency.
1. Voltage
The open-circuit voltage of a fully-charged cell is approximately 2.2 volt. This value is not fixed but depends on
(a) length of time since it was last charged
(b) specific gravity-voltage increasing with increase in sp. gravity and vice versa. If sp. gravity comes near to density of water i.e. 1.00 then voltage of the cell will disappear altogether
(c) temperature-voltage increases, though not much, with increase in temperature.
The variations in the terminal p.d. of a cell on charge and discharge are shown in Figure given below. (Figure A)
The voltage-fall depends on the rate of discharge. Rates of discharge are generally specified by the number of hours during which the cell will sustain the rate in question before falling to 1.8 V. The voltage falls rapidly in the beginning (rate of fall depending on the rate of discharge), then vary slowly up to 1.85 and again suddenly to 1.8 V.
The voltage should not be allowed to fall to lower than 1.8 V, otherwise hard insoluble lead sulphate is formed on the plate which increases the internal resistance of the cell. The general form of the voltage-time curves corresponding to 1-, 3- 50 and 10- hour rates of corresponding to the steady currents which would discharge the cell in the above mentioned times (in hour). It will be seen that both the terminal voltage and the rate at which the voltage and the rate at which the voltage falls, depend on the rate of discharge. The more rapid fall in voltage at higher rates of discharge is due to the rapid increase in the internal resistance of the cell.
During charging, the p.d. increases (Figure A). The curve is similar to the discharge curve reversed but is everywhere higher due to the increased density of H2SO4 in the pores of the positive plate.
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2. Capacity
It is measured in amp-hours (Ah). The capacity is always given at a specified rate of discharge (10-hour discharge in U.K., 8-hour discharge in U.S.A.). However, motor-cycle battery capacity is based on a 20-hour rate (at 30° C). The capacity depends upon the following :
(a) Rate of discharge.
The capacity of a cell, as measured in Ah, depends on the discharge rate. It decreases with increased rate of discharge. Rapid rate of discharge means greater fall in p.d. of the cell due to internal resistance of the cell. Moreover, with rapid discharge the weakening of the acid in the pores of the plates is also greater. Hence, the chemical change produced at the plates by 1 ampere for 10 hours is not the same as produced by 2 A for 5 hours or 4 A for 2.5 hours. It is found that a cell having a 100 Ah capacity at 10 hour discharge rate, has its capacity reduced to 82.5 Ah at 5-hour rate and 50 Ah at 1-hour rate. The variation of capacity with discharge rate is shown in Figure C.
(b) Temperature.
At high temperature, (i) chemical reactions within the cell take place more vigorously. (ii) the resistance of the acid is decreased and (iii) there is a battery diffusion of the electrolyte.
Hence, high temperature increases the capacity of the lead-acid cell. Apparently, it is better to operate the battery at a high temperature. However, at high temperatures :
(a) the acid attacks the antimony-lead alloy grid, terminal posts and wooden separators.
(b) the paste is rapidly changed into lead sulphate. Sulphation is always accompanied by expansion of paste particularly at the positive plates and results in buckling and cracking of the grid.
Hence, it is not advisable to work batteries above 40° C. As temperature is lowered, the speed of chemical reactions is decreased. Moreover, cell resistance also increases. Consequently, the capacity of the cell decreases with decrease in temperature till at freezing point the capacity is reduced to zero even though the battery otherwise be fully charged.
(c) Density of electrolyte.
As the density of electrolyte affects the internal resistance and the vigour of chemical reaction, it has an important effect on the capacity. Capacity increases with the density.
(d) Quantity of active material.
Since production of electricity depends on chemical action taking place within the cells, it is obvious that the capacity of the battery must depend directly upon the kind and amount of the active material employed. Consider the following calculations:
The gram-equivalent of lead is 103.6 gram and Faraday’s constant is 96,500 coulombs which is
= 96,500/3600 = 26.8 Ah.
Hence, during the delivery of one Ah by the cell, the quantity of lead expended to form lead sulphate at the negative plate is 103.6/26.8 = 3.86 gram.
Similarly, it can be calculated that, at the same time, 4.46 gram of PbO2 would be converted into lead sulphate at the positive plate while 3.66 gram of acid would be expended to form 0.672 gram of water. It is obvious that for obtaining a cell of a greater capacity, it is necessary to provide the plates with larger amounts of active material.
3. Efficiency
It has already been explained in the Article Two Efficiencies of the Cell.
Read article – Units of Resistivity
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