Electrical Engineering ⇒ Topic : Characteristics of a Lead-acid Cell
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| Maninder
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Characteristics of a Lead-acid Cell A lead-acid cell is the most popular type of secondary cell. Therefore, it is profitable to study its important characteristics. 1. E.M.F :-The e.m.f of a fully charged lead-acid cell is about 2.2 volts. As the cell delivers current, its e.m.f. also decreases, though by a very small amount. The magnitude of e.m.f. depends upon :
When a cell has been recently charged, its e.m.f. is high but it gradually decreases even if left on open-circuit. The e.m.f. of the cell rises with the increase in specific gravity of the electrolyte. The surrounding temperature also affects the e.m.f. ; there being a slight increase in its value with the rise in temperature. 2. Terminal voltage.The open-circuit terminal voltage (i.e. e.m.f.) of a fully charged cell is about 2.2 V. When the cell delivers current, the terminal voltage is less than its e.m.f. due to voltage drop in the internal resistance of the cell. If the discharge is continued, the terminal voltage falls rapidly for a short time, then slowly for some time and again rapidly towards the end of discharge.It may be noted that when the terminal voltage has fallen to about 1.8 volts, the discharge should be stopped. If it is continued, too much lead sulphate (PbS04) is formed, clogging the pores of the plates, possibly **damaging them and making the recharging increasingly difficult. Also excess lead sulphate in the plates raises the internal resistance of cell, thus lowering the terminal voltage. The Specific Gravity of the electrolyte too is reduced by the water that is formed and this lowers the cell's voltage even more. 3. Internal resistance.The opposition offered to current within the cell is called the internal resistance of the cell. It is made up of resistances of the plates, the active material and the electrolyte.The internal resistance of a lead-acid cell is very small (typical value being 0.01 Ω) and depends upon the following factors :
The internal resistance of a lead-acid cell should be minimum in order to reduce the internal drop.This is achieved by using multiplate construction in a cell. As explained in Art. , the negative plates of a cell are connected together as are the positive plates. The effect of this arrangement is as if we have connected a number of cells in parallel. At the same time, the length of electrolyte between the plates is reduced. The result is that the internal resistance of the cell is lowered 4. Capacity.The capacity of a cell is the quantity of electricity which it can give out during single discharge until its terminal voltage falls to 1.8 volts. It is measured by the product of current in amperes and the time in hours i. e. Capacity of cell = Id x Td ampere-hours (or Ah) where Id = Steady discharging current in amperes Td = Time in hours for which the cell can supply current until its p.d. falls to 1.8 volts The capacity of a lead-acid cell is taken up to a point till it discharges down to 1.8 volts. It is because a lead-acid cell is not permitted to discharge beyond this point to avoid permanent damage to the cell. Consider a cell of capacity 120 Ah. This means that theoretically the cell can deliver 1 A for 120 hours, 2 A for 60 hours or any combination of amperes and hours that, when multiplied together, gives 120. After that much time, the terminal voltage of the cell falls to 1.8 volts, requiring recharging. The capacity of a cell depends upon the following factors Rate of discharge - Higher the rate of discharge, less the capacity Temperature - Increases with temperature Area of plates - Increases with plate area Sp. gravity of electrolyte - Increases with Sp. gravity 5. Efficiency . Efficiency. There are two ways of expressing the efficiency of a secondary cell viz (i) Ampere hour efficiency (ii) Watt-hour efficiency. (i) Ampere-hour efficiency. It is the ratio of output ampere-hours to the input ampere-hours of the cell i.e 
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