The product of current in amperes and the period in hours that the cell can supply current until its e.m.f. drops to 1.8 volt determine a cell’s capacity. A battery’s capacity is measured in amp-hours (Ah). The capacity is always stated at a certain discharge rate (10-hour discharge in the U.K., 8-hour discharge in the U.S.A.). Vehicle battery capacity, on the other hand, is based on a 20-hour rate (at 30° C).
The capacity is determined by the following factors:
Table of Contents
Rate of discharge:
The discharge rate affects a cell’s capacity, capacity is measured in Ah. Capacity decreases when the rate of discharge rises. Because of the cell’s internal resistance, a rapid rate of discharge produces a higher drop in the cell’s p.d. Furthermore, rapid discharge causes the acid in the pores of the plates to weaken faster. As a result, the chemical change caused by 1 ampere for 10 hours is not the same as the chemical change caused by 2 A for 5 hours or 4 A for 2.5 hours. A cell with a 100 Ah capacity at a 10-hour discharge rate has its capacity dropped to 82.5 Ah at a 5-hour rate and 50 Ah at a 1-hour rate, as per the analysis.
The following graph depicts the capacity fluctuation as a function of the discharge rate.
Chemical changes within the cell are more intense at high temperatures, the acid resistance is reduced, and the electrolyte diffusion is increased. As a result, a high temperature boosts the capacity of a lead-acid battery. It appears that operating the battery at a high temperature is better. The acid, on the other hand, damages the antimony-lead alloy grid, terminal posts, and wooden separators at high temperatures. The paste quickly degrades into lead sulfate. Sulphation is always followed by paste expansion, especially at the positive plates, resulting in grid buckling and cracking. As a result, working batteries at temperatures exceeding 40° C is not recommended.
On the contrary, the speed of chemical reactions slows down when the temperature drops. Furthermore, cell resistance rises. As a result, the capacity of the cell drops with decreasing temperature until it reaches zero at the freezing point, even though the battery is fully charged.
The density of electrolytes:
The density of the electrolyte has a significant impact on the capacity because it influences the internal resistance and the intensity of the chemical reaction. The capacity of a battery grows in proportion to its electrolyte density.
Quantity of active material:
Because the creation of energy is dependent on chemical reactions within the cells, the battery’s capacity must be exactly proportional to the type and amount of active material used.