Electrical Engineering ⇒ Topic : Characteristic Curves of a Lead-acid Cell

Characteristic Curves of a Lead-acid Cell

The behaviour of a lead-acid cell can be easily studied from its characteristic curves.

(i) Charging and discharging curves. Fig. (a) shows the variations in the terminal P.D. of a cell with time on charge and discharge. When the cell is charged, its terminal voltage rises from 1.8  V (See charging curve) to 2.2 V during the first 2 hours and then increases slowly. Finally, after the charging period, the terminal voltage is about 2.7 V When the cell is disconnected from the charging source, its terminal voltage rapidly falls to about 2.2 V without any discharge.

                                                                                        figure (a)

On discharge, the terminal voltage falls to 2.0 V in the beginning, remains constant for sufficient time and finally drops to 1.8 V at which the cell should be recharged. If the cell is allowed to discharge below 1.8 V repeatedly, or it is left in an uncharged condition for a considerable time, a hard insoluble lead sulphate (PbS0₄) is formed on the plates of the cell. This increases the internal resistance of the cell considerably and finally ruins the cell

(ii) Capacity and rate of discharge. Fig. (a) shows the effect of rate of discharge on the capacity of the cell. The ampere-hour capacity of the cell decreases with the increase in the rate of discharge. Rapid rate of discharge means heavier load current. As a result, the voltage of the cell falls more rapidly due to the increased voltage drop in the internal resistance of the cell. Moreover,the rapid discharge weakens the acid in the pores of the plates and chemical action becomes brisk.These effects reduce the capacity of the cell

                                                                    figure (a)

The capacity of a cell is given in ampere-hours for a discharge of 8 hours or 10 hours. A rate of discharge greater than this value decreases the ampere-hour capacity of the cell while a slower rate increases its capacity.

(iii) Capacity and temperature. The ampere-hour capacity of a cell increases with the increase in temperature and vice-versa. It is because higher temperatures permit more diffusion of acid and increase the velocities of the ions taking part in the chemical changes.