Charge and discharge characteristics of nickel cadmium cell:
During charging and discharging the electrolyte acts only as an ionised conductor, transporting electrons from one plate to the other, its specific gravity remaining constant.
On discharge the terminal voltage initially falls rapidly and then remains constant for most of the discharge cycle, dropping rapidly again
when the battery is nearly fully discharged. When charged, the terminal voltage initially rises rapidly and then settles to a gradual increase. A second rapid rise takes place as the battery reaches the fully charged condition, at this time gassing takes place, hydrogen being released at the negative plates, oxygen at the positive plates, this combination of gases is explosive. Prolonged gassing should be avoided as it reduces the water content of the electrolyte and causes overheating of the battery, a slight amount of gassing, however, is necessary to ensure charging is complete.
The terminal voltage remains constant for most of the batteries life and the specific gravity of the electrolyte remains unchanged, the only way of determining the state of charge of the battery therefore, is to carry out a full charge followed by a capacity test.
During discharge the plates absorb electrolyte to such an extent that the level may disappear from view. As the battery is charged, the electrolyte is forced back out of the plates, a point to note when topping up the cells.