Figure 1: Discharge rate of Ni-Cd battery
One Ni-Cd cell gives 1.2V during discharge. These batteries have a flat discharge rate that falls rapidly at the end of the cycle as shown in Figure 1
. Thus, it is difficult to estimate the amount of charge left. The advantage of this kind of battery is that it can withstand deep discharges without damaging the cell.
Along with the advantages mentioned, Ni-Cd comes with disadvantages as well, resulting in shrinking market share. Some of the major disadvantages include:
• Memory Effect:
If the battery is repeatedly overcharged after partial discharging, it loses its capability to hold the maximum energy. This condition is referred as the memory effect. Thus, it is advised to either go for precisely controlled charging or regularly do a deep discharge such that the battery can recover its capacity. However, care must be taken since excessive deep discharge can permanently damage the cells.
• Cell Reversing
: A battery consists of many cells connected together where each cell differs from the others slightly in terms of capacity. Thus, when one cell completely discharges before the other cells, the remaining cells will still force current through the discharged cell. This is known as cell reversing and results in undesirable and irreversible chemical reactions which permanently damage the cell.
When fully charged, the positive electrode (nickel) generates oxygen while the negative electrode (cadmium) generates hydrogen gas. These gases must be properly vented out from the system or operating conditions can become hazardous. To address this problem in sealed Ni-Cd batteries, the negative electrode (cadmium) is built with higher capacity. This causes the positive electrode to reach its fully charged state before negative electrode does. Thus, the oxygen released by positive electrode gets absorbed by the negative electrode and oxidizes.