|
|
|
|
|
The NiCd system offers good power density and can be used very effectively in battery packs with multiple cells. Major drawbacks are the low energy density and a memory effect (see below) which lowers the usable capacity if the battery is repeatedly partially discharged or continually overcharged. More importantly, there is growing environmental concern over the presence of the toxic metal Cd in the battery. This may limit its future use in small portable batteries, but not in large, high rate, industrial ones.
Although they cost 2 to 3 times more than equivalent lead acid batteries, nickel-cadmium batteries are widely used in portable power applications, e.g. electric tools and TV cameras: they are the major battery type in the IRIS system e.g. BB521, BB590. The main advantages are a higher energy density, especially at high discharge rates, and a longer cycle life, typically 500 to 2000 cycles, as opposed to the 200 to 700 cycles for automobile and sealed lead acid batteries. Unlike lead-acid cells, individual Ni-Cd cells can be deeply discharged and left in this condition without harm. It is, however, bad practice to deeply discharge a battery of series-connected cells due to possible damage through voltage reversal of weaker cells.
Unfortunately, Ni-Cd cells tend to lose their charge fairly rapidly, especially above 35oC. Furthermore, if partially discharged to a given level and then recharged on a regular basis, some types of cell suffer a temporary loss of capacity called the memory effect. This effect is reversible and full capacity can be restored by several deep discharge/charge cycles at a low rate and some modern chargers include a facility to do this at intervals (called charger-analyzers). The IRIS battery charger is equipped for this. The memory effect is usually not a problem with modern portable, sealed cells. The memory effect has been attributed to an alloying reaction that occurs between cadmium metal and high surface area nickel (sintered), which is used as a substrate in some batteries.
Sintered nickel has been used with both electrodes to increase the discharge rate, because the discharge products (cadmium hydroxide and nickel hydroxide) are insulating and the high surface area increases contact to the active material. Sintered nickel is still used as a cadmium substrate in large industrial batteries but small portable cells now have a polymer-bonded cadmium electrode, in order to avoid the nickel-cadmium alloy. Another operational problem of NiCd batteries is that the voltage varies very little during discharge making it difficult to estimate the state of charge.
