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Nickel-cadmium cells were the most commonly used rechargeable batteries until recently. NiCads come in sizes like non-rechargeable cells and can directly replace non-rechargeables like alkalines. NiCads do have a lower voltage output 1.2 volts as opposed to non-rechargeable cells at 1.5 volts. That voltage drop will not make much of a difference under most applications.
NiCad packs generally have voltages of 2.4 through 10.8 volts. These voltages correspond to the number of cells the battery contains, generally 2 through 9. NiCads do work best in temperatures between 60 and 78 degrees Fahrenheit. Capacity is reduced at higher temperatures. Below 32 degrees Fahrenheit, hydrogen gas emitted may cause an explosion when the cells are active.
NiCads do have a low internal resistance. This is good for Ham Radio HT's which draw large amounts of current. Low internal resistance means a high current draw however. As much as 30 amps can be emitted should the cells become short-circuited! As you might imagine, short-circuiting causes heat build-up and permanent cell damage.
In the recent past, most portable transceivers came with NiCad packs. The packs are basically cells welded to metal connecting straps. There is good reason for this configuration. In high-current applications, the resistance between cells and battery holder contacts can result in erratic operation. By encasing the HT battery pack properly, the erratic behavior is overcome and the battery pack provides more stable operation.
A standard charging rate for NiCads is 10% of the battery's capacity for 14 hours. So a battery pack with a 600mAh rating, the proper charging current is 60mA. Since charging is never 100% efficient, charging for 14 hours is required instead of the 10 hours resulting from the 10% rule. NiCads can be left on a trickle charger indefinitely if the charging current is reduced to 2% of the battery's amp-hour rating.
To best preserve NiCad cell life, do not discharge cells to less than 1.0 volt per cell. When charging, NiCads should read 1.45 volts per cell. Higher cell voltages during charging suggests the cell is faulty and should be discarded.
The 'memory effect' of NiCad cells refers to the claim that charged battery packs no longer deliver their rated output. The 'memory effect' is technically referred to as voltage depression. The 'memory effect' is actually caused by long, continuous overcharging, which causes crystals to grow inside the cell. The 'memory effect' and related voltage depression can be overcome by performing one or more deep charge/discharge cycles.
NiCads do short circuit because of the build up of crystals inside the battery. The use of a fully-charged electrolytic capacitor placed across the cell can provide a temporary cure. When batteries are over-discharged, it can yield short circuiting. Batteries should be stored charged. A 200 to 800 charge lifespan is typical for NiCad batteries.