This type of rechargeable batteries are usually termed as “NiCd”, which was a brand name of SAFT Corporation and should not pertain generically to nickel-cadmium batteries. Charging NiCd batteries requires constant current source by using a transistor. The BD140 power transistor has the features of high current (max. 1.5 A) and low voltage (max. 80 V). It is generally used for power applications purposes as used in television circuits and hi-fi amplifiers. Two high conductance fast diode 1N4148 applies a direct voltage to the majority-carrier of the transistor while stabilizing the voltage drop across them. Voltage drop can be defined as the reduction in voltage across a conductor where current is flowing. When the switch is closed, the charging current is estimated as 15mA to 45mA which makes it appropriate for rechargeable batteries of 1.5 V and 9 V. For high quality NiCd’s, the charging stops when the battery gets too hot. To obtain the best results, charging should be done in a room temperature or cool place. The discharge rate for NiCd batteries depends on the size.
There are other available types of rechargeable batteries in the market. While NiCd uses nickel oxide hydroxide and metallic cadmium as electrodes, the nickle-metal hydride cell (NiMH) uses hydrogen-absorbing alloy for negative electrode. The Lithium-ion (Li-ion) uses the anode, cathode and electrolyte to function properly.
The disdavantage of NiCd batteries is the severe toxicity and high cost. Inaccurate use of these batteries can lead to damage to itself or to humans.
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