Nickel Cadmium Batteries
1.2 Volt secondary cells using an alkaline chemistry withenergy density about double that of lead acidbatteries.
Invented in 1899 but only introduced in volume in the early1960's
They use nickel hydroxideNi(OH)
2for the positive electrode (cathode), cadmium Cd as the negative electrode (anode) and an alkaline potassium hydroxide KOH electrolyte. (KOH is also one of the main ingredients in drain cleaners and soft soaps)
Their small size and high rate discharge capacity madeportable tools and other consumer applications practical for thefirst time.
The cells are sealed and utilise a recombinant system toprevent electrolyte loss and extend the useful life.
Once the battery of choice for low power portable productsthey have lost market share to the newer Nickel Metal Hydridebatteries.
Low internal resistance
High rate charge and discharge rate possible
Up to 10C discharge rates for short periodstypical
Flat discharge characteristic (but falls off rapidly at theend of the cycle)
Tolerates deep discharges - can be deepcycled.
Wide temperature range (Up to 70°C)
Typical cycle life is over 500 cycles.
Charging process is strongly endothermic-the battery coolsduring charging. This makes it possible to charge very quickly, asthe I2R heating and endothermic chemical reactioncounteract each other.
Rapid charge typically 2 hours, but can be as low as 10 to15 minutes.
The coulombic efficiency of nickel cadmium is over 80% fora fast charge but can drop to below 50% for slowcharging.
The sealed nickel-cadmium cell can be stored in the chargedor discharged state without damage. It can be restored for serviceby recharging several charge/discharge cycles.
Available in a large variety of sizes andcapacities
A major drawback of this technology is its susceptibilityto memory effect.
Originally, the terms memory effect or memory problem wascoined to describe a cyclic memory problem where the NiCad batterywould "remember" the amount of discharge for previous dischargesand limit the recharge life of the battery. The problem is lessprevalent with modern Ni-Cd batteries, which are designed to avoidcyclic memory issues.
The memory effect is caused by a change in crystallineformation from the desirable small size to a large size whichoccurs when a NiCad battery is recharged before it is fullydischarged. The growth of large crystals increases the cellimpedance and can eventually prevent the battery from dischargingbeyond that point and/or cause rapid self-discharge of thebattery.
The growth of large crystals can be avoided by eithercompletely discharging it each time it is used or by using a NiCadbattery charger which has a built-in dischargecircuit.
Memory effect can sometimes be reversed by putting thebattery through several complete discharge and recharge cycleswhich helps to recover the smaller crystal formations. This iscalled reconditioning.
NiCad batteries are also prone to damage byovercharging.
Low cell voltage of 1.2 Volts compared with primaryalkaline cells 1.5 Volts and only quarter of the capacity of thealkaline cells.
Self re-sealing safety vents must be incorporated toprevent damage due to overheating and pressure buildup.
The use of Cadmium in consumer products is now deprecatedon environmental grounds.
Gradually being phased out in favour of Nickel metalhydride and Lithium technologies which have superior energy densitycharacteristics and performance characteristics.
Run down fully once per month to avoid memoryeffect.
Do not leave battery in charger.
Slow charging method: Constant current followed by tricklecharge.
Rapid charging method uses Negative delta V (NDV) chargetermination.
Two way radios
Commercial and industrial portable products
Relatively inexpensive for low power applications butbetween three and four times more expensive than lead acid for thesame capacity.
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