Nickel Cadmium Batteries

Characteristics

1.2 Volt secondary cells using an alkaline chemistry with energy density about double that of lead acid batteries.
Invented in 1899 but only introduced in volume in the early 1960's
They use nickel hydroxide Ni(OH)2 for the positive electrode (cathode), cadmium Cd as the negative electrode (anode) and an alkaline potassium hydroxide KOH electrolyte.
Their small size and high rate discharge capacity made portable tools and other consumer applications practical for the first time.
The cells are sealed and utilise a recombinant system to prevent electrolyte loss and extend the useful life.

Once the battery of choice for low power portable products they have lost market share to the newer Nickel Metal Hydride and Lithium batteries.

Advantages

Low internal resistance (less than half the equivalent NiMH cells)
High rate charge and discharge rates possible
Up to 10C discharge rates for short periods typical
Flat discharge characteristic (but falls off rapidly at the end of the cycle)
Tolerates deep discharges - can be deep cycled.
Wide temperature range (Up to 70°C)
Typical cycle life is over 500 cycles.
Charging process is strongly endothermic-the battery cools during charging. This makes it possible to charge very quickly, as the I²R heating and endothermic chemical reaction counteract each other.
Rapid charge typically 2 hours, but can be as low as 10 to 15 minutes.
The coulombic efficiency of nickel cadmium is over 80% for a fast charge but can drop to below 50% for slow charging.
The sealed nickel-cadmium cell can be stored in the charged or discharged state without damage. It can be restored for service by recharging several charge/discharge cycles.
The electrolyte is commonly availalable, low cost potassium hydroxide KOH.
Available in a large variety of sizes and capacities.

Shortcomings

A major drawback of this technology is its susceptibility to memory effect.
Originally, the terms memory effect or memory problem was coined to describe a cyclic memory problem where the NiCad battery would "remember" the amount of discharge for previous discharges and limit the recharge life of the battery. The problem is less prevalent with modern Ni-Cd batteries, which are designed to avoid cyclic memory issues.
The memory effect is caused by a change in crystalline formation from the desirable small size to a large size which occurs when a NiCad battery is recharged before it is fully discharged. The growth of large crystals increases the cell impedance and can eventually prevent the battery from discharging beyond that point and/or cause rapid self-discharge of the battery.
The growth of large crystals can be avoided by either completely discharging it each time it is used or by using a NiCad battery charger which has a built-in discharge circuit.
Memory effect can sometimes be reversed by putting the battery through several complete discharge and recharge cycles which helps to recover the smaller crystal formations. This is called reconditioning.
NiCad batteries are also prone to damage by overcharging.
Low cell voltage of 1.2 Volts compared with primary alkaline cells 1.5 Volts and only quarter of the capacity of the alkaline cells.
Self re-sealing safety vents must be incorporated to prevent damage due to overheating and pressure build up.
Cadmium is a high cost heavy metal and its use in consumer products is now deprecated on environmental grounds.
The
Gradually being phased out in favour of Nickel metal hydride and Lithium technologies which have superior energy density characteristics and performance characteristics.

Charging

Run down fully once per month to avoid memory effect.
Do not leave battery in charger.
Slow charging method: Constant current followed by trickle charge.
Rapid charging method uses Negative delta V (NDV) charge termination.

Applications

Motorised equipment
Power tools
Two way radios
Electric razors
Commercial and industrial portable products
Medical instrumentation
Emergency lighting
Toys

Costs

Relatively inexpensive for low power applications but between three and four times more expensive than lead acid for the same capacity.