The nickel cadmium battery (NiCd) is a popular type of rechargeable battery. It was invented in 1899 by physicist Walther Hermann Nernst. Today, more than 50% of all rechargeable batteries for portable devices are NiCd. They are suitable for use in small, portable devices such as two-way radios, barcode scanners and power tools. NiCd batteries have an edge over other battery chemistries because of their ability to deliver 500 to 1000 full charge/discharge cycles, they are lightweight and have a good energy storage density. They usually perform well in rigorous conditions and are economically priced. However, these batteries develop a condition called “memory” if not properly maintained and require proper disposal due to the toxicity of cadmium.

Examples of NiCd batteries include two way radio batteries, barcode scanner batteries, and barcode printer batteries.

Applications
Nickel cadmium dry cells are used either individually, or are assembled into battery packs containing two or more cells to obtain a higher terminal voltage.

NiCd batteries are a popular choice as rechargeable batteries in portable applications such as two-way radios, barcode scanners, power tools, barcode printers and emergency medical equipments as these devices require high discharge rates and rapid discharging modes. They are also used in hobby electronic devices like remote controlled model cars, airplanes and boats.

How NiCd Battery Works
A battery is a chemical device that generates electrical energy through electrochemical reaction. The basic components of a battery are the anode, cathode and the electrolyte.

A NiCd battery cell has a nickel hydroxide cathode, a metallic cadmium anode and a potassium hydroxide electrolyte. When a load is connected to the cell, the anode is oxidized and the cathode is reduced. The reaction continues to take place until the anode and cathode can no longer oxidize and reduce, respectively. When electric charge is applied to recharge the battery, the reverse reaction takes place.

Voltage
Nickel cadmium cells have a nominal cell potential of 1.2 Volt. More than one battery cell can be assembled in a series, parallel or a mixture of both to obtain a higher terminal voltage, capacity or power density.

For example, a 12 V, NiCd battery would be made up of 10 battery cells connected in a series.

NiCd cells have very low internal resistance. This allows NiCd cells to produce a nearly constant output voltage until it is almost completely discharged. The low internal resistance makes it nearly impossible to detect the charge state of NiCd batteries. Often, the charge state of NiCd batteries is deduced from the time they have been in operation.

Charging method
Nickel cadmium batteries can be charged at different rates. However, they prefer fast charge to slow charge and pulse charge to DC charge. Battery manufacturers recommend that a new battery be slow trickle charged for around 24 hours in its first charge. During storage, the cells within a battery pack self-discharge to different capacity levels. The initial slow trickle charge helps to bring all the cells to an equal charge level.

Nickel cadmium batteries should be charged in room temperatures between 120°F and 40°F. Charging at temperatures lower or higher than this range can hinder the charge acceptance capacity of the batteries. Most NiCd batteries have a thermal cut-off of around 122°F, so, if the battery gets too hot the charger automatically stops charging.

Recharging NiCd batteries when not fully discharged and overcharging leads to crystalline formations on the cell plates. This creates “memory effect” in the batteries, which reduces their charge carrying capacity. Recharging without fully discharging and overcharging should be avoided to keep the batteries performing optimally.

Problems with NiCd
There are several problems associated with nickel cadmium batteries, including toxicity of cadmium and "memory." As NiCd batteries contain cadmium, which is a toxic heavy metal, special care has to taken when disposing them. Improper disposal of NiCd batteries can cause substantial cadmium pollution, threatening both wildlife and human life.

Nickel cadmium batteries are known for another drawback: memory. These batteries develop “memory” or “memory effect” when charged without first being completely discharged, or when overcharged. They need to be periodically exercised and reconditioned to perform optimally.

Advantages

1. Nickel cadmium is one of the most rugged rechargeable battery chemistries, tolerating deep discharge for long periods.


2. NiCd batteries provide the highest number of charge/discharge cycles. A properly maintained NiCd battery can provide over 1000 battery cycles.


3. NiCd batteries can be stored for a relatively longer period in any state-of-charge.


4. NiCd batteries are more economically priced in comparison to other battery chemistries like NiMH and Li-Ion. It is the lowest cost battery in terms of cost per battery cycle.

Maintenance Tips for NiCd Batteries:

• Never leave a NiCd battery in the charger for more than 48 hours.
• Use high quality battery chargers.
• Slow trickle charge a new battery in its first charge.
• Charge in room temperatures between 120°F and 40°F.
• Do not recharge if not fully discharged.
• Never leave the battery in the charger after being fully charged.
• Store in cool and dry locations.
• Discharge to about 50% of their capacity before storage.
• Apply a full discharge cycle once every month. More frequent full discharge would put undue stress on the battery.
• Periodically analyze and recondition the batteries.