NiCd / NiCad Nickel Cadmium Battery Technology Overview

- information, tutorial about the basics of nickel cadmium or Nicad / NiCd battery technology giving an overview of how they work and their applications.

This overview or tutorial about battery technology is split into several pages, each addressing a particular type of aspect of battery technology:

Nickel cadmium or NiCd batteries and cells have been widely used in applications where electrical rechargeable power sources are needed. These NiCd cells have been used for many applications where electronic equipment such as laptop computers, electronic games, mobile phones and many other items of electronics equipment have needed a form of re-chargeable power source. In addition to this nickel cadmium cells have also been widely used for torches and other small items of electronic equipment.

NiCd cells are less widely used these days because of their use of cadmium which has to be disposed of carefully when the battery life has been finished. These environmental concerns along with the fact that there are more efficient cells available has brought about a decline in the use of nickel cadmium cells.

Often the abbreviation NiCad is used to describe nickel cadmium cells. The abbreviation NiCad is a registered trademark of SAFT Corporation and therefore it should not be used to refer generically to nickel-cadmium batteries. The abbreviation NiCd is therefore the recognized generic abbreviation for these cells and batteries.

NiCd basics

NiCd cells are able to provide almost direct replacements for zinc carbon or alkaline primary batteries. They generally are able to retain less charge than these cells, but have the obvious advantage that they are able to be re-charged. This means that although the initial purchase cost is higher than the equivalent primary cells, costs can be saved after a few charge / discharge cycles.

The nominal cell voltage for a NiCd / Nickel cadmium cell is 1.2 volts. It holds this voltage well for most of the discharge cycle, only falling when most of the charge has been used. It holds the output voltage better than the equivalent zinc carbon primary types which have a steady fall over the life of the cell. Whilst a flat curve shows the advantage that the output voltage from the cell is very stable, it does mean that when the cell nears the end of its discharge cycle, the output voltage falls off rapidly, often giving little warning to the user.

NiCd cells have a very low level of internal resistance. A good quality alkaline cell might have an internal resistance of about 300 milli-ohms when new. This figure might rise to about 900 milli-ohms when 20% discharged and several ohms when almost completely discharged. A NiCd has very much lower figures, and any internal resistance can be ignored for most purposes as it is of the order of only a few milli-ohms, dependent upon the exact type of cell and the manufacturer. This does mean that the cell is capable of producing very high currents, especially if the cell is accidentally short-circuited. In view of this care must be taken to ensure this does not happen as large amounts of heat can be generated.

NiCd construction

The cells basically consist of positive and negative plates with a separator and electrolyte. In the discharged state the positive active material is consists of nickel hydroxide (Ni(OH)2 and the negative material is cadmium hydroxide. During charging these convert to NiO OH and cadmium together with some water. Although the separator does not take place in the reaction it serves to insulate between the plates. An electrolyte is also needed. Potassium hydroxide is used for this. It does not participate in the reaction, but enables electron transfer to take place between the two plates.

NiCd cell sizes

NiCd cells can be obtained in a variety of sizes, and often special NiCd battery packs may be manufactured for particular equipments. However the most popular NiCd cells are those in the standard battery or cell sizes: AAA, AA, C, and D cells packages. These standard sizes for this are given below, although it has occasionally been found that some NiCds have exceeded these sizes making fitment to standard slots rather tight.

Charging Nicds

Unlike the lead acid cells, NiCds are charged using a constant current source. Their internal resistance is such that if a constant voltage was used, they would draw excessively large currents which would damage the cells.

Normally cells are charged at a rate of around C/10. In other words if their capacity is 1 amp hour then they would be charged at a rate of 100mA. The charge time is usually longer than ten hours because not all the energy entering the cell is converted into stored electrical energy.

Today many applications require that the cells should be charged faster than this. Accordingly it is possible to obtain some cells which can be charged in an hour or two. It is found that the life expectancy of these cells when they are repeatedly fast charged is less than one which is charged at a slower rate. However for many commercial users the cost of replacing the cells is worth the convenience of being able to fast charge them.

Nickel Cadmium / NiCd memory effect

There is a lot of talk with Nickel Cadmium batteries about the memory effect, and whether it is of any importance to the average user. The NiCd memory effect was discovered when satellites started to use Nickel cadmium batteries. In this application these NiCd batteries were repeatedly partially discharged. Soon it was discovered that their overall capacity was reduced as they "remembered" the amount by which they were normally discharged.

For most normal applications it appears that the NiCd memory effect is not a major issue, although if the cell is run through a complete cycle occasionally, ensuring that it is completely discharged. If the cells are contained within a larger battery, it is helpful to discharge them separately (if possible) as this will ensure that no individual cells become reverse charged as some cells will contain slightly more charge than others. By performing the occasional complete discharge / charge cycle this may help reduce the NiCd memory effect if it is suspected.

Precautions to ensure long life of NiCds

There are a number of precautions that can be observed to help extend the life of NiCd batteries. A short list is shown below:

1. Do not short circuit the cells as very large currents can be drawn. This can be dangerous as large amounts of heat can be generated. It is also advisable not to discharge the cells at very high rates.


2. Never overcharge the cells at a rate greater than or equal to their normal charging current. Trickle charging is permissible.


3. Never reverse charge the cells. This can occur when a battery consisting of several cells in series is completely discharged. As some cells will hold less charge than the rest, as the total battery becomes discharged, some cells will be put into the reverse charge situation.


4. Never discard cells in a fire.


5. Cells operate best under normal room temperature conditions. High and low temperatures reduce their effectiveness. High temperatures can cause permanent damage to the cell.

NiCd summary

Nicd cells and NiCd batteries have been in use for many years as the main form of secondary cell or rechargeable cell for electronics equipment and for small electrical appliances such as torches, etc. Although they are now not favoured for environmental reasons and there are more efficient forms or rechargeable batteries and cells available, they will remain in use, even if at a reduced levels for some years to come as NiCds still offer an effective form of secondary cell.

Further pages from this tutorial
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