The abbreviation NiCad is a registered trademark of SAFT Corporation and should not be used to refer generically to nickel-cadmium batteries, although this brand-name is commonly used to describe all nickel-cadmium batteries. On the other hand, the abbreviation NiCd is derived from the chemical symbols of nickel (Ni) and cadmium (Cd), though it is not to be confused with a chemical formula.
There are two types of NiCd batteries: sealed and vented. This article mainly deals with sealed cells.
With a relatively low internal resistance, a NiCd battery can supply high surge currents. This makes them a favourable choice for remote controlled electric model aeroplanes, boats, and cars, as well as cordless power tools and camera flash units. Larger flooded cells are used for aircraft starting batteries, electric vehicles, and standby power. `
NiCd batteries used to replace nominally 9-V "transistor radio" batteries usually only have six cells, for a terminal voltage of 7.2 volts. While most pocket radios will operate satisfactorily at this voltage, some manufacturers such as Varta made 8.4 volt batteries with seven cells, for more critical applications.
12 V NiCd batteries are made up of 10 cells connected in series
Recently, Nickel-Metal Hydride (Ni-MH) and lithium ion batteries (Li-ion) have become more commercially available and cheaper, the former type now rivalling Ni-Cds in cost. Where energy density is important, Ni-Cds batteries are now at a distinct disadvantage over Ni-MH and Li-ion batteries. This and environmental considerations have largely relegated the Ni-Cd construction to history. However, the Ni-Cd battery is still very useful in applications requiring very high discharge rates because the Ni-Cd can endure such discharge with no damage or loss of capacity.
The primary trade-off with NiCd batteries is their higher cost and the extreme toxicity. They require extra labour to manufacture, and thus, are typically more costly than lead-acid batteries. Typically nickel and cadmium are more costly materials than those used for lead-acid cells. Specifically, if the battery is consistently discharged to the same level, then fully recharged, the battery will eventually stop discharging on its own upon reaching this threshold. (See Memory and lazy battery effects below for more details on this effect).
One of the Nickel-Cadmium's biggest disadvantages was that the battery exhibited a very marked negative temperature coefficient. This meant that as the cell temperature rose, the internal resistance fell. This could pose considerable charging problems particularly with the relatively simple charging systems employed for lead-acid type batteries. Whilst lead-acid batteries could be charged by simply connecting a dynamo to it, with a simple electromagnetic cut out system for when the dynamo is stationary, or an over current occurs, the nickel-cadmium under a similar charging scheme would exhibit thermal runaway, where the charging current would continue to rise until the over current cut out operated or the battery destroyed itself. This was the principal factor that prevented its use for engine starting batteries. Today with alternator based charging systems with solid state regulators, the construction of a suitable charging system would be relatively simple, but the car manufacturers are reluctant to abandon tried and tested technology. In any event, nickel-cadmium technology is falling out of favour.
NiCd cells are available in the same general purpose physical sizes as alkaline batteries, from AAA through D, as well as several multi-cell sizes, including the equivalent of a 9 volt battery. Each cell has a nominal potential of 1.2 volts, compared to the nominal 1.5 volt potential for alkaline batteries. More specifically, a fully charged single NiCd cell, under no load, carries a potential difference of between 1.25 and 1.35 volts, which stays relatively constant as the battery is discharged. Since an alkaline battery near fully discharged may see its voltage drop to as low as 0.9 volts, NiCd cells and alkaline cells are typically interchangeable for most applications.
Miniature button cells are sometimes used in photographic equipment, hand held lamps (flashlight or torch), computer memory standby, toys, and novelties.
In addition to single cells, batteries exist that contain up to 300 cells (nominally 360 volts, actual voltage under no load between 380 and 420 volts). This many cells are mostly used in automotive and heavy duty industrial applications. For portable applications, the number of cells is normally below 18 cells (24 V). Industrial-sized flooded batteries are available with capacities ranging from 12.5Ah up to several hundred Ah.
The maximum discharge rate for a NiCd battery varies by size. For a common AA-size cell, the maximum discharge rate is approximately 18 amps; for a D size battery the discharge rate can be as high as 35 amps.
Model aircraft or boat builders often take much larger currents of up to a hundred amps or so from specially constructed small batteries, which are used to drive main motors. 5-6 minutes of model operation is easily achievable from quite small batteries, so a reasonably high power-to-weight figure is achieved, comparable to internal combustion motors, though of lesser duration.
The safe temperature range for a NiCd battery in use is between −20 °C and 45 °C. During charging, the battery temperature typically stays low, around 0°C (the charging reaction absorbs heat), but as the battery nears full charge the temperature will rise to 45–50 °C. Some battery chargers detect this temperature increase to cut off charging and prevent over-charging.
When not under load or charge, a NiCd battery will self-discharge approximately 10% per month at 20 °C, ranging up to 20% per month at higher temperatures. It is possible to perform a "trickle charge" at current levels just high enough to offset this discharge rate; to keep a battery fully charged. However, if the battery is going to be stored unused for a long period of time, it should be discharged down to at most 40% of capacity (some manufacturers recommend fully discharging, or even short-circuiting), and stored in a cool, dry environment.
NiCd batteries usually have a metal case with a sealing plate equipped with a self-sealing safety valve. The positive and negative electrode plates, isolated from each other by the separator, are rolled in a spiral shape inside the case.
The chemical reactions in a NiCd battery during discharge are:
When Jungner built the first nickel-cadmium batteries, he used nickel oxide in the cathode and iron and cadmium materials in the anode. It was not until later that pure cadmium metal and nickel hydroxide were used. Until about 1960, the reaction in nickel-cadmium batteries was not completely understood. There were several speculations as to the reaction products. The debate was finally resolved by spectrometry, which revealed cadmium hydroxide and nickel hydroxide.
Another historically important variation on the basic nickel-cadmium cell is the addition of lithium hydroxide to the potassium hydroxide electrolyte. This was believed to prolong the service life by making the cell more resistant to electrical abuse. The nickel-cadmium battery in its modern form is extremely resistant to electrical abuse anyway, so this practice has been discontinued.
For this reason, the anode is always designed for a higher capacity than the cathode, to avoid releasing hydrogen gas. There is still the problem of eliminating oxygen gas, to avoid rupture of the cell casing. NiCd cells are vented, with seals that fail at high internal gas pressures. The sealing mechanism must allow gas to escape from inside the cell, and seal again properly when the gas is expelled. This complex mechanism, unnecessary in alkaline batteries, contributes to their higher cost.
NiCd cells dealt with in this article are of the sealed type (see also vented type). Cells of this type consist of a pressure vessel that is supposed to contain any generation of oxygen and hydrogen gasses until they can recombine back to water. Such generation typically occurs during rapid charge and discharge and exceedingly at overcharge condition. If the pressure exceeds the limit of the safety valve, water in the form of gas is lost. Since the vessel is designed to contain an exact amount of electrolyte this loss will rapidly affect the capacity of the cell and its ability to receive and deliver current. To detect all conditions of overcharge demands great sophistication from the charging circuit and a cheap charger will eventually damage even the best quality cells.
Individual cells may be fully discharged to zero volts and some of the battery manufacturers recommend this if the cells are to be stored for lengthy intervals. At least one manufacturer even recommends short-circuiting each cell for storage. However, it is normally recommended that NiCd Batteries be charged to around 40% capacity for long-term storage.
A common form of this deprication occurs when cells connected in series develop unequal voltages and discharge near zero voltage. The first cell that reaches zero is pushed beyond to negative voltage and gasses generated open the seal and dry the cell.
In modern cells an excess of anti-polar material (basically active material ballast at positive electrode) is inserted to allow for moderate negative charge without damage to the cell. This excess material slows down the start of oxygen generation at the negative plate. This means a cell can survive a negative voltage of about -0.2 to -0.4 volts. However if discharge is continued even further, this excess ballast is used up and both electrodes change polarity, causing destructive gassing (gas generation).
Battery packs with multiple cells in series should be operated well above 1 volt per cell to avoid placing the lowest capacity cell in danger of going negative. Battery packs that can be disassembled into cells should be periodically zeroed and charged individually to equalize the voltages. However, this does not help if old and new cells are mixed, since their different capacities will result in different discharge times and voltages.
It is sometimes claimed that NiCd batteries suffer from a "memory effect" if they are recharged before they have been fully discharged. The apparent symptom is that the battery "remembers" the point in its charge cycle where recharging began and during subsequent use suffers a sudden drop in voltage at that point, as if the battery had been discharged. The capacity of the battery is not actually reduced substantially. Some electronics designed to be powered by NiCds are able to withstand this reduced voltage long enough for the voltage to return to normal. However, if the device is unable to operate through this period of decreased voltage, the device will be unable to get as much energy out of the battery, and for all practical purposes, the battery has a reduced capacity.
There is controversy about whether the memory effect actually exists, or whether it is as serious a problem as is sometimes believed. Some critics claim it is used to promote competing NiMH batteries, which apparently suffer this effect to a lesser extent. Many nickel-cadmium battery manufacturers either deny the effect exists or are silent on the matter.
It has been sugested that the memory effect story originated from orbiting satellites, where they were typically charging for twelve hours out of twenty-four for several years. After this time, it was found that the capacities of the batteries had declined significantly, but were still perfectly fit for use. It is thought unlikely that this precise repetitive charging (e.g. 1000 charges / discharges with less than 2% variability) would ever be reproduced by consumers using electrical goods.
An effect with similar symptoms to the memory effect is the so-called voltage depression or lazy battery effect. (Some people use this term as a synonym for "memory effect") This results from repeated overcharging; the symptom is that the battery appears to be fully charged but discharges quickly after only a brief period of operation. Sometimes, much of the lost capacity can be recovered by a few deep discharge cycles, a function often provided by automatic NiCd battery chargers. However, this process may reduce the shelf life of the battery. If treated well, a NiCd battery can last for 1000 cycles or more before its capacity drops below half its original capacity.
NiCd batteries, when not used regularly, tend to develop dendrites which are thin, conductive crystals which may penetrate the separator membrane between electrodes. This leads to internal short circuits and premature failure, long before the 800–1000 charge/discharge cycle life claimed by most vendors. Sometimes, applying a brief, high-current charging pulse to individual cells can clear these dendrites, but they will typically reform within a few days or even hours. Cells in this state have reached the end of their useful life and should be replaced. Many battery guides, circulating on the Internet and online auctions, promise to restore dead cells using the above principle, but achieve very short-term results at best.
NiCd batteries contain cadmium, which is a toxic heavy metal and therefore requires special care during battery disposal. In the United States, part of the price of a NiCd battery is a fee for its proper disposal at the end of its service lifetime. In the European Union, the Restriction of Hazardous Substances Directive (RoHS) bans the use of cadmium in electrical and electronic equipment products since July 2006, though NiCd batteries are not restricted.
Cadmium, being a heavy metal, can cause substantial pollution when landfilled or incinerated. Because of this, many countries now operate recycling programs to capture and reprocess old NiCd batteries.