The nickel-cadmium battery (commonly abbreviated NiCd or NiCad) is a popular type of rechargeable battery for portable electronics and toys. They are sometimes used as a replacement for so called primary batteries, such as heavy duty or alkaline, being available in many of the same sizes. In addition, specialty NiCd batteries have a niche market in the area of cordless and wireless phones, Emergency Lighting, as well as power tools.
Due to their beneficial weight/energy ratio as compared to lead based technologies, nickel-cadmium batteries of large capacities with a wet electrolyte are used for electric cars and as start batteries for aeroplanes.
Nickel-cadmium cells have a nominal cell voltage of 1.2 V. This is lower than the 1.5 V of many popular primary batteries, and consequently they are not appropriate as a replacement in all applications. However, unlike most primary batteries, NiCds keep a near constant voltage throughout their service life. Because many electronic devices are designed to work throughout the lifetime of the battery, they must operate on voltages as low as 0.9 to 1.0 V per cell, and the 1.2 V of a NiCd is more than enough. Note that some would consider the near constant voltage a drawback, as it makes it difficult to detect when the battery charge is low; this is usually a minor concern. Despite their lower nominal voltage, NiCds are actually better suited for high current applications. Due to a significantly lower series resistance, they can supply high surge currents. This makes them a favourable choice for remote controlled electric toy aeroplanes, boats and cars, as well as cordless power tools.
Besides 1.2V single cells, 7.2, 9.6, and 12 V NiCd batteries made up of several cells connected in series are widely available. 7.2 V batteries are the most common replacement for 9 V primary batteries.
In 1899, Waldemar Jungner of Sweden created the first nickel-cadmium battery. At this time, the only direct competitor was the lead acid battery. The nickel-cadmium battery offered several advantages in certain applications. Even early nickel-cadmium batteries were physically and chemically robust. With minor improvements to the first prototypes, energy density rapidly increased to about half of that of primary batteries, significantly better than lead acid batteries.
In 1910, a company was formed to produce industrial nickel-cadmium batteries in Sweden. The first production in the United States began in 1946. Up to this point, the batteries were "pocket type," constructed of nickel-plated steel pockets containing nickel and cadmium active materials. Around the middle of the twentieth century, sintered plate nickel-cadmium batteries became increasingly popular. Sintered plates are created by fusing nickel powder at a temperature well below the melting point using high pressures. The plates thus formed are highly porous, with about 80 percent pore volume. Positive and negative plates are produced by soaking the nickel plates in nickel and cadmium active materials, respectively. Sintered plates are usually much thinner than the pockets of pocket type batteries, allowing more surface area per volume, in turn allowing higher currents for batteries of comparable size. In general, the more surface area of reactive materials in a battery, the lower the internal resistance. In the past few decades, this fact has allowed for nickel-cadmium batteries with internal resistance as low as that for alkaline batteries. Today, all consumer nickel-cadmium batteries use the "jelly-roll" design. As might be expected, this design incorporates several layers of anode and cathode material rolled into a cylindrical shape.
Advances in both battery and manufacturing technology throughout the second half of the twentieth century have made batteries increasingly cheaper to produce. Battery powered devices in general have increased in popularity. As of 2000, about 1.5 billion nickel cadmium batteries were produced annually. While NiCd never became widely used as a replacement for lead acid batteries in the areas where those batteries dominate, up until the mid 1990s, NiCds were an overwhelming majority of the market share for rechargeable batteries in consumer electronics. Recently, however, Nickel Metal Hydride (NiMH) and lithium ion batteries have become more commercially available and cheaper, though still more expensive than NiCds. Where energy density is important, those types of batteries have become favorable to NiCds, especially when the cost of the battery is small compared to the cost of the device, such as in cell phones.
NiCd batteries contain a nickel hydroxide positive electrode plate, a cadmium hydroxide negative electrode plate, a separator, and an alkaline electrolyte. 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 reaction which occurs in a NiCd battery is:
2 NiO(OH) + Cd + 2 H2O ↔ 2 Ni(OH)2 + Cd(OH)2
This reaction goes from left to right when the battery is being discharged and from right to left when it is being recharged. The alkaline electrolyte (commonly KOH) is not consumed in this reaction.
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.
Overcharging must be considered in the design of most rechargeable batteries. In the case of NiCds, there are two possible results of overcharging. If the anode is overcharged, hydrogen gas is produced; if the cathode is overcharged, oxygen gas is produced. 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.
Another potential problem is reverse charging. This can occur due to an error by the user, or more commonly, when a battery of several cells is fully discharged. Because there is a slight variation in the capacity of cells in a battery, one of the cells will usually be fully discharged before the others, at which point reverse charging begins seriously damaging the other cells, reducing battery life. The byproduct of reverse charging is hydrogen gas, which can in some circumstances be dangerous. Some commentators advise that one should never discharge multi-cell nickel-cadmium batteries to zero voltage, for example torches should be turned off when they yellow, before they go out completely.
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.
NiCd batteries contain cadmium, which is a toxic heavy metal and therefore requires special care when the batteries are disposed of. In the United States, part of the price of a NiCd battery is for its proper disposal at the end of its service lifetime.
It is sometimes claimed that NiCd batteries suffer from a so-called "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 NiCads 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 this effect exists or are silent on the matter.
An effect with similar symptoms to the memory effect is the so-called "lazy battery effect" (note however that some people use this term as a synonym for "memory effect"). This effect is the result of 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 NiCd battery chargers. If treated well, NiCd batteries can last for 1000 cycles or more before capacity drops below 50% of original.
Comparison to other batteries
Lead-acid batteries are the most commonly used rechargeable batteries, found in nearly all automobiles. However, they have a much lower energy density than NiCds. NiCds have found some limited use in transportation applications where lead acid batteries used to dominate, but due to higher cost, they are only practical when size and weight are important considerations.
NiCds have lower capacities than alkaline batteries, against which they are a direct competitor in many applications. However, the total lifetime of NiCds is longer, as most alkalines cannot be recharged. In the mid-1990s, Rayovac introduced a rechargeable alkaline, Renewal, which, although more expensive, began to replace NiCds.
Nickel metal hydride (NiMH) batteries are similar to NiCd, but are less toxic and offer higher capacities. As they became commercially available in the 1990s, NiMH batteries took over a large portion of the rechargeable battery market share. However, NiCds appear to have two advantages over NiMH. Most important to consumers is a lower cost. The other advantage is that the self discharge rate for NiCd rechargeables ranges around 20% per month, whereas in Nickel metal hydride batteries it is around 30% per month. In both types of battery, the self discharge rate is highest for a full charge state and drops off somewhat for lower charge states.
In the future, another new rechargeable alkaline technology, the super iron battery, may take the forefront. As of 2000, only working prototypes have been constructed, but it appears the batteries will have a capacity about 50% higher than that of alkalines and be rechargeable up to 300 times.
- Bergstrom, Sven. "Nickel-Cadmium Batteries – Pocket Type." Journal of the Electrochemical Society, September 1952. 1952 The Electrochemical Society.
- Ellis, G. B., Mandel, H., and Linden, D. "Sintered Plate Nickel-Cadmium Batteries." Journal of the Electrochemical Society, September 1952. 1952 The Electrochemical Society.