Nickel-cadmium battery
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The nickel-cadmium battery (commonly abbreviated NiCd and pronounced "nye-cad") is a popular type of rechargeable battery for portable electronics and toys using the metals nickel (Ni) and cadmium (Cd) as the active chemicals. The abbreviation NiCad is a registered trademark of SAFT corporation and should not be used to refer generically to nickel-cadmium batteries. They are sometimes used as a replacement for primary cells, 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 telephones, emergency lighting, as well as power tools.
Due to their beneficial weight/energy ratio as compared to lead based technologies and good service lifetimes, nickel-cadmium batteries of large capacities with a wet electrolyte (wet NiCds) are used for electric cars and as start batteries for aeroplanes.
Nickel-cadmium cells have a nominal cell potential of 1.2 V. This is lower than the 1.5 V of many popular primary cells, and consequently they are not appropriate as a replacement in all applications. However, unlike most primary cells, 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 0.90 to 1.0 V per cell, and the 1.2 V of a NiCd is more than enough. 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 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 model aeroplanes, boats and cars, as well as cordless power tools and camera flash units.
Besides 1.2 V single cells, 7.2, 9.6, and 12 V NiCd batteries made up of several cells connected in series are widely available. The 7.2 V batteries are the most common replacement for 9 V primary batteries, although 8.4 V batteries have been made by some manufacturers, e.g., VARTA, to better match the performance of carbon-zinc and alkaline "transistor radio" batteries.
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[edit] History
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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. This design incorporates several layers of anode and cathode material rolled into a cylindrical shape.
Advances in both battery and manufacturing technologies 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 had 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.
Nickel Cadmium (NiCd)
Advantages and disadvantages:
- -NiCds are relatively expensive
- -require much more labour to manufacture, hence their extra expense
- -can develop a false bottom effect where they won't accept a full discharge if they are routinely discharged to the same level, then charged.
- +require less care and are difficult to damage
- +usually last a longer time (more cycles)
- +can often be discharged or charged at a faster rate than gel-cell lead acid batteries.
- +Are not damaged if left in a deep discharge or complete uncharged condition for a long period of time (in fact discharged is the correct way to store unused batteries)
- +Incur virtually no loss of capacity when subject to high discharge currents
Size, capacity and package ranges: Each cell is nominally 1.2 Volts (actual unloaded fully charged about 1.25-1.35 Volts), so 10 cells would total nominally 12 Volts (actual unloaded about 12.5-13.5 Volts). NiCd’s have the ability to be 11 cells which is nominally 13.2 Volts (actual unloaded about 13.75-14.85 Volts) or 12 cells which is nominal 14.4 Volts (actual unloaded about 15.0-16.2 Volts).
Consumer NiCds are available in standard “AAA” all the way through to “D” and 9v sizes. Can be built to have 10,11, or 12 cells instead of 1 standard size. Industrial flooded versions are available in sizes like 12.5Ah, 25Ah, 100Ah, and more.
Maximum charge rate: A rough rule of thumb is that a high quality NiCd battery can accept a charge of up to 80% of its amp-hour rating i.e. a “BYD” AA NiCd is 0.9 Ah and takes roughly 45 minutes to charge the fully discharged battery and at 1.26A
Maximum discharge rate: For a common AA size roughly 18A , for a C size battery about 22A and for a D size about 35A  Temperature range: Standard application -20°C / +45°C In charge 0°C / +45°C
Price: AA, AAA’s roughly $1-2 ea C’s roughly $3-4 ea D’s roughly $4-5 ea 9v’s roughly $6-9 ea
Typical applications: Most portable electronics, calculators, radios and toys
Care and maintenance: NiCd batteries self-discharge at 10% per month at 20 degrees C so a periodic top up is necessary. Ideally, NiCd batteries should be stored discharged. Don’t overcharge it, as the excessive overheating will damage battery. Remove it from the device and store it in a cool, dry, clean place. If the battery will not be in use for a month or longer, recharge it after a storage period.
Inspecting: The battery should have no external damage and depending on the number of cells it should have 1.2v per cell when fully charged and about 0.8-1v when discharged.
Battery state: The larger NiCd’s contain a liquid much like flooded batteries while smaller ones e.g. those used in flashlights are relatively "dry"
Charge condition: High quality NiCd’s have a thermal cut-off so if the battery gets too hot the charger stops. If a NiCd is still warm from discharging and been put on charge, it will not get the full charge it can get. In that case let the battery cool to room temperature then charge. Watch for the correct polarity. Leave charger in a cool place or room temperature when charging to get best results.
Charging method: A NiCd battery requires a charger with a slightly different voltage charge level than a lead-acid battery, especially if the NiCd has 11 or 12 cells. Also the charger requires a more intelligent charge termination method if a fast charger is used. Often NiCd batteries have a thermal cut-off inside that feeds back to the charger telling it to stop the charging once the battery has heated up and/or a voltage peaking sensing circuit. A rough rule of thumb is that a high quality NiCd battery can accept a charge not less than 80% of its Amp/hour rating so for a 0.9 Ah takes 45 minutes to fully charge and takes 140% of its Ah rating as current taken to charge so for the same battery 1.26 A is required. At room temperature during normal charge conditions the cell voltage increases from an initial 1.2V to an end-point of about 1.45V. The rate of rise increases markedly as the cell approaches full charge. The end-point voltage decreases slightly with increasing temperature.
[edit] Chemistry
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 were 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 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 after July 2006, though NiCd batteries will not be restricted.
[edit] Problems with NiCd
[edit] Memory effect
- Main article: Memory effect
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 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 this effect exists or are silent on the matter.
The memory effect story first came from satelites in orbit, where they were typically charging for twelve hours out of twenty four for very long periods of time - several years. After this time, it was found that the capacity 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 "lazy battery effect". (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 automatic NiCd battery chargers. However, this process may reduce the shelf-life of the battery [2]. If treated well, a NiCd battery can last for 1000 cycles or more before its capacity drops below half its original capacity.
[edit] Dendritic shorting
NiCd batteries, when not used regularly, tend to develop dendrites (thin, conductive crystals), causing internal short circuits and premature battery failure, long before the 800-1000 charge/discharge cycles claimed by most vendors. Sometimes, these dendrites can be cleared by applying a brief, high-current charging pulse to individual cells, but once dendrites have begun to form, they will typically recur soon thereafter.
[edit] Environmental consequences
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.
[edit] Safety
- Never short-circuit the battery because this may cause the battery to explode. (A short-circuit is a direct electrical connection between the + and - battery terminals, such as with a wire. You should not short-circuit any type of battery.)
- Never incinerate NiCd batteries; besides the possibility of explosion, this will release toxic cadmium into the environment. Recycle the battery instead.
- Avoid dropping, hitting, or denting the battery because this may cause internal damage including short-circuiting of the cell.
- Avoid rapid overcharging of the battery; this may cause leakage of the electrolyte, outgassing, or possibly an explosion.
[edit] 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 (except with chargers specifically designed to recharge alkalines). In the mid-1990s, Rayovac introduced a rechargeable alkaline, Renewal, which, although more expensive, began to replace NiCd; however, this product was discontinued several years later.
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, NiCd appear to have three advantages over NiMH. Most important to consumers is a lower cost. The second 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. The third advantage is that the NiCd battery maintains a constant voltage so that power tools can work properly even though some people cannot tell how discharged the battery is. 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 may be rechargeable up to 300 times (or more).
[edit] References
- 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.
