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A battery is a galvanic cell or a series of combined galvanic cells that can be used as a source of direct electric current at a constant electric voltage. Batteries store electricity in a chemical form, inside a closed-energy system. They can be re-charged and re-used as a power source in small appliances, machinery and remote locations. Advances in battery technology may one day help to solve our energy crisis. We think of a battery today as a source of portable power, but it is no exaggeration to say that the battery is one of the most important inventions in the history of mankind. Volta's pile was at first a technical curiosity but this new electrochemical phenomenon very quickly opened the door to new branches of both physics and chemistry and a myriad of discoveries, inventions and applications. The electronics, computers and communications industries, power engineering and much of the chemical industry of today were founded on discoveries made possible by the battery. It is often overlooked that throughout the nineteenth century, most of the electrical experimenters, inventors and engineers who made these advances possible had to make their own batteries before they could start their investigations. They did not have the benefit of cheap, off the shelf, mass produced batteries. For many years the telegraph, and later the telephone, industries were the only consumers of batteries in modest volumes and it wasn't until the twentieth century that new applications created the demand that made the battery a commodity item. In recent years batteries have changed out of all recognition. No longer are they simple electrochemical cells. Today the cells are components in battery systems, incorporating electronics and software, power management and control systems, monitoring and protection circuits, communications interfaces and thermal management.
In 250 B.C.Â TheÂ Baghdad BatteryÂ - In 1936 several unusual earthenware jars, dating from about 250 B.C., were unearthed during archeological excavations at Khujut Rabu near Baghdad. A typical jar was 130 mm (5-1/2 inches) high and contained a copper cylinder, the bottom of which was capped by a copper disk and sealed with bitumen or asphalt. An iron rod was suspended from an asphalt stopper at the top of the copper cylinder into the centre of the cylinder. The rod showed evidence of having been corroded with an acidic agent such as wine or vinegar. 250 BC corresponds to the Parthian occupation ofÂ MesopotamiaÂ (modern day Iraq) and the the jars were held in Iraq's State Museum in Baghdad. In 1938 they were examined by German archeologist WilhelmÂ KonigÂ who concluded that they were Galvanic cells or batteries supposedly used for gilding silver by electroplating. A mysterious anachronism. Backing up his claim, Konig also found copper vases plated with silver dating from earlier periods in the Baghdad Museum and other evidence of (electro) plated articles from Egypt. Since then, several replica batteries have been made using various electrolytes including copper sulphate and grape juice generating voltages from half a Volt to over one Volt and they have successfully been used to demonstrate the electroplating of silver with gold. One further, more recent, suggestion by Paul T. Keyser a specialist in Neat Eastern Studies from the University of Alberta is that the galvanic cells were used for analgesia. There is evidence that electric eels had been used to numb an area of pain, but quite how that worked with such a low voltage battery is not explained. Apart from that, no other compelling explanation of the purpose of these artifacts has been proposed and the enigma still remains.
Rechargeable batteries or accumulators are the oldest form of electricity storage and are widely used. Batteries store electric energy in a chemical form. Progress in battery technology is slow and the transfer of laboratory results into commercial applications is sometimes risky. Lithium ion and nickel-metal-hydride (NiMH) batteries are the only new battery technologies that have achieved significant market penetration in the last decade.
There are different types of rechargeable batteries.
NiCad: Nickel Cadmium:
NiMH: Nickel Metal Hydride
NiCad: Nickel Cadmium:
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 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.
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.
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.
NiMH: Nickel Metal Hydride:
Nickel Metal Hydride, NiMH batteries and cells have come into widespread use in recent years as a viable form of rechargeable battery. These NiMH cells offer almost identical characteristics to those provided by the older NiCad technology, but with the advantage that the NiMH cells do not have the same adverse environmental effects, and they are also able to provide a slightly higher level of energy density and therefore overall charge capacity. As a result, NiMH cells are now widely used, offering high levels of performance.
NiMH cell basics
The NiMH cell bears many similarities to the older NiCd cell technology, using many similar constituents. The NiMH cell consists of three main elements, namely the positive electrode, the electrolyte and the negative electrode. These will be addressed separately below:
Positive electrode:Â Â The positive electrode of the NiMH battery is nickel hydroxide havingthesame composition as the positive electrode in a NiCd cell. The nickel oxide -hydroxide electrode only exchanges a proton in the charge-discharge reaction, and this results in a very small change in size, resulting in a high level of mechanical stability, and this in turn results in a longer cycle life.
Electrolyte:Â Â The electrolyte in the NiMH cell is an aqueous solution of potassium hydroxide (KOH) which has a very high conductivity. The solution does not enter into the NiMH cell reaction to any significant extent. It is found that the electrolyte concentration remains almost constant over the charge / discharge cycle. This is important because the concentration of the electrolyte is the main contributor to the cell resistance. This means that the performance of the cell remains almost the same over the entire charge range
Negative electrode:Â Â The active material for the negative electrode is actually hydrogen. However it is not physically possible to use hydrogen directly and therefore the hydrogen is stored in the NiMH cell as a metal hydride which also serves as the negative electrode. As a point of interest, the metal hydrides used in NiMH cells can normally hold between 1% and 2% hydrogen by weight.
NiMH charge / discharge characteristics
In operation the NiMH cell has many similar characteristics to the more familiar NiCd. It follows a very similar discharge curve to that of the NiCad allowing for the extra charge it can take. However it is very intolerant of overcharging, suffering a reduced capacity if this occurs. This presents a significant challenge to battery charger designers.
Many intelligent chargers for NiCds sense a small but distinct "bump" in the output voltage when a NiCad is fully charged. However for NiMH cells this increase is very much smaller, making it more difficult to detect. As a result the temperature of the cells is also detected as well, because once fully charged the cell dissipates much of the additional charge as heat. A further complication is that the characteristics of NiMH cells vary significantly from one manufacturer to the next making charge performance more difficult to detect.
Self discharge characteristics
One of the problems with NiMH cells is that they self-discharge over a relatively short period of time. All cells will loose their charge over time, even if they are not used, but this is a particular problem for NiMH cells.
Typically it might be expected that a fully charged cell might self discharge over a period of a few weeks. NiCds are better than NiMH cells but in turn they not as good as normal primary cells but NiCds will typically retain charge over several months dependent upon the type of battery or cell.
There are several factors that contribute to the self discharge of an NiMH cell dependent upon the state of charge. These can broadly be described as an oxygen cycle that occurs at high states of charge, and then ion movement that contributes to the self discharge over longer periods of time.
One important factor in the rate of self discharge is the temperature at which the cell is held. It is found that at higher temperatures, the rate of discharge significantly increases. Therefore cells should be kept cool if it is necessary for them to hold their charge over longer periods.
Alkaline batteriesÂ andÂ alkaline cellsÂ (a battery being a collection of multiple cells) are a type of disposableÂ batteryÂ orÂ rechargeableÂ battery dependent upon the reaction betweenÂ zincÂ andÂ manganese dioxideÂ (Zn/MnO2).
Compared withÂ zinc-carbon batteriesÂ of the Leclanche or zinc chloride types, while all produce approximately 1.5Â voltsÂ per cell, alkaline batteries have a higher energyÂ and longerÂ shelf-life. Compared withÂ silver-oxide batteries, which alkalines commonly compete against inÂ button cells, they have lower energy density and shorter lifetimes but lower cost.
The alkaline battery gets its name because it has anÂ alkalineÂ electrolyte ofÂ potassium hydroxide, instead of the acidic ammonium chloride or zinc chloride electrolyte of the zinc-carbon batteries which are offered in the same nominal voltages and physical size. Other battery systems also use alkaline electrolytes, but they use different active materials for the electrodes.
Voltage and Current:
The nominal voltage of a fresh alkaline cell is 1.5 V. Multiple voltages may be achieved with series of cells. The effective zero-load voltage of a non discharged alkaline battery varies from 1.50 to 1.65 V, depending on the chosen manganese dioxide and the contents of zinc oxide in the electrolyte. The average voltage under load depends on discharge and varies from 1.1 to 1.3 V. The fully discharged cell has a remaining voltage in the range of 0.8 to 1.0 V.
The amount of current an alkaline battery can deliver is roughly proportional to its physical size. This is a result of decreasing internal resistance as the internal surface area of the cell increases. A general rule of thumb is that an AA alkaline battery can deliver 700 mA without any significant heating. Larger cells, such as C and D cells, can deliver more current. Applications requiring high currents of several amperes, such as high powered flashlight and portable stereos, will require D-sized cells to handle the increased load.
Alkaline batteries are manufactured in standardized cylindrical forms interchangeable with zinc-carbon batteries, and in button forms. Several individual cells may be interconnected to form a true "battery", such as those sold for use with flashlights and the 9 volt transistor-radio battery.
A cylindrical cell is contained in a drawn steel can, which is the cathode current collector. The cathode mixture is a compressed paste of manganese dioxide with carbon powder added for increased conductivity. The paste may be pressed into the can or deposited as pre-molded rings. The hollow center of the cathode is lined with a separator, which prevents mixing of the anode and cathode materials and short-circuiting of the cell. The separator is made of a non-woven layer of cellulose or a synthetic polymer. The separator must conduct ions and remain stable in the highly alkaline electrolyte solution.
The anode is composed of a dispersion of zinc powder in a gel containing the potassium hydroxide electrolyte. To prevent gassing of the cell at the end of its life, more manganese dioxide is used than required to react with all the zinc.
When describing standard AAA, AA, C, Sub-C and D size cells, the anode is connected to the flat end while the cathode is connected to the end with the raised button.
Recharging Of Alkaline Battery:
Some alkaline batteries are designed to be recharged, but most are not. Attempts to recharge may cause rupture, or the leaking of hazardous liquids which will corrode the equipment.
Lithium ion, Li-ion battery:
Lithium ion, or Li-ion batteries are now being widely used for applications such as powering laptop computers, mobile phones cameras and many more devices. The high energy density that Li-ion batteries provide, enables the electronic devices they power to be recharged less frequently. Also Li-ion batteries are comparatively light when compared to other forms of rechargeable cells and batteries.
In view of their convenience, Li-ion, lithium ion batteries are widely used and there are a number of different manufacturers for these batteries. Accordingly costs have fallen from their original high levels, although Li-ion batteries are still expensive..
Lithium ion, Li-ion battery basics
The lithium ion cell is comparatively complicated to manufacture, requiring over 30 components. Basically it consists of a lithium cobalite cathode and a graphite anode. These are wound tightly together so that they can be placed into a cylinder along with a plastic insulator. The cylinder is then filled with an electrolyte and the end is capped.
Li-ion charge discharge characteristics
Unlike the NiCd and NiMH cells which exhibit very similar characteristics the Li-ion cell is quite different. The cell potential starts at around four volts, decaying to around three volts before it is said to be discharged. This has a number of advantages. Items like cell phones require a minimum operating voltage of around three volts. This can be provided by a single Li-ion cell. Having a single cell also simplifies charging.
Other characteristics of the Li-ion cell show improvements over its competitors. It has been shown to be able to withstand over 1000 charge/discharge cycles and still be able to hold 80% of its initial capacity. NiCds offer up to around 500 cycles, although this is very dependent upon the way they are used. A badly treated cell may only give 50 or 100. NiMH cells are even worse, and this is one of the main areas receiving development. They are only able to give 500 cycles at the very best before their capacity drops to 80% of the initial charge rating.
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.
With many new battery and cell technologies being developed, NiMH technology is looked upon as an interim solution by many. NiMH cells have the advantage that they offer very similar characteristics to NiCads and in that way they can be used as a direct replacement. For the longer term, Li-ion batteries are finding widespread use, but despite this the use of NiMH batteries and cells is still widespread.
Li-ion batteries and cells or li-ion batteries and cells are now in widespread use. They have taken a position of dominance in the rechargeable battery market and as a result many mobile phones, laptop computers and cameras, etc use them. Although there are many new battery developments taking place, lithium ion batteries, li-ion batteries will remain one of the main types of battery for many years to come.
In the relatively short time since the last International Power Sources Symposium, progress on lithium-ion batteries has been generally incremental with considerable interest in the new lithium electrolyte salt (LiBOB) and in the new cathode material, lithium iron phosphate (LiFePO4). Developing technologies include layered LiMnNi oxides, ionic liquids and N-containing polymer electrolytes. The conclusions at the last symposium that cost and safety are the main factors limiting the usage of lithium-ion batteries in larger sizes, such as for electric vehicles, remain.
Not only has the world has became more dependent on batteries, in general it has also come to appreciate the economy and reliability of rechargeable batteries. Battery is one of the most important invention of mankind. Today battery is a source of portable power. Advances in battery technology may one day help to solve our energy crisis. The electronics, computers and communications industries, power engineering and much of the chemical industry of today were founded on discoveries made possible by the battery.