|[ Team LiB ]|
Outside of the physical difference in packaging, notebook computers differ from desktop machines most significantly in that they are designed with self-contained power systems. They hold their own electricity and can run on batteries. The power versatility is what gives the notebook machine the ability to compute anywhere.
But batteries play a larger role in modern computing. Wireless keyboards and mice, digital cameras, and MP3 players all rely on batteries as their primary source of power (as likely does your cell phone and a host of other electronic gadgets you take for granted). It's enough to make you want to buy stock in a battery company—and shudder to think what all those throw-away batteries are doing to landfills.
Batteries represent the primitive side of electricity, the chemical side. It's a territory strewn with things that get your hands dirty, including smudgy black carbon and a host of toxic materials you hope you never do get on your fingers. Batteries have stubbornly refused to give in to micro-miniaturization, yielding only small increases in capacity with every investment in research and development.
We often forget the chemical nature of batteries because the chemistry is all sealed away, usually permanently. To use the battery usually is a small cylinder that produces electricity (or, in the case of batteries for notebook computers, an expensive plastic shell that doesn't hold nearly enough electricity). In any case, the outward manifestations of the battery are physical and electrical. Its physical size and shape determine where it will fit. Its electrical ratings determine what it can run.
The most popular batteries for small electronic devices come in standard sizes. The battery packs for notebook computers are usually tailored to a specific model of machine, although many such packs comprise a set of standard batteries permanently connected together. All batteries produce the same kind of electricity—direct current—but they vary in the amount of energy they can store and several other electrical characteristics.
Batteries can be divided into two types: primary and secondary or storage. In primary batteries, the creation of electricity is irreversible; one or both of the electrodes is altered and cannot be brought back to its original state except by some complex process (such as re-smelting the metal). Secondary or storage batteries are rechargeable; the chemical reaction is reversible by the application of electricity. The electrons can be coaxed back from whence they came. After the battery is discharged, the chemical changes inside can be reversed by pumping electricity into the battery again. The chemicals revert back to their original, charged state and can be discharged to provide electricity once again.
In theory, any chemical reaction is reversible. Clocks can run backwards, too. And pigs can fly, given a tall enough cliff. The problem is that when a battery discharges, the chemical reaction affects the electrodes more in some places than others; recharging does not necessarily reconstitute the places that were depleted. Rechargeable batteries work because the chemical changes inside them alter their electrodes without removing material. For example, an electrode may become plated with an oxide, which can be removed during recharging.
Primary and secondary (storage) batteries see widely different applications, even in computers. Nearly every modern computer has a primary battery hidden somewhere inside, letting out a tiny electrical trickle that keeps the time-of-day clock running while the PC is not. This same battery also maintains a few bytes or kilobytes of CMOS memory to store system configuration information. Storage batteries are used to power just about every notebook computer in existence. (A few systems use storage batteries for their clocks and configuration memory.)
The most important of these is voltage, which describes the electrical potential of a battery, the force with which the battery can move electrons through circuitry. The technical term is electromotive force (EMF), but most people usually talk about its direct measure, volts.
All batteries have a voltage rating that is both unchangeable and varying. That is, the voltage of a battery cell is characteristic of the cell design and the chemical reaction taking place inside, and this reaction does not change. But the voltage produced by the reaction varies with temperature (most batteries produce lower voltage as the temperature declines), the age of the cell (most batteries produce lower voltage as they age), and load (most batteries produce lower voltages when they are called upon to deliver more current).
These factors result in battery voltage varying widely from the nominal or rated voltage. Cells may start life producing 1.8 volts and remain useful until their output falls to half that. Because of the wide variance of cell voltage, most equipment that uses battery power is either insensitive to the exact voltage supplied or regulates the supplied voltages so that the internal circuitry of the equipment sees a constant voltage no matter the exact voltage produced by battery cells. Consequently, typical commercial cells that use carbon-zinc (nominally rated at 1.5 volts), nickel-cadmium (nominally rated at 1.2 volts), and lithium disulfide (nominally rated at 1.6 volts) are essentially interchangeable.
Depending on the chemistry used, a single cell can produce anywhere from a small fraction of a volt to somewhat more than three volts. Batteries rated with voltages higher than about three are composites of several cells linked together. (Technically, the term battery describes a collection of several individual electrochemical cells, although in common usage a commercial battery may only be a single cell.)
Current describes the number of electrons the potential can push, the quantity of electricity. Current is measured in amperes (named after the French mathematician and physicist André Marie Ampére, 1775–1836), usually clipped to the term amps.
Battery cells are limited in the current they can produce by their designs and chemistries. In theory, if the entire chemical reaction in a battery cell occurred instantly, the cell would produce unlimited current—for an instant. Practical factors limit the chemical reaction rate and the current a cell can produce. Chief among these are the basic reaction rate of the chemicals, the design of the cell, and the area over which the reaction takes place. Consequently, some cells are inherently able to produce high currents. Others can only product weak currents. For example, the currents produced by lead-acid batteries and nickel-cadmium cells are so high, such batteries can melt metals and start fires when shorted out. Put an unpackaged ni-cad battery in your pocket, and it may short out against your keys and loose change. The high current and heat from the short circuit could start a fire. Consequently, these high-current cells often wear warning labels.
Cell size is also an important factor in determining the reaction area of the cell chemistry and consequently the current-creating capabilities of the cell. Making a cell larger increases the current it can produce, so heavy-duty applications often require large cells. "D" cells can produce more current than "AA" cells.
The various factors in cell design and chemistry essentially reduce to a single mathematical factor—the equivalent internal resistance of the cell, which determines current capabilities. A low internal resistance allows high currents.
Energy and Capacity
Voltage and current are instantaneous values that describe battery characteristics that are relevant when determining what kind of device the battery can power. Electrical circuits and motors vary in their voltage and current needs, and they must be tailored to match the battery used to power them.
The actual power that a battery can produce is the product of the voltage and current and is described in watts (named in honor of Scottish inventor and engineer James Watt, 1736–1819). A battery's power is independent of its size—a battery designed to produce a high current can generate tremendous power, although briefly. For example, even a small AA ni-cad battery can create enough current to melt metal, but it wouldn't last very long when challenged with the task of running a notebook computer.
A more relevant measure is the energy a given battery can produce. Energy is the amount of power a battery can produce over an extended period. A common measure is the watt-hour, the steady production of one watt of power for one hour.
The rated capacity of a cell or battery is the amount of electricity or electric charge it can produce when fully charged under specified conditions. As with voltage, the actual amount of charge the battery can produce varies with its temperature and the discharging current.
In science, the standard unit for measuring battery capacity is the coulomb (named after French physicist C. A. de Coulomb, 1736–1806), which describes the time the battery can produce a given current. One coulomb is one ampere produced for one second. In practice, however, cell or battery capacity is more commonly expressed in ampere-hours (AH) or milliampere-hours (mAH), equal to 3600 times the coulomb rating. The total energy in a battery is its capacity multiplied by its voltage (which results in a measurement of watt-hours).
The ratio of capacity to the weight (or size) of the battery is called the storage density of the battery. For you, as a battery user and the person charged with carrying around a notebook computer weighed down with batteries, this is a most important measure. The storage density of a battery determines how heavy a load of batteries your computer needs for a given runtime. The higher the storage density of a battery, the more energy that can be stored in a given size or weight of a cell and, hence, the more desirable the battery.
The chemistry used by a battery to store or produce electricity is the primary factor in determining the storage density of a battery. Table 31.2 lists the storage density of the major chemical systems used in storage batteries for personal computer and cell phone applications, expressed in watt-hours per kilogram of weight (Wh/kg).
Some of the chemical reactions permanently affect the ability of the cell to store chemical energy. After a while, the battery loses its usefulness and becomes nothing more than a colorful piece of clutter. The period during which the cell remains useful is termed its shelf life. The chemistry and construction of the cell determine its shelf life, as do the conditions of storage. Some cells, such as modern lithium designs, have shelf lives in excess of ten years, whereas other cells may deteriorate in a matter of weeks (for example, zinc-air batteries once activated). Poor storage conditions—especially high temperatures—usually accelerate the degradation of cells, whereas refrigeration (and, with some chemistries, freezing) often prolongs shelf life.
In secondary cells, the reversible chemical reactions that produce electricity slowly take place even when the cell is not used. These reactions discharge the cell as if it were being used and are consequently called self-discharge. As with normal discharge of the cells, the reactions of self-discharge are reversible by simply recharging the cells. The self-discharge rates of batteries vary with the same factors affecting shelf life, although in modern cells the chemistry and cell design are the major determinants. Some chemistries lose as much as 10 percent of their charge in a day; others less than 1 percent.
The chemical reactions in the cell are the most important factor constraining energy density and the usefulness of batteries. In fact, the entire history of battery technology has been mostly a matter of finding and refining battery chemistries to pack more energy in ever-smaller packages. Today's batteries use a variety of chemical systems, some dating from the late 19th Century, as mentioned previously, and some hardly a decade old. The diversity results from each having distinct benefits for particular applications. The following battery chemistries are the most popular for portable computer, cell phone, power system, and peripheral applications.
The starting point for battery technology is the carbon-zinc cell, the heir of Georges Leclanché's 1866 invention of the wet cell for producing electricity. Carbon-zinc cells are probably the most common batteries in the world, known under a variety of names, including dry cell and flashlight battery. When you think of batteries, it's likely that carbon-zinc cells first come to mind. One company alone, Energizer, sells over six billion carbon-zinc cells each year. They are the lowest priced primary cells. They also have the lowest storage density of any common battery.
One reason carbon-zinc cells are so popular is that the name actually describes two or three different chemistry systems. These include Leclanché cells, zinc chloride cells, and alkaline batteries.
The name describes the basic chemistry of the cells. In the basic carbon-zinc cell, the "carbon" in the name is a cathode current collector—a carbon rod in the center of the cell. The actual material of the cathode is a mixture of manganese dioxide, carbon conductor, and electrolyte. The zinc serves as the anode and often serves as the metal shell of the battery. The electrolyte is a complex mixture of chemicals that typically includes ammonium chloride, manganese dioxide, and zinc chloride.
The electrolyte is the chief difference between Leclanché and zinc-chloride cells. The former use a slightly acidic mix of ammonium chloride and zinc chloride in water. The electrolyte in zinc-chloride cells is mostly zinc chloride. Zinc chloride cells produce a slightly higher open-circuit voltage than Leclanché cells (1.6 versus 1.55 volts).
Although zinc-chloride cells typically have a greater capacity than Leclanché cells, this difference shrinks under lighter loads, so zinc-chloride cells are often termed heavy-duty. In any case, the efficiency of any carbon-zinc cell decreases as the load increases—doubling the current drain more than cuts in half the capacity of the cell. The most efficient strategy is to use as large a cell as is practical for a given application. That's why power-hungry toys demand "D" batteries and low-drain transistor radios make do with "AA" cells.
Alkaline batteries, no matter the advertised claims, are little more than an enhancement of 19th Century carbon-zinc technology. The biggest change in chemistry is an alteration to the chemical mix in the electrolyte that makes it more alkaline (what did you expect?). This change helps to increase storage density and shelf life of the cells.
The construction (as opposed to chemistry) of alkaline cells differs significantly from ordinary carbon-zinc cells, however. Alkaline cells are effectively turned inside-out. The shell of the alkaline battery is nothing more than that—a protective shell—and it does not play a part in the overall chemical reaction. The anode of the cell is a gelled mixture of powered zinc combined with the electrolyte (itself a mixture of potassium hydroxide—a strong alkali—and water), and the combination is linked to the negative terminal of the cell by a brass spike running up the middle of the cell. The cathode, a mixture of carbon and manganese dioxide, surrounds the anode and electrolyte, separated by a layer of nonwoven fabric such as polyester.
Depending on the application, alkaline cells can last for four to nine times the life of more traditional carbon-zinc cells. The advantage is greatest under heavy loads that are infrequently used—that is, something that draws heavy current for an hour once a day rather than a few minutes of each hour.
Carbon-zinc cells nominally produce 1.5 volts, but this full voltage is available during the initial discharge of the cell. The voltage of the cell diminishes as the load to the cell increases and as the charge of the cell decreases.
Standard nine-volt batteries also use carbon-zinc chemistry. To produce the higher voltage, six separate carbon-zinc cells are stacked and connected in series inside each battery. Higher-voltage carbon-zinc cells can be made similarly, such as the "B" batteries of the 1950s, which stacked from 45 to 90 volts of cells to power vacuum-tube portable radios.
Ordinarily, alkaline batteries cannot be recharged because the chemical reactions in the cell cannot be readily reversed. If you attempt to recharge an ordinary carbon-zinc cell, it acts more like a resistor than a storage cell, turning the electricity you apply to it into heat. Apply too much power to a cell and it will heat up enough to explode—a good reason never to attempt to recharge carbon-zinc or alkaline batteries.
The exceptions to this rule are the Renewal batteries produced under license by Rayovac Corporation. The Renewal design relies on a two-prong attack on carbon-zinc technology. The Renewal cell is fabricated differently from a standard cell. More importantly, Renewal batteries are part of a system that requires a special battery charger. Instead of applying a nearly constant current to recharge the cells, the Renewal charger adds power in a series of pulses. A microprocessor in the charger monitors how each pulse affects the cell to prevent overheating. Even with the novel charger, however, Renewal cells have a limited life, typically between 25 and 100 charge/discharge cycles. In that Renewal cells cost only about twice as much as standard alkaline cells, they can be very cost-effective in some applications.
The most common storage batteries in the world are the lead-acid batteries used to start automobiles. Gaston Planté developed the first lead-acid cell in France in 1859, and the design remains virtually unchanged today. Lead-acid cells use anodes made from porous lead and cathodes made from lead oxide, both soaked in a sulfuric acid electrolyte.
The lead in the cells makes these batteries inherently heavy. Filled with highly corrosive acid (which is also heavy), they are cumbersome and dangerous. Not only can the acid and its fumes damage nearby objects (particularly metals), but overcharging cells also results in electrolysis of the water component of the internal acid. The hydrogen released by the electrolysis is highly combustible and, mixed with air and a spark, prone to a Hindenburg-like explosion.
The breakdown of the water in the cells also has another effect: It reduces the overall amount of water in the cell. Too little water reduces the reaction area inside the cell, reducing its capacity. It also allows the cells to deteriorate by atmospheric action. The electrodes can flake and possibly short out a cell entirely, reducing its capacity to zero. Early lead-acid cells consequently required regular maintenance to keep the water/acid inside the cell at the proper level. Only the water electrolyzes in the battery, so only it needs to be replaced. To avoid contaminating the battery chemistry, manufacturers recommend you use only distilled water to replenish the battery. Judging the proper amount to add requires only refilling the battery to its normal level. If the battery carries no mark as to the proper level, you should fill it so that the liquid in the battery just covers the electrode plates inside the cell.
In stationary applications, lead-acid batteries were once cased in glass. Not only would it resist the internal acid, but it also allowed maintenance workers to quickly assess the condition of the cells. Automotive applications required a more shatterproof case, for which engineers developed hard rubber or plastic enclosures.
The convenience of using lead-acid batteries is immensely increased by sealing the cells. The result is the so-called maintenance-free battery. Because the vapors within the cell cannot escape, electrolysis losses are minimized. The maintenance-free battery never needs water (or at least it shouldn't).
However, maintenance-free batteries are not entirely trouble free. They still have acid sloshing around inside them, after all, and it can leak out the battery vent, damaging the battery compartment or even the equipment in which the battery is located. Engineers developed two ways of eliminating the slosh. One keeps the liquid acid inside a plastic separator between cell electrodes (typically a micro-porous polyolefin or polyethylene). The other alternative chemically combines the liquid electrolyte with other compounds that turn it into a gel—a colloidal form like gelatin—which is less apt to leak out.
Lead-acid cells have several other drawbacks besides the cantankerous nature of their acidic contents. As noted before, they are heavy. The energy a lead-acid battery stores per pound of battery is lower than just about any technology short of a potato wired with zinc plates. This is the chief frustration of automotive engineers who would like to use low-cost lead-acid batteries to power electric cars. Once a sufficient number of lead-acid batteries to move the car a worthwhile distance get piled into the car, it weighs more like an electric truck.
On the other hand, besides being cheap, lead-acid batteries have over 150 years of technical development behind them. They can be custom-tailored to specific applications, such as those requiring deep discharge cycles (for example, where the batteries are used as the sole power source for electrical equipment) or for battery backup uses, such as in large uninterruptible power supply systems in data centers. Lead-acid cells also have a low internal resistance and therefore can produce enormous currents. They suffer no memory effect as do some more exotic cell designs, such as nickel-cadmium cells. (This effect, discussed in relation to nickel-cadmium batteries, reduces the capacity of a cell if it gets recharged before being fully discharged). The cells also have a moderately long, predictable life. And, of course, they are cheap.
Most uninterruptible power systems rely on gelled lead-acid cells for their power reserves. In this application, they require little maintenance. That is, you don't have to do anything to keep them going. The power system, however, must be tailored to the needs of the cells. Gel cells are degraded by the application of continuous low-current charging after they have been completely charged. (Most lead-acid batteries are kept at full capacity by such "trickle" charging methods.) Consequently, gel cells require special chargers that automatically turn off after the cells have been fully charged. The chargers switch back on when the battery discharges—either under load or by self-discharge—to a predetermined level. Uninterruptible power systems typically check their batteries periodically (usually weekly) to ensure they maintain a full charge.
In consumer electronic equipment, the most popular rechargeable/storage batteries are nickel-cadmium cells, often called ni-cads. These batteries use cathodes made from nickel and anodes from cadmium, as the name implies. Their most endearing characteristic is the capability to withstand a huge number of full charge/discharge cycles, in the range of 500 to 1000, without deteriorating past the point of usefulness. Ni-cads are also relatively lightweight, have a good energy storage density (although about half that of alkaline cells), and tolerate trickle charging (when properly designed). On the downside, cadmium is toxic, thus the warning labels that implore you to be cautious with them and properly dispose of them.
The output voltage of most chemical cells declines as the cell discharges, because the reactions within the cell increase its internal resistance. Ni-cads have a very low internal resistance—meaning they can create high currents—which changes little as the cell discharges. Consequently, the ni-cad cell produces a nearly constant voltage until it becomes almost completely discharged, at which point its output voltage falls precipitously.
This constant voltage is an advantage to the circuit designer because fewer allowances need to be made for voltage variations. However, the constant voltage also makes determining the state of a ni-cad's charge nearly impossible. As a result, most battery-powered computers deduce the battery power they have remaining from the time they have been operating and known battery capacity rather than by actually checking the battery state.
Ni-cads are known for another drawback: memory. When some ni-cads are partly discharged and then later recharged, they may lose capacity. Chemically, recharging ni-cads before they are fully discharged often results in the formation of cadmium crystals on the anodes of the cell. The crystals act like a chemical memory system, marking a second discharge state for the cell. When the cell gets discharged to this secondary discharge state, its output abruptly falls despite further capacity being available within the cell. In subsequent cycles, the cell remembers this second discharge level, which further aggravates the situation by reinforcing the memory of the second discharge state. The full capacity of the cell can only be recovered by nudging the cell past this second discharge state. This will erase the memory and restores full cell capacity.
As a practical matter, the cure for the memory problem is deep discharge—discharging the battery to its minimum working level and then charging the battery again. Deep discharge does not mean totally discharging the battery, however. Draining nearly any storage battery absolutely dry will damage it and shorten its life. If you discharge a ni-cad battery so that it produces less than about one volt (its nominal output is 1.2 volts), it may suffer such damage. Notebook computers are designed to switch off before their batteries are drained too far, and deep discharge utilities do not push any farther, so you need not worry in using them. But don't try to deeply discharge your system's batteries by shorting them out—you risk damaging the battery and even starting a fire.
According to battery-makers, newer ni-cads and nickel-metal hydride cells are free from memory effects, although this has not been proven in practice. Some lithium battery–makers claim that the memory effect results from the use of nickel rather than cadmium (a view not supported by the chemistry), and some users also report contrary experiences with both nickel-based battery types. In any case, to get the longest life from ni-cads, the best strategy is to operate them between extremes—operate the battery through its complete cycle. Charge the battery fully, run it until it is normally discharged, and then fully charge it again.
As with lead-acid batteries, nickel-cadmium cells are also prone to electrolysis breaking down water in the electrolyte into potentially explosive hydrogen and oxygen. Battery-makers take great steps to reduce this effect. Commercially available ni-cads are sealed to prevent leakage. They are also designed so that they produce oxygen before hydrogen, which reacts internally to shut down the electrolysis reaction.
To prevent sealed cells from exploding should gas somehow build up inside them, their designs usually include resealable vents. You risk the chance of explosion if you encase a ni-cad cell in such a way it cannot vent. The vents are tiny and usually go unnoticed. They operate automatically. The warning against blocking the vents applies mostly to equipment-makers. Standard battery holders won't block the vents, but encapsulating the battery epoxy to make a solid power module certainly will.
Chemically, one of the best cathode materials for battery cells would be hydrogen. But hydrogen is problematic as a material for batteries. At normal temperatures and pressures, hydrogen is a lighter-than-air gas, as hard to hold to as grabbing your breath in your hands.
In the late 1960s, however, scientists discovered that some metal alloys have the ability to store atomic hydrogen 1000 times their own volume. These metallic alloys are termed hydrides and typically are based on compounds such as LiNi5 and ZrNi2. In properly designed systems, hydrides can provide a storage sink of hydrogen that can reversibly react in battery cell chemistry.
The most common cells that use hydride cathodes carry over the nickel anodes from ni-cad cell designs. These cells typically have an electrolyte of a dilute solution of potassium hydroxide, which is alkaline in nature.
Substituting hydrides for cadmium in battery cells has several advantages. The most obvious is that such cells eliminate one major toxic material, cadmium. No cadmium also means that the cells should be free from the memory effect that plagues ni-cad cells. In addition, hydrogen is so much better as a cathode material that cells based on nickel and metal hydrides have a storage density about 50 percent higher than nickel-cadmium cells. In practical terms, that means cells of the same size and about the same weight can power a notebook computer for about 50 percent longer.
Cells based on nickel and metal hydrides—often abbreviated as NiMH cells—are not perfect. Their chief drawback is that most such cells have a substantially higher self-discharge rate than do ni-cad cells. Some NiMH cells lose as much as five percent of their capacity per day, although this figure is coming down with more refined cell designs.
As with ni-cads, NiMH cells have a nominal output voltage of 1.2 volts that remains relatively flat throughout the discharge cycle, falling precipitously only at the end of the useful charge of the cell. (Fully charged, a NiMH cell produces about 1.4 volts, but this quickly falls to 1.2 volts, where it remains throughout the majority of the discharge cycle.)
In many ways NiMH cells are interchangeable with ni-cads. They have a similar ability to supply high currents, although not quite as much as ni-cads. NiMH cells also endure many charge/discharge cycles, typically up to 500 full cycles, but they are not a match for ni-cads.
Although the discharge characteristics of NiMH and ni-cads are similar, the two cell types react differently during charging. Specifically, ni-cads are essentially endothermic while being charged, and NiMH cells are exothermic—they produce heat. As the NiMH cell approaches full charge, its temperature can rise dramatically. Consequently, chargers are best designed for one or the other type of cell. NiMH cells work best in chargers designed for them. NiMH cells do, however, readily accept trickle charging (discussed later).
Lithium is the most chemically reactive metal and provides the basis for today's most compact energy storage for notebook computer power systems. Nearly all high-density storage systems use lithium because it has an inherent chemical advantage. Lithium has a specific capacity to store 3860 ampere-hours per kilogram of mass, compared to 820 AH/kg for zinc and 260 AH/kg for lead.
Lithium is also very reactive. Depending on the anode, cells with lithium cathodes can produce anywhere from 1.5 volts to 3.6 volts per cell, higher voltage than any other chemistry.
The problem with lithium is that it is too reactive. It reacts violently with water and can ignite into flame. Batteries based on lithium metal were developed and manufactured in the 1970s, and in the 1980s some companies introduced commercial rechargeable cells based on metallic lithium. Such batteries quickly earned a reputation for doubtful safety.
To prevent problems caused by reactive metallic lithium, battery-makers refined their designs to keep the lithium in its ionic state. In this way, they were able to reap the electrochemical benefits of lithium-based cells without the safety issues associated with the pure metal. In lithium ion cells, the lithium ions are absorbed into the active material of the electrodes rather than being plated out as metal.
The typical lithium ion cells use carbon for its anode and lithium cobalt dioxide as the cathode. The electrolyte is usually based on a lithium salt in solution.
Lithium batteries offer higher storage densities than nickel-metal hydride cells, which equates to using them in notebook computer systems for about fifty percent longer without a recharge. Lithium ion cells also lack the memory effect that plagued early nickel-cad cells.
On the other hand, current lithium cells have a higher internal resistance than nickel-cadmium cells and consequently cannot deliver high currents. A ni-cad could melt a screwdriver, but a lithium cell cannot—that's why lithium cells don't wear the same warnings as ni-cads. The available power is sufficient for a properly designed notebook computer that minimizes surge requirements (meaning that certain devices, such as disk drives, may require a fair surge of power during certain phases of operation, notably spin-up). Moreover, the life of lithium cells is more limited than that of nickel-based designs, although lithium ion cells withstand hundreds of charge/recharge cycles.
Because lithium ion cells use a liquid electrolyte (although one that may be constrained in a fabric separator), cell designs are limited to the familiar cylindrical battery form. Although such designs are no more handicapped than they are with other battery chemistries, the lithium ion chemistry lends itself to other, space-saving designs based on polymerized electrolytes.
A refinement of familiar lithium chemistry, called the lithium solid polymer cell, promises more power for portable applications, but through packaging rather than high-energy density. Whereas conventional lithium ion cells require liquid electrolytes, solid polymer cells integrate the electrolyte into a polymer plastic separator between the anode and cathode of the cell. As an electrolyte, lithium polymer cells use a polymer composite such as polyacrylonitrile containing a lithium salt. Because there's no liquid, the solid polymer cell does not require the chunky cylindrical cases of conventional batteries. Instead, the solid polymer cells can be formed into flat sheets or prismatic (rectangular) packages better able to fit the nooks and crannies of notebook computers.
Although the energy density of solid polymer cells is similar to ordinary lithium ion cells, computer manufacturers can shape them to better fit the space available in a notebook machine, squeezing more capacity into otherwise unused nooks and crannies. For example, simply by filling the empty space that would appear in the corners around a cylindrical cell, a solid polymer battery can fit in about 22 percent more chemistry and energy capacity. In addition, solid polymer batteries are environmentally friendly, lighter because they have no metal shell and safer because they contain no flammable solvent. Most battery-makers and computer-makers are switching to the lithium solid polymer cell design.
Unlike other lithium cells that have chemistries tuned to obtaining the greatest capacity in a given package, lithium-iron disulfide cells are a compromise. To match to existing equipment and circuits, their chemistry has been tailored to the standard nominal 1.5-volt output (whereas other lithium technologies produce double that). These cells are consequently sometimes termed voltage-compatible lithium batteries. Unlike other lithium technologies, lithium-iron disulfide cells are not rechargeable.
Internally, the lithium-iron disulfide cell is a sandwich of a lithium anode, a separator, and iron disulfide cathode with an aluminum cathode collector. The cells are sealed but vented.
Compared to the alkaline cells with which they are meant to compete, lithium-iron disulfide cells are lighter (weighing about 66 percent of same-size alkaline cells), higher in capacity, and longer in life. Even after ten years of shelf storage, lithium-iron disulfide cells still retain most of their capacity.
Lithium-iron disulfide cells operate best under heavier loads. In high-current applications, they can supply power for about 260 percent the time of a same-size alkaline cell. This advantage is less at lower loads, however, and at very light loads may disappear entirely. For example, under a 20 mA load, one manufacturer rates its AA-size lithium-iron disulfide cells as providing power for about 122 hours, whereas its alkaline cells will last for 135 hours. With a one-ampere load, however, the lithium-iron disulfide cells last for 2.1 hours versus only 0.8 for alkaline.
You can use lithium-iron disulfide cells wherever you might use zinc-carbon batteries, although they are cost-effective only under high-current loads—flashlights, motor-driven devices, and powerful electronics. They are not a wise choice for clocks and portable radios.
Of the current battery technologies, the one offering the densest storage is zinc-air. One reason is that one of the components of its chemical reaction is external to the battery. Zinc-air batteries use atmospheric oxygen as their cathode reactant, hence the "air" in the name. Small holes in the battery casing allow air in to react with a powered zinc anode through a highly conductive potassium hydroxide electrolyte.
Originally created for use in primary batteries, zinc-air batteries were characterized by their long stable storage life, at least when kept sealed from the air and thus inactive. A sealed zinc-air cell loses only about 2 percent of its capacity after a year of storage. Once air infiltrates the cell, zinc-air primary cells last only for months, whether under discharge or not.
Some battery-makers have adapted zinc-air technology for secondary storage. Zinc-air cells work best when frequently or continuously used in low-drain situations. The chief drawback of zinc-air batteries is, however, a high internal resistance, which means zinc-air batteries must be huge to satisfy high-current needs—for notebook computers that means an auxiliary battery pack about the size of the computer itself.
Contrary to appearances, most rechargeable batteries used by notebook computers are standard sizes. Computer manufacturers, however, package these standard-size batteries in custom battery packs that may fit only one model of computer. Usually you cannot change the batteries in the pack but must buy a new replacement if something goes awry.
Attempts at standardizing batteries for notebook computers have fallen flat, as witnessed by the near extinction of Duracell's "standard" designs. Computer-makers find proprietary cells more profitable. They can put a few dollars of individual cells in a cheap but odd plastic package and sell the assembly for a hundred dollars or more.
Anyone who has ever received a holiday gift and discovered the batteries weren't included knows the basic rule about the different standard sizes for batteries: The one you need is the one you don't have or cannot find. Frustrating as the multiplicity in standard sizes may be, the situation is better than having no existing standards. For example, nearly all battery packs for notebook computers are nonstandard and consequently very costly. You're captive to the manufacturer's pricing policy in many cases, although there are lower-cost second sources for batteries for various notebook computers, cellular phones, and other common portable electronic devices that use rechargeable batteries. Many people remain suspicious of the build quality, longevity, and safety of some of these second-source batteries, although most are likely fine for the majority of targeted applications. Standard batteries, on the other hand, are readily available from multiple sources and far less expensive than rechargeables.
The chief battery standards now in use originally applied to carbon-zinc cells, but other technologies (some varieties of lithium, nickel-cadmium, and nickel-metal hydride cells) now follow the same size standards. These standards specify the basic dimensions of the batteries, allowing many manufacturers to produce interchangeable cells. Table 31.3 lists the dimensions of many standard battery types.
Note that these sizes are a physical characteristic only. You might find any battery chemistry in any cell size, some of which might not be suited to a give application. In other words, just because a battery fits into a holder is no indication that it will work properly there.
Just as your electronic gear is sensitive to the kind of electricity you supply it, battery chemistry is extremely sensitive to the electricity used for charging cells. If the voltage applied is too low, the cell will output current instead of accepting it—that is, discharging rather than charging. If the voltage is too high, undesirable reactions can take place that can destroy the cell. For example, raising the voltage inevitably raises the current, and too much current can cause the cell to overheat. In addition, trying to charge a cell beyond its capacity can result in the production of explosive gases—and an explosion itself.
Modern battery chargers are consequently sophisticated electronic devices with many different types of safety circuitry to protect both you and the batteries you want to charge. Most batteries require a specific charger tailored to their needs. Plug the wrong batteries into the wrong charger (or the wrong charger into your battery pack), and you may destroy the batteries, the charger, and even the equipment connected to the batteries.
The chief difference between battery chargers is the mode in which they operate. The choice is between constant voltage and constant current.
Constant voltage chargers are the simplest. They always produce a specific voltage level but deliver a current that depends on the charge level of the battery (and environmental factors). As the battery becomes charged, its voltage increases while that of the charger remains the same, so there is less difference in potential between the charger and battery. As a result, less current can flow through the system. When the two are equal, no current flows.
A constant-voltage charger requires little more than a transformer (to reduce line voltage to the level required for battery charging) and a rectifier (to change the alternating current of utility power into the direct current used to charge batteries). Such simple designs are often found in the battery chargers used for charging car and boat batteries.
The lead-acid cells used for cars and backup power systems typically use constant-voltage chargers. In addition, lithium ion cells often use constant-voltage systems, although these usually are more complex with added circuitry to protect both the batteries and your safety.
The alternate design for battery chargers maintains a constant current and alters the voltage applied to the batteries to maintain this current. These constant-current chargers vary the voltage they apply to the batteries to maintain a constant current flow, switching off when the voltage reaches the level of a full charge. (Remember, the voltage produced by any cell falls as it becomes discharged.) This design is usually used for nickel-cadmium and nickel-metal hydride cells or batteries.
In addition to charging cells at the proper rate, battery chargers also face another vital issue: the proper time to switch off. A charger can destroy a battery by overcharging it. Depending on the requirements of the battery to be charged and the sophistication of the charger, the charger may use any of several technologies to determine the proper time to turn off.
The most straightforward way of determining charge is by the voltage produced by the battery. The charger monitors the battery's voltage and switches off when it reaches the cutoff voltage. The voltage-sensing technique is not adequate for many kinds of batteries—in particular, ni-cads have a very linear discharge curve that makes the cutoff voltage difficult to determine.
More advanced charging systems use temperature cutoff. That is, the charger monitors the temperature of the battery cells and switches off or reduces the charging rate when the battery begins to heat up (which indicates an overcharge condition). Typically battery packs using temperature cutoff have built-in thermometers that relay a control signal to the charger circuitry.
More sophisticated chargers combine the voltage and temperature cutoff (VTCO). Chargers using this technology may switch from high-current charging to a lower or maintenance charge rate using circuitry that senses both temperature and voltage.
Standard battery chargers supply less current than the battery's discharge rate. High-current chargers supply a current greater than the battery's nominal discharge rate. Trickle chargers supply a charging current at a rate so low it only compensates for the self-discharge of the battery (by definition, a trickle charger is one that compensates for self-discharge). The trickle charging rate typically is about one-twentieth to one-thirtieth the battery's nominal discharge rate. Modern battery chargers often operate at several charging rates, starting at high current and switching to low current as the battery nears full charge. If the battery is of a type that tolerates trickle charging (ni-cads, for example, do not), the charge will switch to a trickle rate at the end of the charging cycle.
Smart Battery Specifications
Battery charging is actually an interaction between the battery and the charger. The charger must attend to the state of the battery if it is to avoid damaging the battery by overcharging the battery and damaging your state of mind by undercharging the battery so that you run out of power long before you run out of the need for your computer.
Charging and monitoring the charge of batteries has always been problematic. Both capacity and charge characteristics vary with the battery type and over the life of a given battery. The smartest conventional battery chargers monitor not the voltage but the temperature of their subjects, because a sharp rise in temperature is the best indication available to the charger of the completion of its work. Even this rise varies with battery chemistry, so ni-cad and NiMH batteries present different—and confusing—temperature characteristics that would lead to a charger mistaking one for the other, possibly damaging the battery.
The Smart Battery system, developed jointly by battery-maker Duracell and Intel and first published as the Smart Battery Data Specification, Version 1.0, on February 15, 1995, eliminates these problems by endowing batteries with enough brains to tell of their condition. When matched to a charger with an equivalent I.Q. that follows the Smart Charger specification, the Smart Battery gets charged perfectly every time with never a worry about overcharging.
The Smart Battery system defines a standard with several layers that distribute intelligence between battery, charger, and your computer. It provides for an inexpensive communication link between them and outlines the information that a battery can convey to its charger and the message format for doing so.
Among other data that the battery can relay are its chemistry, its capacity, its voltage, and even its physical packaging. Messages warn not only about the current status of the battery's charge but even how many charge/recharge cycles the battery has endured so that the charger can monitor its long-term prognosis. The specification is independent of the chemistry used by the battery and even the circuitry used to implement its functions.
The battery packs of nearly all new notebook computers follow the Smart Battery specifications or a similar replacement standard called the Control Method Battery Interface, because battery management is required for compatibility with the latest versions of Windows. Use of either of the two battery-management standards is required by the Advanced Configuration and Power Interface (ACPI). The Control Method Battery Interface is part of the ACPI standard and effectively supercedes Smart Battery.
The maximum current any battery can produce is limited by its internal resistance. Zinc-carbon batteries have a relatively high resistance and produce small currents, on the order of a few hundred milliamperes. Lead-acid, nickel-cadmium, and nickel-hydride batteries have very low internal resistances and can produce prodigious currents. If you short the terminals of one of these batteries, whatever produces the short circuit—wires, a strip of metal, a coin in your pocket—becomes hot because of resistive heating. For example, you can melt a wrench by placing it across the terminals of a fully charged automotive battery. You can also start a fire with something inadvertently shorting the terminals of the spare nickel-cadmium battery for your notebook or subnotebook computer. Be careful and never allow anything to touch these battery terminals except the contacts of your notebook computer.
When a battery is charged, a process called electrolysis takes place inside. If you remember your high-school science experiments, electrolysis is what you did to break ordinary water into hydrogen and oxygen using electricity. Hydrogen is an explosive gas; oxygen is an oxidizer. Both are produced when charging batteries. Normally these gases are absorbed by the battery before they can do anything (such as explode), but too great a charging current (as results from applying too high a voltage) can cause them to build up. Trying to charge a primary battery produces the same gas build-up. As a result, the battery can explode from too great an internal pressure or from combustion of the gases. Even if the battery does not catastrophically fail, its life will be greatly reduced. In other words, use only the charger provided with a portable computer battery and never try to hurry things along.
Nearly all batteries contain harmful chemicals of some kind. Even zinc-carbon batteries contain manganese, which is regarded as hazardous. All batteries present some kind of environmental hazard, so be sure to dispose of them properly. Some manufacturers are beginning to provide a means of recycling batteries. Encourage them by taking advantage of their offers.
|[ Team LiB ]|