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.

Electrical Characteristics

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.

Cell Types

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.)

Voltage

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

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).

Storage Density

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).

The storage density of each chemistry falls into a range of values because the actual construction of a battery cell—the materials used and the layout—also affects its storage density.

Shelf Life

No battery stores energy forever. The chemicals inside the cells inevitably react and slowly degrade. As a result, the charge stored by the battery degrades as well. The degradation takes two forms.

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.

Chemical Systems

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.

Carbon-Zinc

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 incr