Acid vs. Alkaline
Batteries are often classified by the type of electrolyte used in their construction. There are three common classifications; acid, mildly acid, and alkaline.
Acid-based batteries often use sulphuric acid as the major component of the electrolyte. Automobile batteries are acid-based. The electrolyte used in mildly acidic batteries is far less corrosive than typical acid-based batteries and usually includes a variety of salts that produce the desired acidity level. Inexpensive household batteries are mildly acidic batteries.
Alkaline batteries typically use sodium hydroxide or potassium hydroxide as the main component of the electrolyte. Alkaline batteries are often used in applications where long-lasting, high-energy output is needed, such as cellular phones, portable CD players and radios, pagers, and flash cameras.
Wet vs. Dry
"Wet" cells refer to galvanic cells where the electrolyte is liquid in form and is allowed to flow freely within the cell casing. Wet batteries are often sensitive to the orientation of the battery. For example, if a wet cell is oriented such that a gas pocket accumulates around one of the electrodes, the cell will not produce current. Most automobile batteries are wet cells.
"Dry" cells are cells that use a solid or powdery electrolyte. These kind of electrolytes use the ambient moisture in the air to complete the chemical process. Cells with liquid electrolyte can be classified as "dry" if the electrolyte is immobilized by some mechanism, such as by gelling it or by holding it in place with an absorbent substance such as paper.
In common usage, "dry cell" batteries will usually refer to zinc-carbon cells (Sec. 2.3.1) or zinc-alkaline-manganese-dioxide cells (Sec. 2.3.2), where the electrolyte is often gelled or held in placed by absorbent paper.
Some cells are difficult to categorize. For example, one type of cell is designed to be stored for long periods without its electrolyte present. Just before power is needed from the cell, liquid electrolyte is added.
Batteries can further be classified by their intended use. The following sections discuss four generic categories of batteries; "vehicular" batteries, "household" batteries, "specialty" batteries, and "other" batteries. Each section will focus on the general properties of that category of battery. Note that some battery types (acidic or alkaline, wet or dry) can fall into several different categories. For this guideline, battery types are placed into the category in
which they are most likely to be found in commercial usage.
This section discusses battery types and configurations that are typically used in motor vehicles. This category can include batteries that drive electric motors directly or those that provide starting energy for combustion engines. This category will also include large, stationary batteries used as power sources for emergency building lighting, remote-site power, and computer back up. Vehicular batteries are usually available off-the-shelf in standard designs or can be
custom built for specific applications.
Lead-acid batteries, developed in the late 1800s, were the first commercially practical batteries. Batteries of this type remain popular because they are relatively inexpensive to manufacture. The most widely known uses of lead-acid batteries are as automobile batteries. Rechargeable lead-acid batteries have been available since the 1950s and have become the most widely used type of battery in the world—more than 20 times the use rate of its nearest rivals. In fact, battery
manufacturing is the single largest use for lead in the world1. Equation 1 shows the chemical reaction in a lead-acid cell.
PbO2 + Pb + 2H2SO4 —> 2PbSO4 + 2H2O
Lead-acid batteries remain popular because they can produce high or low currents over a wide range of temperatures, they have good shelf life and life cycles, and they are relatively inexpensive to manufacture and purchase. Lead-acid batteries are usually rechargeable.
Battery manufacturing is the single largest use for lead in the world. Lead-acid batteries come in all manner of shapes and sizes, from household batteries to large batteries for use in submarines. The most noticeable shortcomings of lead-acid batteries are their relatively heavy weight and their falling voltage during discharge (Sec. 3.5).
Sealed vs. Flooded
In "flooded" batteries, the oxygen created at the positive electrode is released from the cell and vented into the atmosphere. Similarly, the hydrogen created at the negative electrode is also vented into the atmosphere. The overall result is a net loss of water (H2O) from the cell. This lost water needs to be periodically replaced. Flooded batteries must be vented to prevent excess pressure from the build up of these gasses. Also, the room or enclosure housing the battery
must be vented, since a concentrated hydrogen and oxygen atmosphere is explosive.
In sealed batteries, however, the generated oxygen combines chemically with the lead and then the hydrogen at the negative electrode, and then again with reactive agents in the electrolyte, to recreate water. The net result is no significant loss of water from the cell.
Deep-cycle batteries are built in configurations similar to those of regular batteries, except that they are specifically designed for prolonged use rather than for short bursts of use followed by a short recycling period. The term "deep-cycle" is most often applied to lead-acid batteries. Deep-cycle batteries require longer charging times, with lower current levels, than is appropriate for regular batteries.
As an example, a typical automobile battery is usually used to provide a short, intense burst of electricity to the automobile's starter. The battery is then quickly recharged by the automobile's electrical system as the engine runs. The typical automobile battery is not a deep-cycle battery. A battery that provides power to a recreational vehicle (RV), on the other hand, would be expected to power lights, small appliances, and other electronics over an extended period of
time, even while the RV's engine is not running. Deep-cycle batteries are more appropriate for this type of continual usage.
Battery Categories for Vehicular Batteries
Vehicular, lead-acid batteries are further grouped (by typical usage) into three different categories;
Starting-Lighting-Ignition (SLI) -- Typically, these batteries are used for short, quick-burst, high-current applications. An example is an automotive battery, which is expected to provide high current, occasionally, to the engine's starter.
Traction -- Traction batteries must provide moderate power through many deep discharge cycles. One typical use of traction batteries is to provide power for small electric vehicles, such as golf carts. This type of battery use is also called Cycle Service.
Stationary -- Stationary batteries must have a long shelf life and deliver moderate to high currents when called upon. These batteries are most often used for emergencies. Typical uses for stationary batteries are in uninterruptible power supplies (UPS) and for emergency lighting in stairwells and hallways. This type of battery use is also called Standby or Float.
"Household" batteries are those batteries that are primarily used to power small, portable devices such as flashlights, radios, laptop computers, toys, and cellular phones. The following subsections describe the technologies for many of the formerly used and presently used types of household batteries. Typically, household batteries are small, 1.5 V cells that can be readily purchased off the shelf. These batteries come in standard shapes and sizes as shown in Table 2.
They can also be custom designed and moulded to fit any size battery compartment (e.g., to fit inside a cellular phone, camcorder, or laptop computer). Most of the rest of this guideline will focus on designs, features, and uses of household batteries.
Various Popular Household-Battery Sizes
Size Shape and Dimensions Voltage
D Cylindrical, 61.5 mm tall, 34.2 mm diameter. 1.5 V
C Cylindrical, 50.0 mm tall, 26.2 mm diameter. 1.5 V
AA Cylindrical, 50.5 mm tall, 14.5 mm diameter. 1.5 V
AAA Cylindrical, 44.5 mm tall, 10.5 mm diameter. 1.5 V
9-Volt Rectangular, 48.5 mm tall, 26.5 mm wide, 17.5 mm deep. 9 V
Note: Three other standard sizes of household batteries are available, AAAA, N, and 6-Volt (lantern) batteries. It is estimated that 90% of portable, battery-operated devices require AA, C, or D battery sizes.
Zinc-carbon cells, also known as "Leclanché cells" are widely used because of their relatively low cost. The equation below shows the chemical reaction in a Leclanché cell.
Zn + 2MnO2 + 2NH4Cl —> Zn(NH3)2Cl2 + 2MnOOH
They were the first widely available household batteries. Zinc-carbon cells are composed of a manganese-dioxide-and-carbon cathode, a zinc anode, and zinc chloride (or ammonium chloride) as the electrolyte. Generally, zinc-carbon cells are not rechargeable and they have a sloping discharge curve (i.e., the voltage level decreases relative to the amount of discharge). Zinc-carbon cells will produce 1.5 V, and they are mostly used for non-critical uses such as small household
devices like flashlights and portable personal radios. One notable drawback to these kind of batteries is that the outer, protective casing of the battery is made of zinc. The casing serves as the anode for the cell and, in some cases, if the anode does not oxidize evenly, the casing can develop holes that allow leakage of the mildly acidic electrolyte, which can damage the device being powered.
Zinc-Manganese-Dioxide Alkaline Cells ("Alkaline Batteries")
When an alkaline electrolyte—instead of the mildly acidic electrolyte—is used in a regular zinc-carbon battery, it is called an "alkaline" battery. An alkaline battery can have a useful life of 5 to 6 times that of a zinc-carbon battery. One manufacturer estimates that 30% of the household batteries sold in the world today are zinc-manganese dioxide (i.e., alkaline) batteries. Like zinc-carbon batteries, alkaline batteries are not generally rechargeable.
Nickel-cadmium cells are the most commonly used rechargeable household batteries. They are useful for powering small appliances, such as garden tools and cellular phones. The basic galvanic cell in a Ni-Cd battery contains a cadmium anode, a nickel-hydroxide cathode, and an alkaline electrolyte. The equation below shows the chemical reaction in a Ni-Cd cell.
Cd + 2H2O + 2NiOOH —> 2Ni(OH)2 + Cd(OH)2
Batteries made from Ni-Cd cells offer high currents at relatively constant voltage and they are tolerant of physical abuse. Nickel-cadmium batteries are also tolerant of inefficient usage cycling. If a Ni-Cd battery has incurred memory loss (Sec. 3.4), a few cycles of discharge and recharge can often restore the battery to nearly "full" memory.
Unfortunately, nickel-cadmium technology is relatively expensive. Cadmium is an expensive metal and is toxic. Recent regulations limiting the disposal of waste cadmium (from cell manufacturing or from disposal of used batteries) has contributed to the higher costs of making and using these batteries. These increased costs do have one unexpected advantage, however: it is more cost effective to recycle and reuse many of the components of a Ni-Cd battery than it is to recycle
components of other types of batteries. Several of the major battery manufacturers are leaders in such recycling efforts.
Nickel-Metal Hydride (Ni-MH)
Battery designers have investigated several other types of metals that could be used instead of cadmium to create high-energy secondary batteries that are compact and inexpensive. The nickel-metal-hydride cell is a widely used alternative. The anode of a Ni-MH cell is made of a hydrogen storage metal alloy, the cathode is made of nickel oxide, and the electrolyte is a potassium hydroxide solution. According to one manufacturer, Ni-MH cells can last 40% longer than the same
size Ni-Cd cells and will have a life-span of up to 600 cycles.5 This makes them useful for high-energy devices such as laptop computers, cellular phones, and camcorders. Ni-MH batteries have a high self-discharge rate and are relatively expensive to purchase.
Nickel-iron cells, also known as the Edison battery, are much less expensive to build and to dispose of than nickel-cadmium cells. Nickel-iron cells were developed even before the nickel-cadmium cells. The cells are rugged and reliable, but do not recharge very efficiently. They are widely used in industrial settings and in eastern Europe, where iron and nickel are readily available and inexpensive.
Another alternative to using cadmium electrodes is using zinc electrodes. Although the nickel-zinc cell yields promising energy output, the cell has some unfortunate performance limitations that prevent the cell from having a useful lifetime of more than 200 or so charging cycles. When nickel-zinc cells are recharged, the zinc does not redeposit in the same "holes" on the anode that were created during discharge. Instead, the zinc redeposits in a somewhat random fashion,
causing the electrode to become misshapen. Over time, this leads to the physical weakening and eventual failure of the electrode.
Lithium and Lithium Ion
Lithium is a promising reactant in battery technology, due to its high electropositivity. The specific energy of some lithium-based cells can be five times greater than an equivalent-sized lead-acid cell and three times greater than alkaline batteries.6 Lithium cells will often have a starting voltage of 3.0 V. These characteristics translate into batteries that are lighter in weight, have lower per-use costs, and have higher and more stable voltage profiles. The equation
below shows the chemical reaction in one kind of lithium cell.
Li + MnO2 —> LiMnO2
Lithium will ignite or explode on contact with water. Unfortunately, the same feature that makes lithium attractive for use in batteries—its high electrochemical potential—also can cause serious difficulties in the manufacture and use of such batteries. Many of the inorganic components of the battery and its casing are destroyed by the lithium ions and, on contact with water, lithium will react to create huge volumes of hydrogen which can ignite or can create excess pressure
in the cell. Many fire extinguishers are water based and will cause disastrous results if used on lithium products. Special D-class fire extinguishers must be used when lithium is known to be within the boundaries of a fire. Lithium also has a relatively low melting temperature for a metal 180°C (356°F). If the lithium melts, it may come into direct contact with the cathode, causing violent chemical reactions.
Some manufacturers are having success with lithium-iron sulfide, lithium-manganese dioxide, lithium-carbon monoflouride, lithium-cobalt oxide, and lithium-thionyl cells. In recognition of the potential hazards of lithium components, manufacturers of lithium-based batteries have taken significant steps to add safety features to the batteries to ensure their safe use. Lithium primary batteries (in small sizes, for safety reasons) are currently being marketed for use in flash
cameras and computer memory. Lithium batteries can last three times longer than alkaline batteries of the same size. But, since the cost of lithium batteries can be three times that of alkaline batteries, the cost benefits of using lithium batteries are marginal.
Button-size lithium batteries are becoming popular for use in computer memory back-up, in calculators, and in watches. In applications such as these, where changing the battery is difficult, the longer lifetime of the lithium battery makes it a desirable choice. One company now produces secondary lithium-ion batteries with a voltage of 3.7 V, "four times the energy density of Ni-Cd batteries," "one-fifth the weight of Ni-Cd batteries," and can be recharged 500 times. In
general, secondary (rechargeable) lithium-ion batteries have a good high-power performance, an excellent shelf life, and a better life span than Ni-Cd batteries. Unfortunately, they have a very high initial cost and the total energy available per usage cycle is somewhat less than Ni-Cd batteries.
Specialty Batteries ("Button" and Miniature Batteries)
"Button" batteries are the nickname given to the category of batteries that are small and shaped like a coin or a button. They are typically used for small devices such as cameras, calculators, and electronic watches.
Miniature batteries are very small batteries that are can be custom built for devices, such as hearing aids and electronic "bugs," where even button batteries can be too large. Industry standardization has resulted in 5 to 10 standard types of miniature batteries that are used throughout the hearing-aid industry. Together, button batteries and miniature batteries are referred to as specialty batteries.
Most button and miniature batteries need a very high energy density to compensate for their small size. The high energy density is achieved by the use of highly electropositive—and expensive—metals such as silver or mercury. These metals aren't cost effective enough to be used in larger batteries.
Several compositions of specialty batteries are described in the following sections.
A very practical way to obtain high energy density in a galvanic cell is to utilize the oxygen in air as a "liquid" cathode. A metal, such as zinc or aluminum, is used as the anode. The oxygen cathode is reduced in a portion of the cell that is physically isolated from the anode. By using a gaseous cathode, more room is available for the anode and electrolyte, so the cell size can be very small while providing good energy output. Small metal-air cells are available for
applications such as hearing aids, watches, and clandestine listening devices.
Metal-air cells have some technical drawbacks, however. It is difficult to build and maintain a cell where the oxygen acting as the cathode is completely isolated from the anode. Also, since the electrolyte is in direct contact with air, approximately 1 to 3 months after it is activated, the electrolyte will become too dry to allow the chemical reaction to continue. To prevent premature drying of the cells, a seal is installed on each cell at the time of manufacture. This seal
must be removed by the customer prior to first use of the cell. Alternately, the manufacturer can provide the battery in an air-tight package.
Silver-oxide cells use silver oxide as the cathode, zinc as the anode, and potassium hydroxide as the electrolyte. Silver-oxide cells have a moderately high energy density and a relatively flat voltage profile. As a result, they can be readily used to create specialty batteries. Silver-oxide cells can provide higher currents for longer periods than most other specialty batteries, such as those designed from metal-air technology. Due to the high cost of silver, silver-oxide
technology is currently limited to use in specialty batteries.
Mercury-oxide cells are constructed with a zinc anode, mercury-oxide cathode, and potassium hydroxide or sodium hydroxide as the electrolyte. Mercury-oxide cells have a high energy density and flat voltage profile resembling the energy density and voltage profile of silver-oxide cells. These mercury-oxide cells are also ideal for producing specialty batteries. The component, mercury, unfortunately, is relatively expensive and its disposal creates environmental problems.
This section describes battery technology that is not mature enough to be available off-the-shelf, has special usage limitations, or is otherwise impractical for general use.
Nickel-hydrogen cells were developed for the U.S. space program. Under certain pressures and temperatures, hydrogen (which is, surprisingly, classified as an alkali metal) can be used as an active electrode opposite nickel. Although these cells use an environmentally attractive technology, the relatively narrow range of conditions under which they can be used, combined with the unfortunate volatility of hydrogen, limits the long-range prospects of these cells for terrestrial
A thermal battery is a high-temperature, molten-salt primary battery. At ambient temperatures, the electrolyte is a solid, non-conducting inorganic salt. When power is required from the battery, an internal pyrotechnic heat source is ignited to melt the solid electrolyte, thus allowing electricity to be generated electrochemically for periods from a few seconds to an hour. Thermal batteries are completely inert until the electrolyte is melted and, therefore, have an excellent
shelf life, require no maintenance, and can tolerate physical abuse (such as vibrations or shocks) between uses.
Thermal batteries can generate voltages of 1.5 to 3.3 V, depending on the battery's composition. Due to their rugged construction and absence of maintenance requirements, they are most often used for military applications such as missiles, torpedoes, and space missions and for emergency-power situations such as those in aircraft or submarines.
The high operating temperatures and short active lives of thermal batteries limit their use to military and other large-institution applications.