Acid vs. Alkaline
Wet vs. Dry
Vehicular Batteries
Lead Acid Batteries
Sealed vs. Flooded
Deep-Cycle Batteries
Vehicle Battery Categories
Household Batteries
Zinc Carbon Batteries
Alkaline Batteries
Rechargeable Alkaline Batteries
Nickel Cadmium Batteries
Nickel Metal Hydride Batteries
Nickel Iron Batteries
Nickel Zinc Batteries
Lithium & Lithium Ion Batteries
Specialty Batteries
Metal-Air Cells
Silver Oxide Cells
Mercury Oxide Cells
Other Battery Types
Nickel Hydrogen Cells
Thermal Batteries
Super Capacitor
The Potato Battery
The Sea Battery

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 kinds 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 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 or zinc-alkaline-manganese-dioxide cells, where the electrolyte is often gelled or held in place 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.

Categories
Batteries can be further classified by their intended use. The following sections discuss four generic categories of batteries: "vehicular" batteries, "household" batteries, "specialty" batteries and "other" batteries.

Vehicular Batteries
Vehicular batteries are battery types and configurations typically used in motor vehicles. This category can include batteries that drive electric motors directly or those that provide starting energy for combustion engines. It also includes large, stationary batteries used as power sources for emergency building lighting, remote-site power or computer backup.

Lead-Acid
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 battries 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 world. The following equation 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, have good shelf life and life cycles, and are relatively inexpensive to manufacture and purchase. Lead-acid batteries are usually rechargeable.

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.

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 replenished. Flooded batteries must be vented to prevent excess pressure from the build up of these gases. Also, the room or enclosure housing the battery must be vented, as 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
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.

For example, a typical automobile battery is usually used to provide a short, intense burst of electricity to the automobile's starter, after which the battery is 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) would be expected to power lights, small appliances and other electronics over an extended period of time, even when the RV's engine is not running. Deep-cycle batteries are more appropriate for this type of continual usage.

Categories for Vehicular Batteries
Vehicular lead-acid batteries can be further grouped (by typical usage) into three categories:

• Starting-Lighting-Ignition (SLI): used for frequent, short, quick-burst, high-current applications. Most automobile batteries fall into this category.
• Traction: used to 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 in emergency power situations such as Uninterruptible Power Supplies (UPS) and for emergency lighting in stairwells and hallways. This type of battery is also called "Standby" or "Float".

Household Batteries
Household batteries are those batteries that are primarily used to power small, portable devices such as flashlights, radios, toys, laptop computers and cellular phones. 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 1. The can also be custom designed and molded to fit any size battery compartment (eg. to fit inside a cellular phone, camcorder or laptop computer).

Zinc-Carbon (Z-C)
Zinc-carbon cells, also known as "Leclanch cells", are widely used because of their relatively low cost. The following equation shows the chemical reaction in a Leclance 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.

One notable drawback to these 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 or "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 (Duracell International Inc.) estimates that 30% of the household batteries sold in the world today are alkaline batteries.

Rechargeable Alkaline Batteries
Like zinc-carbon batteries, alkaline batteries are not generally rechargeable. One major battery manufacturer (Rayovac Corporation), however, has designed a "reusable alkaline" battery that they market as being rechargeable 25 times or more. This manufacturer states that its batteries do not suffer from memory effects as the Ni-Cd batteries do, and that their batteries have a shelf life that is much longer than Ni-Cd batteries and almost as long as the shelf life of primary alkaline batteries. Also, the manufacturer states that their rechargeable alkaline batteries contain no toxic metals such as mercury or cadmium that contribute to the poisoning of the environment.

Rechargeable alkaline batteries are most appropriate for low and moderate power portable equipment such as hand-held toys and radio receivers.

Nickel-Cadmium (Ni-Cd)
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 following equation 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. However, Ni-Cd batteries suffer from problem known as the memory effect. If a Ni-Cd battery is only partially discharged before recharging, and this happens several times in a row, the amount of energy available for the next cycle will only be slightly greater than the amount of energy discharged in the cell's most recent cycle. This characteristic makes it appear as if the battery is "remembering" how much energy is needed for a repeated application.

The physical process that causes the memory effect is the formation of potassium hydroxide crystals inside the cells which interferes with the chemical process of generating electrons during the next battery-use cycle. The build up of potassium hydroxide crystals can be reduced by periodically reconditioning the battery. Reconditioning of a Ni-Cd battery is accomplished by controlled power cycling - deeply discharging and then recharging the battery several times - a process that causes most of the potassium hydroxide crystals to redissolve back into the electrolyte.

Unfortunately, Ni-Cd technology is relatively expensive. Cadmium is an expensive metal and is toxic. Recent regulations limiting the disposal of waste cadmium has contributed to the higher costs of making and using these batteries. These increased costs do have one unexpected advantage, however. It is now 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.

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 potassium hydroxide solution. According to one manufacturer (Duracell), 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. 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 (Ni-I)
Nickel iron cells, also known as the Edison battery, are much less expensive to build and to dispose of then Ni-Cd cells. Nickel iron cells were developed even before the Ni-Cd 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.

Nickel Zinc (Ni-Z)
Another alternative to using nickel cadmium electrodes is using zinc electrodes. Although the nickel zinc cell yields promising energy output, the cell has some unfortunate performance limitations that prevent it from having a useful lifetime of more than 200 or so charging cycles. When Ni-Z 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 physical weakening and eventual failure of the electrode.

Lithium and Lithium Ion
Lithium is a promising reactant in battery technology, due to its high electroporosity. 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. 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 following equation shows the chemical reaction in one kind of lithium cell:

Li + MnO₂ - LiMnO₂

Unfortunately, the same feature that makes lithium attractive for use in batteries - its high electrochemical potential - 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. 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^oC). If the lithium melts, it may come into direct contact with the cathode, causing violent chemeical reactions. 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 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 backup, calculators and watches. In applications such as these, where changing the battery is difficult, the longer lifetime of the lithium battery makes it a desirable choice.

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" batteries are small coin-shaped batteries typically used for cameras, calculators and watches. "Miniature" batteries are very small batteries for devices such as hearing aids and electronic "bugs", and are even smaller than button batteries. There are now about 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 are not cost effective enough to be used in larger batteries.

Several compositions are used for specialty batteries, including:

Metal-Air Cells
High energy density can be achieved by using 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 the 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. It is difficult to build and maintain a cell where the oxygen acting as a 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 dry out. To prevent premature drying of the cells, a seal is installed on each cell at the time of manufacture, which must be removed by the customer prior to first use.

Silver Oxide
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 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
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 similar to silver oxide cells, and are suited for producing specialty batteries. Mercury, however, is relatively expensive and its disposal creates environmental problems.

Other Batteries
Some other battery technologies are available which may have special usage applications, but are generally not mature enough to be available off-the-shelf for consumer use.

Nickel Hydrogen (Ni-H)
Nickel hydrogen cells were developed for the U.S. space program. Under certain pressures and temperatures, hydrogen 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 terretrial uses.

Thermal Batteries
A thermal battery is a high-temperature, molten salt primary battery. At ambient temperatures, the electrolyte is a solid, non-conducting inorganic salt. Whe 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 and shocks) between uses.

Thermal batteries are most often used for military applications such as missiles, torpedoes, space missions and for emergency power situations such as those in aircraft or submarines, where their rugged construction, extended shelf life and absence of maintenance are important assets.

Super Capacitor
This kind of battery uses no chemical reactions at all. Instead, a special kind of carbon (carbon aerogel) with a large molecular surface area, is used to create a capacitor that can hold a large amount of electrostatic energy. This energy can be released very quickly, providing a specific energy of up to 4000 watt-hours per kilogram (Wh/kg), or it can be regulated to provide smaller currents typical of many commercial devices such as flashlights, radios and toys. Because there are no chemical reactions, the battery can be recharged an unlimited number of times without degradation. Other potential advantages of this kind of cell are its low cost and wide temperature range. One disadvantage, however, is its high self-discharge rate.

The Potato Battery
One interesting science experiment involves sticking finger-length pieces of copper and zinc wire, one at at time, into a raw potato to create a battery. The wires will carry a very weak current which can be sued to power a small electrical device such as a digital clock. One vendor sells a novelty digital watch that is powered by a potato battery. The wearer must put a fresh slice of potato in the watch every few days.

The Sea Battery
Another interesting battery design uses a rigid framework, containing anode and cathode, which is immersed into the ocean to use sea water as the electrolyte. This configuration seems promising as an emergency battery for marine use.