A battery, in concept, can be any device that stores energy for later use. The common use of the word, "battery," is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. When the electrodes are placed in the electrolyte solution, they generate different voltages. When the electrodes are connected to a load then electricity is generated. The electrolyte solution is a chemical of some type, either liquid (wet battery) or solid/powdery (dry battery).

Batteries consist of one or more galvanic cells connected in series or in parallel to create a battery with almost any current capacity at any voltage level. A battery composed of two 1.5 V galvanic cells connected in series, for example, will produce 3 V. A typical 9 V battery is simply six 1.5 V cells connected in series. Such a series battery, however, will produce a current that is the equivalent to just one of the galvanic cells. A battery composed of two 1.5 V galvanic cells connected in parallel, on the other hand, will still produce a voltage of 1.5 V, but the current provided can be double the current that just one cell would create. Such a battery can provide current twice as long as a single cell.

Batteries are either sealed or unsealed (aka flooded). Unsealed batteries require water to be added to the battery periodically because the electricity producing process causes water to evaporate. Sealed batteries have a slightly different chemistry that does not allow water to evaporate so no additional water must be added to them.

Factors to consider when designing a product that requires a battery:

• Primary (not rechargeable) or secondary (rechargeable) battery
• energy density (weight of battery versus power supplied)
• whether chemicals in battery are dangerous, toxic or explosive (individually or when combined)
• sealed or unsealed battery
• cost of battery
• battery’s tolerance to temperature (some are more tolerant than others)
• battery’s discharge rate – how long does the battery hold a charge when used

Types of batteries:

• Lead-acid batteries come in all manners 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. Uses of lead batteries include automobile batteries and UPSs.

• Zinc-carbon (Z-C) 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 can develop holes that allow leakage of the mildly acidic electrolyte, which can damage the device being powered. This is a typical flashlight battery.

• Zinc-Manganese-Dioxide Alkaline Cells ("Alkaline Batteries") uses an alkaline electrolyte, instead of the mildly acidic electrolyte, in a regular zinc-carbon battery. An alkaline battery can have a useful life of 5 to 6 times that of a zinc-carbon battery.

• Rechargeable alkaline batteries are most appropriate for low- and moderate-power portable equipment, such as hand-held toys and radio receivers. Drawback is only about 25 recharges.

• Nickel-Cadmium (Ni-Cd) 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. Batteries made from Ni-Cd cells offer high currents at relatively constant voltage and they are tolerant of physical abuse. Ni-Cd batteries are also tolerant of inefficient usage cycling. If a Ni-Cd battery has incurred memory loss, a few cycles of discharge and recharge can often restore the battery to nearly "full" memory. Memory effect is that in some batteries if the battery is not completely discharge, but charge again it may reduce the span of use in the battery. Unfortunately, nickel-cadmium technology is relatively expensive. Cadmium is an expensive metal and is toxic.

• Nickel-Metal Hydride (Ni-MH) cell uses different metals to give greater energy density than nickel-cadmium. 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. 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. Ni-MH batteries are widely available.

• Nickel-Iron (Ni-I) 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.

• Nickel-Zinc (Ni-Z) cells yield 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 electrical properties – it generate a higher voltage than other batteries. 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. These characteristics translate into batteries that are lighter in weight, have lower per-use costs. The high electrochemical potential of lithium yields some disadvantages such as (1) the inorganic components of the battery and its casing are destroyed by the lithium ions, (2) on contact with water, lithium will react to create huge volumes of hydrogen which can ignite or can create excess pressure in the cell and (3) lithium also has a relatively low melting temperature and if it comes into direct contact with the cathode there may be a violent chemical reaction. Lithium 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 cost three times that of alkaline batteries, the cost benefits of using lithium batteries are marginal. 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.

• Metal-Air Cells utilize the oxygen in air as a "liquid" cathode. A metal, such as zinc or aluminum, is used as the anode. 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 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. Silver-oxide cells can provide high currents for longer periods. Due to the high cost of silver, silver-oxide cell use is limited.

• 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. The component, mercury, unfortunately, is relatively expensive and its disposal creates environmental problems.

Some developmental batteries are:

• Nickel-hydrogen (Ni-H) 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 uses.

• Thermal battery is a high-temperature, molten-salt primary battery. Unfortunately it is so hot it can not be used in consumer goods.

• 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 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 hundreds of thousands 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 voltage of some prototypes is approximately 2.5 V.

• Sea Battery, uses a rigid framework, containing the 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.

• Lithium-polymer battery cell doesn't employ the hard metal casing as does conventional nickel-metal hydride or lithium-ion batteries. Instead, the electrodes are covered with a flexible plastic or aluminum laminate foil. The battery contains no liquid but instead the electrolyte is polymerized into a gel-like form. This allows for batteries to be designed in a variety of sizes and shapes that would be impossible using conventional battery cells. The ability to shape the battery as need may lend itself to electric vehicle applications.

• Sodium/sulfur (NaS) - this technology has more than four times the available power compared to current nickel cadmium batteries. NaS batteries are very tolerant to high temperatures and are economical to construct.

Novel batteries:

• Potato Battery, one interesting science experiment involves sticking finger-length pieces of copper and zinc wire, one at a time, into a raw potato to create a battery. The wires will carry a very weak current which can be used 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.

Energizer has a good site that lists the currently used batteries. Most useful is the chart that compares the types of batteries, the table labeled "characteristics" and the glossary.