How a Battery Works

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Definition of a Battery

Close up of batteries
ose Luis Pelaez/ The Image Bank/ Getty Images

A battery, which is actually an electric cell, is a device that produces electricity from a chemical reaction. Strictly speaking, a battery consists of two or more cells connected in series or parallel, but the term is generally used for a single cell. A cell consists of a negative electrode; an electrolyte, which conducts ions; a separator, also an ion conductor; and a positive electrode. The electrolyte may be aqueous (composed of water) or nonaqueous (not composed of water), in liquid, paste, or solid form. When the cell is connected to an external load, or device to be powered, the negative electrode supplies a current of electrons that flow through the load and are accepted by the positive electrode. When the external load is removed the reaction ceases.

A primary battery is one that can convert its chemicals into electricity only once and then must be discarded. A secondary battery has electrodes that can be reconstituted by passing electricity back through it; also called a storage or rechargeable battery, it can be reused many times.

Batteries come in several styles; the most familiar are single-use alkaline batteries.

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What is a Nickel Cadmium Battery?

Nickel Cadmium Battery
From top to bottom: "Gumstick", AA, and AAA Nickel-Cadmium rechargeable batteries. GNU Free Documentation License

The first NiCd battery was created by Waldemar Jungner of Sweden in 1899.

This battery uses nickel oxide in its positive electrode (cathode), a cadmium compound in its negative electrode (anode), and potassium hydroxide solution as its electrolyte. The Nickel Cadmium Battery is rechargeable, so it can cycle repeatedly. A nickel cadmium battery converts chemical energy to electrical energy upon discharge and converts electrical energy back to chemical energy upon recharge. In a fully discharged NiCd battery, the cathode contains nickel hydroxide [Ni(OH)2] and cadmium hydroxide [Cd(OH)2] in the anode. When the battery is charged, the chemical composition of the cathode is transformed and the nickel hydroxide changes to nickel oxyhydroxide [NiOOH]. In the anode, cadmium hydroxide is transformed to cadmium. As the battery is discharged, the process is reversed, as shown in the following formula.

Cd + 2H2O + 2NiOOH —> 2Ni(OH)2 + Cd(OH)2

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What is a Nickel Hydrogen Battery?

Nickel Hydrogen Battery
Nickel Hydrogen Battery - Example and example in use. NASA

The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's navigation technology satellite-2 (NTS-2).

The Nickel-Hydrogen battery can be considered a hybrid between the nickel-cadmium battery and the fuel cell. The cadmium electrode was replaced with a hydrogen gas electrode. This battery is visually much different from the Nickel-Cadmium battery because the cell is a pressure vessel, which must contain over one thousand pounds per square inch (psi) of hydrogen gas. It is significantly lighter than nickel-cadmium, but is more difficult to package, much like a crate of eggs.

Nickel-hydrogen batteries are sometimes confused with Nickel-Metal Hydride batteries, the batteries commonly found in cell phones and laptops. Nickel-hydrogen, as well as nickel-cadmium batteries use the same electrolyte, a solution of potassium hydroxide, which is commonly called lye.

Incentives for developing nickel/metal hydride (Ni-MH) batteries comes from pressing health and environmental concerns to find replacements for the nickel/cadmium rechargeable batteries. Due to worker's safety requirements, processing of cadmium for batteries in the U.S. is already in the process of being phased out. Furthermore, environmental legislation for the 1990's and the 21st century will most likely make it imperative to curtail the use of cadmium in batteries for consumer use. In spite of these pressures, next to the lead-acid battery, the nickel/cadmium battery still has the largest share of the rechargeable battery market. Further incentives for researching hydrogen-based batteries comes from the general belief that hydrogen and electricity will displace and eventually replace a significant fraction of the energy-carrying contributions of fossil-fuel resources, becoming the foundation for a sustainable energy system based on renewable sources. Finally, there is considerable interest in the development of Ni-MH batteries for electric vehicles and hybrid vehicles.

The nickel/metal hydride battery operates in concentrated KOH (potassium hydroxide) electrolyte. The electrode reactions in a nickel/metal hydride battery are as follows:

Cathode (+): NiOOH + H2O + e- Ni(OH)2 + OH- (1)

Anode (-): (1/x) MHx + OH- (1/x) M + H2O + e- (2)

Overall: (1/x) MHx + NiOOH (1/x) M + Ni(OH)2 (3)

The KOH electrolyte can only transport the OH- ions and, to balance the charge transport, electrons must circulate through the external load. The nickel oxy-hydroxide electrode (equation 1) has been extensively researched and characterized, and its application has been widely demonstrated for both terrestrial and aerospace applications. Most of the current research in Ni/Metal Hydride batteries has involved improving the performance of the metal hydride anode. Specifically, this requires the development of a hydride electrode with the following characteristics: (1) long cycle life, (2) high capacity, (3) high rate of charge and discharge at a constant voltage, and (4) retention capacity.

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What is a Lithium Battery?

What is a Lithium Battery?
What is a Lithium Battery?. NASA

These systems are different from all of the previously mentioned batteries, in that no water is used in the electrolyte. They use a non-aqueous electrolyte instead, which is composed of organic liquids and salts of lithium to provide ionic conductivity. This system has much higher cell voltages than the aqueous electrolyte systems. Without water, the evolution of hydrogen and oxygen gases is eliminated and cells can operate with much wider potentials. They also require a more complex assembly, as it must be done in a nearly perfectly dry atmosphere.

A number of non-rechargeable batteries were first developed with lithium metal as the anode. Commercial coin cells used for today's watch batteries are mostly a lithium chemistry. These systems use a variety of cathode systems that are safe enough for consumer use. The cathodes are made of various materials, such as carbon monoflouride, copper oxide, or vanadium pentoxide. All solid cathode systems are limited in the discharge rate they will support.

To obtain a higher discharge rate, liquid cathode systems were developed. The electrolyte is reactive in these designs and reacts at the porous cathode, which provides catalytic sites and electrical current collection. Several examples of these systems include lithium-thionyl chloride and lithium-sulfur dioxide. These batteries are used in space and for military applications, as well as for emergency beacons on the ground. They are generally not available to the public because they are less safe than the solid cathode systems.

The next step in lithium ion battery technology is believed to be the lithium polymer battery. This battery replaces the liquid electrolyte with either a gelled electrolyte or a true solid electrolyte. These batteries are supposed to be even lighter than lithium ion batteries, but there are currently no plans to fly this technology in space. It is also not commonly available in the commercial market, although it may be just around the corner.

In retrospect, we have come a long way since the leaky flashlight batteries of the sixties, when space flight was born. There is a wide range of solutions available to meet the many demands of space flight, 80 below zero to the high temperatures of a solar fly by. It is possible to handle massive radiation, decades of service, and loads reaching tens of kilowatts. There will be a continued evolution of this technology and a constant striving toward improved batteries.