digital battery

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.

One of the first space batteries was the Silver-Zinc battery, which dominated the industry in the 60's. This is a premium system with very high specific power and energy, but is quite expensive due to the use of silver. They are still used in selected applications, such as launch vehicles (rockets) and torpedoes. Mars Pathfinder also used a Silver-Zinc battery, but it was designed to be rechargeable. They have a relatively short cycle life, and are not used for multi-year missions. This type of battery is used commonly in the commercial market as hearing aid batteries.

Batteries come in several styles; the most familiar are single-use alkaline batteries. NASA spacecraft usually use rechargeable nickel-cadmium or nickel-hydride batteries like those found in laptop computers or cellular phones. Engineers think of batteries as a place to store electricity in a chemical form.

Battery technology is part of the power system, storing and discharging energy on each orbit of the spacecraft. The batteries help provide a constant source of power to the spacecraft by storing energy when excess is provided by the solar cells and discharging stored energy when the solar cells are not providing any during periods of eclipse.

NICKEL-CADMIUM BATTERIES

Nickel-Cadmium has been the most common space battery since the 70's. They were used in all commercial communications satellites, in most earth orbiters, and in some space probes. They are generally a prismatic (resembling, or being a prism) design, and packaged very efficiently. This means that the batteries can be stored on the spacecraft in a very compact form, eliminating the need for extraneous space. They have been known to last for ten to twenty years in space. They are still in use in selected space applications, including small satellites and for missions that encounter very severe radiation environments.

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

Nickel cadmium is the most commonly used battery for Low Earth Orbit (LEO) missions. A spacecraft battery consists of series-connected cells, the number of which depends upon bus voltage requirements and output voltage of the individual cells.

NICKEL-HYDROGEN BATTERIES

The Nickel-Hydrogen battery is currently the most popular space Sony Digital Camera Battery such as NP-F550, NP-FR1, NP-FM50, NP-FM51, NP-F10, NP-FE1 and DSC-T7. It 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. It is the longest-lived space battery yet built, with 10 to 20 year lifetimes being common. This battery is too expensive for commercial applications, and few terrestrial examples have been built.

Nickel-hydrogen batteries are sometimes confused with Nickel-Metal Hydride batteries, the batteries commonly found in cell phones and laptops. The nickel-metal hydride system is rarely used in space due to its limited life. 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 constant voltage, and (4) retention capacity.