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Glossary of Battery Definitions
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Efficiency: For a secondary cell, the ratio of the output on discharge to the input required to restore it to its initial state of charge under specified conditions. Can be measured in ampere-hour, voltage, and watt-hour efficiency.
Electrolyte: The chemistry of a battery requires a medium that provides the ion transport mechanism between the positive and negative electrodes of a cell.
Energy density (specific energy): These two terms are often used interchangeably. Energy density refers mainly to the ratio of a battery's available energy to its volume (watt hour/liter).Specific energy refers to the ratio of energy to mass (watt hour/kg). The energy is determined by the charge that can be stored and the cell voltage (E=qV).
Electrochemical Couple:The system of active materials within a cell that provides electrical energy storage through an electrochemical reaction.
Electrode: An electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an electrical conductor at the surface of which a change occurs from conduction by electrons to conduction by ions. For gases and vacuum, the electrodes merely serve to conduct electricity to and from the medium.
Electropositivity: The degree to which an element in a galvanic cell will function as the positive element of the cell. An element with a large electropositivity will oxidize faster than an element with a smaller electropositivity.
End-of-Discharge Voltage: The voltage of the battery at termination of a discharge.
Energy - Output Capability: expressed as capacity times voltage, or watt-hours.
Float Charging: Method of recharging in which a secondary cell is continuously connected to a constant-voltage supply that maintains the cell in fully charged condition. Typically applied to lead acid batteries.
Fuel cell: A cell in which one or both of the reactants are not permanently contained in the cell, but are continuously supplied from a source external to the cell and the reaction products continuously removed. Unlike the metal anodes typically used in batteries, the fuels in a fuel cell are usually gas or liquid, with oxygen as the oxidant. The hydrogen/oxygen fuel cell is the most common. In this fuel cell, hydrogen is oxidized at the anode:
half-reaction V vs SHE
2H2 > 4H+ + 4e- 0
4H+ + O2 + 4e- > 2H2O 1.2
Hydrogen/oxygen fuel cell systems work well in space travel applications because of their high efficiency, high power-to-weight and volume ratios, and usable reaction product (water). They can function for many months as long as fuel is supplied and therefore the energy density cannot be measured.
Galvanic Cell: A combination of electrodes, separated by electrolyte, that is capable of producing electrical energy by electrochemical action.
Gassing: The evolution of gas from one or both of the electrodes in a cell. Gassing commonly results from self-discharge or from the electrolysis of water in the electrolyte during charging.
Half-reaction: Refers to the chemical processes occurring at each electrode. The potential of the two half-reactions add to give us the overall cell potential.
Internal Resistance: The resistance to the flow of an electric current within the cell or battery.
Lithium-ion battery: These batteries use lithium for the negative electrode and offer high energy density and the ability to undergo repeated charge cycles.
Lithium-ion-polymer battery: Similar to a lithium-ion battery, a lithium-ion-polymer battery uses a conductive plastic and is more malleable than traditional lithium-ion batteries. Lithium-ion polymer can be molded into different shapes, which can be critically important to the makers of small devices, such as cell phones.
Memory Effect: A phenomenon in which a cell, operated in successive cycles to less than full, depth of discharge, temporarily loses the remainder of its capacity at normal voltage levels (usually applies only to Ni-Cd cells). Note, memory effect can be will be induced in NiCd cells even if the level of discharge is not the same during each cycle. Memory effect is reversable.
Milliamp hour: This is the main battery capacity rating, equal to one-thousandth of an amp-hour, generally referred to by its acronym: mAh. The typical laptop battery has between 2,000 and 6,000 milliamp hours of capacity.
Negative Terminal: The terminal of a battery from which electrons flow in the external circuit when the cell discharges. See Positive Terminal.
Nonaqueous Batteries: Cells that do not contain water, such as those with molten salts or organic electrolytes.
Nickel-cadmium battery: Also known as NiCd, this is the original battery technology used in laptops. In using cadmium as the negative electrode, these batteries have a relatively low energy density and suffer from memory effects.
Nickel-metal-hydride battery: By removing the cadmium and using nickel hydride instead, these batteries are made to hold more energy, but they can't be recharged more than a few hundred times. They are generally referred to as NiMH.
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