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Glossary of RE Terms

Glossary of renewable energy and battery terms

active material — The materials which chemically react within the cell to release free electrons are known as active materials. In most cases, one active material is a metal or metallic compound which is oxidized. The other active material, often a metallic oxide, is reduced.

ampere — The ampere is the standard unit used to measure electrical current. Physically, the ampere is a measure of the number of electrons passing a given point per unit time.

ampere-hour — The ampere-hour is the unit of measurement of the electrical capacity of a cell or battery. Physically, it represents the number of electrons available from the cell or battery.

anode — The anode is the electrode within the cell which undergoes the chemical process of oxidation. Electrically, the anode is the cell’s positive terminal.

antimony — Antimony is a metallic chemical element with the atomic number of 51. Antimony is alloyed with lead to physically strengthen the plates of lead-acid cells.

battery — A battery is a group of interconnected electrochemical cells. Single cells are considered to be a battery if they are used alone.

capacity — Capacity is the amount of electrical energy a cell or battery contains. The ampere-hour is the unit of this capacity.

cathode — The cathode is the electrode within the cell which undergoes the chemical process of reduction. Electrically, the cathode is the negative terminal of the cell.

cell — The cell is the basic unit used to store energy in the battery. The cell contains an anode, a cathode, and the electrolyte.

cutoff voltage — The voltage level at which a cell is considered to be empty, and the discharge process is terminated.

cycle — A cycle is one complete charge/discharge sequence of the battery.

cycle life — Cycle life is the number of cycles a cell or battery will undergo before being considered “worn out.” This point is usually defined as when the battery’s capacity has reached only 80% of its initial rated capacity.

deep cycle — A battery or cell is said to be “deep cycled” if 80% or more of its energy is withdrawn before recharging.

dendrites — Dendrites are microscopic whiskers of metal which form in nickel- cadmium cells. These metallic whiskers may cause internal shorting within the

cell, rendering it useless.

depth of discharge — The amount of energy withdrawn from a battery or cell expressed as a percentage of its rated capacity.

electrochemical couple — An electrochemical couple is two chemical compounds or elements which react together to release free electrons.

electrolyte — The electrolyte is the medium of ion transport within the cell. The electrolyte provides a path for electron transfer between the anode and cathode of the cell. Electrolytes are usually liquids or pastes, which are either acidic or basic.

end of charge voltage — The voltage level at which a cell or battery is considered, while under charge, to be full.

energy density — Energy density is a ratio of a battery or cell’s capacity to either its volume or weight. Volumetric energy density is expressed in watt-hours per cubic inch. Weight energy density is expressed in watt-hours per pound.

energy transfer rate — The energy transfer rate is a measure of the rate at which energy is either being added or withdrawn from a battery or cell. This energy transfer is measured in amperes.

equalizing charge — The equalizing charge is a controlled overcharge of an already full battery to restore all the individual cells within the battery to the same state of charge.

float service — A battery is in float service when it is continually charged at a very slow rate, and only occasionally discharged.

gassing — Gassing is the evolution of hydrogen and oxygen gasses at the cell’s electrodes. These gasses result from the hydrolysis of water in the electrolyte during the charging process.

grid — The grid within a cell is an electrically conductive structure which holds the cell’s active materials. The grid may or may not participate in the chemical reactions of the cell.

hydrometer — The hydrometer is an instrument for measuring the density of liquids in relation to the density of water. The hydrometer is used to indicate the state of charge in lead-acid cells by measuring the specific gravity of the electrolyte.

ion — An ion is an electrically charged particle or molecule.

local action — Local action is the process of self-discharge that is present in all forms of electrochemical cells.

Isc — Amperes short circuit, usually applied to photovoltaic modules.

photovoltaic module — A series connected group of photovoltaic (PV) cells. PV cells make DC electric current directly from sunshine.

primary cell — A primary cell is an electrochemical cell which cannot be recharged. The chemical process within the primary cell is only one way— discharge. When a primary cell is discharged it is discarded.

rate of charge — The amount of energy per unit time that is being added to the battery. Rate of charge is commonly expressed as a ratio of the battery or cell’s rated capacity to charge duration in hours.

resistance — Resistance is the property of materials to impede a flow of electrons through themselves. All materials have some resistance. Those of low resistance are known as conductors, while those of high resistance are known as insulators. The unit used to measure resistance is the Ohm (Ω).

rest voltage — The voltage of a cell or battery that is neither being charged or discharged.

secondary cell — Secondary cells are electrochemical cells which are rechargeable. The chemical reaction within the secondary cell is reversible, allowing the cell to be recharged many times.

self-discharge — Self-discharge is the tendency of all electrochemical cells to lose energy. Self-discharge represents energy lost to internal chemical reactions within the cell. This energy is not and cannot be used from the battery or cell’s output terminals.

specific gravity — Specific gravity is the ratio of a liquid’s density to the density of water.

state of charge — State of charge is a ratio, expressed in percent, of the energy remaining in a battery in relation to its capacity when full.

sulphation — Sulphation is the formation of lead sulphate crystals on the plates of lead-acid cells.

Voc — Volts open circuit, usually applied to photovoltaic modules.

volt — The volt is the unit used in the measurement of the electromotive force. A standard electrical definition of the volt is: an electromotive force of 1 volt is necessary to move a current of 1 ampere through a 1 Ω resistor.

watt — The watt is the unit used to measure power. In electrical terms, it is a volt- ampere.

78 Home Power #47 • June / July 1995

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