Battery Glossary

Battery from A to Z

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    Active material: The chemically reactive materials in an energy cell which react with each other converting from one chemical composition to another while generating electrical energy or accepting electric current from an external circuit.

    Acid: A proton donor. A compound containing hydrogen which dissociates in aqueous solution producing positively charged hydrogen ions (H+). An acidic solution has a pH less than 7,0.

    Aging: Permanent loss of capacity with frequent use or the passage of time due to unwanted irreversible chemical reactions in the cell.

    AGM (Absorbent Glass Mat) battery: A lead acid battery using a glass mat to promote recombination of the gases produced by the charging process.

    Ambient temperature: The average temperature surrounding the battery, typically air.

    Ampere hours (Ah): The unit of measure used for comparing the capacity or energy content of a battery with the same output voltage. For most batteries it defines the battery’s C rate. Strictly – One Ampere hour is the charge transferred by one amp flowing for one hour. 1Ah = 3600 Coulombs. The true capacity of any battery is its energy content and this is measured in WattHours (Wh). It is the battery’s Amphour capacity multiplied by the battery voltage.

    Aqueous solution: Chemical components in liquid or gel form.


    Battery: Two or more cells electrically connected to form a unit. Under common usage, the term “battery” also applies to a single cell.


    C Rate: C is a value which expresses the rated current capacity of a cell or battery. A cell discharging at the C rate will deliver its nominal rated capacity for 1 hour. Charging and discharging currents are generally expressed as multiples of C. The time to discharge a battery is inversely proportional to the discharge rate. NC is a charge or discharge rate which is N times the rated current capacity of the battery where N is a number (fraction or multiple). CN is the battery capacity in AmpHours which corresponds to complete discharge of the battery in N hours (N is usually a subscript).

    Capacity: Number of ampere-hours (Ah) a fully charged cell or battery can deliver under specific conditions of discharge.

    Cell: A cell comprises a number of positive and negative charged plates immersed in an electrolyte that produces an electrical charge by means of an electrochemical reaction.

    Cell reversal: A condition which may occur multi cell series chains in which an over discharge of the battery can cause one or more cells to become completely discharged. The subsequent volt drop across the discharged cell effectively reverses its normal polarity.

    Charge: The process of replenishing or replacing the electrical charge in a rechargeable cell or battery.

    Charge acceptance: The ability of a secondary cell to convert the active material to a dischargeable form. A charge acceptance of 90% means that only 90% of the energy can become available for useful output. Also called Coulombic Efficiency or Charge Efficiency.

    Charge rate: The current at which a cell or battery is charged. Generally expressed as a function of rated capacity C.

    Charge retention: The ability of a battery to retain its charge in zero current conditions. Charge retention is much poorer at high temperatures.

    Charge, state of: The available or remaining capacity of a battery expressed as a percentage of the rated capacity.

    Constant current: A charging scheme which maintains the current through the cell at a charge (CC) constant value.

    Constant voltage (CV): A charging scheme which maintains the voltage across the battery terminals constant value.

    Coup de Fouet: A dramatic initial voltage drop when a battery is suddenly called upon to supply a heavy load. The voltage recovers after a short time once the electro-chemical discharge process stabilizes.

    Current: Flow of electrons equal to one coulomb of charge per second, usually expressed in amperes (A).

    Cutoff voltage: The specified voltage at which the discharge of a cell is considered complete.

    Current limit: The maximum current drain under which the particular battery will perform adequately under a continuous drain.

    Cycle: The discharge and subsequent charge of a secondary battery in a way that it is restored back to its fully charged state.

    Cycle life: The number of cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. See also Float life below.


    Depth of discharge: The ratio of the quantity of electricity or charge removed from a cell on
    (DOD): discharge to its rated capacity.

    Duty cycle: Operating parameters of a cell or battery including factors such as charge and discharge rates, depth of discharge, cycle length, and length of time in the standby mode.


    Electrode: Conducting element within a cell in which an electrochemical reaction occurs.

    Electrode: Electrical conductor and the associated active materials at which an electrochemical reaction occurs. Also referred to as the positive and negative plates of a secondary cell.

    Electrolysis: Chemical dissociation of water into hydrogen and oxygen gas caused by passage of an electrical current.

    Electrolyte: Medium which provides the ion transport function between the positive and negative electrodes of a cell.

    Equalizing charge: The process used during charging to ensure that every cell is charged to the same state of charge.


    Float charge: Method of charging in which a secondary cell is continuously connected to a constant-voltage supply that maintains the cell in a fully charged condition.


    Gassing: Evolution of gas from one or more electrodes as a result of water electrolysis during the charge or self-discharge of a cell. Significant gassing occurs when the battery is nearing the fully charged state while recharging or when the battery is on equalizing charge.


    Potential difference: Work which must be done against electrical forces to move a unit charge from one point to the other, also known as electromotive force (EMF).

    Primary battery: Cell or battery which is not intended to be recharged and is disposed when the cell or battery has delivered its useful capacity.


    Secondary battery: A battery that after discharge may be restored to its charged state by passing an electrical current through the cell in the opposite direction to that of the discharge.

    Separator: Electrically insulating layer of material which physically separates electrodes of opposite polarity. Separators must allow passage to ions in the electrolyte and may also have the function of storing or immobilizing the electrolyte.

    Specific gravity: Ratio of the weight of a solution to an equal volume of water at a specified temperature. Used as an indicator of the state of charge of a cell or battery.

    Sulfation: Formation of lead sulfate crystals on the plates of a lead-acid battery.


    Terminal: External electric connections of a cell or battery, also referred to as “terminal post” or “post.”

    Thermal runaway: A condition that occurs in a battery (especially valve-regulated types) when charging energy results in heat generation within the battery greater than the heat dissipated, causing an uncontrolled rise in battery temperature. This can cause failure through cell dry-out, shortened life, and/or melting of the battery.

    Trickle charge: Method of charging in which a secondary cell is either continuously or intermittently connected to a constant current supply in order to maintain the cell in fully or nearly fully charged condition.


    Voltage: Electromotive force or potential difference, expressed in volts (V).