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VRLA batteries

Technical Glossary




Product of current in amperes multiplied by the time current is flowing. Capacity of a cell or battery is usually expressed in ampere-hours. Abbreviated as Ah.


Electrode at which an oxidation reaction (loss of electrons) occurs. In secondary cells, either electrode may become the anode, depending upon direction of current flow. The negative electrode is the anode on discharge.

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One or more electrochemical cells electrically interconnected to form one unit and having provisions for external electrical connections.

Battery Case

Battery box or enclosure which contains cells, connectors, and associated hardware.

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Rated capacity of the cell or battery. Call charge/discharge current is often specified in terms of a multiple of C. (For example the 0.1C current for a cell rated at 1.4 Ah is 140mA.)

C Rate

Charge or discharge current in amperes that is numerically equal to the rated capacity of a cell in ampere-hours.


Amount of charge available from a battery or cell. Measured in ampere-hours. There are various sub definitions of capacity as listed below depending on the conditioned under which it is measured.

Actual Capacity

Cell capacity of a fully formed and fully charged cell when measured under non-standard conditions including non-standard end of discharge voltage.

Available Capacity

Capacity of a fully formed and fully charged cell delivered under non-standard conditions including non-standard end of discharge voltage.

Dischargeable Capacity

Capacity which a partially discharged cell can deliver before it becomes fully discharged or the amount of capacity that can be withdrawn after a limited charge input.

Nominal Capacity

Capacity typically delivered under standard conditions; a value greater than rated capacity which is a minimum value.

Permanent loss of Capacity

Reduction in cell capacity from the "fully formed, as new" value, measured under standard rating conditions, that is not recoverable by reconditioning.

Rated Capacity

Capacity value shown in the specification sheet. The minimum expected capacity when a new, but fully formed, cell is measured under standard conditions. The basis of the C rate. Note that rated capacities depend on the standard conditions used which may vary from battery to battery.

Residual Capacity

Capacity remaining after discharge.

Retained Capacity

Capacity remaining in a cell after an open circuit rest period. The result of self- discharge.

Standard Capacity

Cell capacity measured under standard conditions.

Temporary Loss of Capacity

Reduction in cell capacity that is recovered when the cell is reconditioned.


Electrochemical unit, composed of positive and negative electrodes, separator, and electrolyte, which is capable of storing electrical energy. When encased in a container and provided with electrical terminals, the cell is the basic "building block" of a battery. Although the capacity of the cell is determined by its size, the cell's voltage is strictly a function of the basic electrochemistry of the couple.

Cell Reversal

Reversing of polarity of terminals of a cell in a multi-cell battery due to over discharging.


Return of electrical energy to a battery.

Charge Acceptance

Willingness of a battery or cell to accept change. May be affected by cell temperature, charge rate and state of charge.

Charge Efficiency

Value obtained when the increase in dischargeable capacity of the battery is divided by the current input. A measure of charge acceptance. Also indicates

Charge Rate

Rate at which current is input to battery. Various sub definitions listed below indicate the speed at which the battery is returned to full capacity from a fully discharged state.

Fast Charge

Fastest return of battery to fully charged state. Specifically refers to a class of nickel-cadmium cell specially designed for rapid recharge. With appropriate choice of cell and charger, full recharge may be obtained in significantly less than one hour. Charge rate cannot be maintained in overcharge without damage to cell. Requires both a cell designed to accept charge at high rates and a charger that will deliver high rates until cell is approximately fully charged then switch to a trickle charge rate.

Float Charge

Charging for batteries used in backup applications that reduces the charge rate to prolong life while maintaining the battery in a ready-to-serve condition.

Quick Charge

Highest charge rate that can be maintained indefinitely in overcharge. Specifically refers to a class of specially designed nickel-cadmium cells that are often able to return cells to fully charged state in 3 to 5 hours.

Standard Charge

Charge rate that can be maintained indefinitely without requiring either special cells or switching chargers. Normally returns cell to full charge overnight.

Trickle Charge

Charge rate that will maintain the battery in the fully charged state while reducing overcharge temperature and thereby prolonging life when compared to other charging rates.

Charge Retention

Capacity remaining after a period of storage of a fully charged battery.


Device that supplies electrical energy to a secondary battery in a form that can be used to reverse the discharge reactions within the cells.


Cyclic charging and discharging of a battery to ensure that it is fully formed and fully charged. Sometimes indicated when a battery is first placed in service or returned to service after prolonged storage.


Device used to make external electrical connections to a battery through mechanical means.

Constant Current

Charging method in which current does not change appreciably in magnitude, regardless of battery voltage or temperature. The preferred charging method for sealed nickel-cadmium batteries. Often abbreviated CC.

Constant Potential

Charging method which input voltage does not change appreciably in magnitude regardless of battery state of charge. The most common method of charging for sealed-lead batteries. Often abbreviated CP.


Cell enclosure in which plates, separator, and electrolyte are held. It is normally made up of the cell jar and cover that are permanently joined.


Undesirable component, usually chemical, within the cell which reduces its capacity or life.


Term often used to describe the gradual oxidation of the metallic lead in the grid for the positive plate into lead dioxide.


Electrochemical or electronic device, capable of integrating current-times, used for charge control and for measurement of charge inputs and discharge outputs. Results usually reported in ampere-hours.

Coup de Fouet

A momentary drop in voltage occurring at the onset of high rate discharges in some batteries. It is followed by a recovery to the loaded voltage.

Current Collector

Structure within the electrode that allows current to be transmitted between cell terminals and the active materials.

Cut-off Voltage

Voltage at which a discharge or charge is terminated.


Charge followed by a discharge, usually repeated on a regular basis.

Cycle Life

Number of cycles a battery survives before its capacity falls below the acceptable level.

Cycling Down

Loss of capacity caused by insufficient charging between repeated discharges.

Cylindrical Cell

Battery cells in cylindrical form. Most commonly associated with primary cells and wound sealed nickel-cadmium and sealed-lead cells.

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Deep Cycling

Charge/discharge cycle where approximately 100 per cent of the available capacity is withdrawn at a low rate.

Deep Discharge

Condition where a cell is fully discharged at a low rate resulting in removal of all dischargeable capacity.

Depth of Discharge

Capacity removed from a battery divided by its actual capacity, expressed as a percentage.

Discharge Rate

Rate at which current is withdrawn from a battery. May be expressed in absolute terms (amps) or in relative terms (as a fraction or multiple of the C rate).


Withdrawing electrical energy from a battery.

Duty Cycle

Condition and usage to which a battery is subjected during operation, consisting of charge, overcharge, rest and/or discharge.

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Effective Internal Resistance, Re

Apparent opposition to current flow within a battery that manifests itself as a drop in battery voltage proportional to the discharge current. Its value is dependant upon battery design, state of charge, temperature, and age.


Conducting body within the cell in which the electrochemical reactions occur. It normally consists of the active material and the structures necessary to collect the charge and to support the active material as required.


Medium, usually liquid, within the cell that permits the movement of ions between electrodes. Sealed-lead cells contain a dilute sulphuric acid electrolyte.

End-of-Charge Voltage (EOCV)

Voltage of the battery at termination of a charge but before the charge is stopped.

End-of-Discharge Voltage (EODV)

Voltage of the battery at termination of a discharge but while still under load. Standard end-of-discharge voltages that depend on discharge rate have been established for rating purposes.

Energy Density

Energy stored within a battery or cell as a function of weight (gravimetric energy density - watt-hours per gram) or volume (volumetric energy density - watt-hours per cubic centimetre). Rate dependant.

Environmental Conditions

External circumstance to which a cell or battery may be subjected, such as ambient temperature, humidity, shock, vibration and altitude.

Equivalent Circuit

Circuit using conventional lumped parameters that simulates the electrical behaviour of a cell.

Equivalent No-Load Voltage, Eo

Numerical value of the source voltage in the equivalent circuit.

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Long-term loss of capability with use.


Condition in which a battery is unable to perform satisfactorily. Various forms of failure are described below.

Function Failure

Condition in which the battery causes the end product to fail to perform as expected.

Permanent Failure

Condition where the cell or battery cannot be restores to satisfactory performance.

Reversible Failure

Failure condition which may be corrected through the application of certain electrical procedures or reconditioning.


Charge of 1C or greater applied to a battery designed to handle that charge rate. Cannot be used as an overcharge rate so the charging system must switch to trickle charge rate as battery approaches overcharge.

Fast Charging

Rapid return of energy to a battery, usually at the 1C rate or greater.


Battery duty cycle (often associated with power backup applications) featuring long periods of time on overcharge and infrequent discharges.

Float Charging

Charging approach that minimises the deleterious effects of prolonged overcharge as experienced in float duty. Float charging often consists of constant-potential charging at relatively low voltages for sealed-lead batteries or switched-rate constant-current charging for sealed nickel-cadmium batteries.

Float Life

Life of a battery measured in calendar time (years) when essentially all of its life is spent in an overcharge condition.

Flooded Cell

Cell where the electrodes are immersed in a pool of electrolyte, thereby eliminating most opportunities for recombination. As a result the cell vents gases through most of its charge cycle. Flooded cell is a term typically used with lead-acid batteries while vented cell is used for the equivalent form of nickel-cadmium battery.

Form Factor

Geometric shape of battery configurations which may be created by interconnecting cells in various arrangements.


Initial electrical charge applied to a sealed-lead cell to convert most of the paste on the plates to active materials.

Fully Formed

Batteries, especially sealed-lead, that have all of the paste on the plates converted to active materials.

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Gas Recombination

Method of suppressing hydrogen generation by recombining oxygen gas on the negative electrode and suppressing hydrogen formation.


Formation of gas by the plates as the cell approaches full charge. This gas can either be recombined or remain within the cell until the pressure increases to the point where the cell vents. Gassing is a plate-related phenomenon while venting (release of gas to the outside environment from the cell) is a cell-related phenomenon.


Framework that supports the active materials within the electrodes. Also serves as the current collector.

Grid Growth

Increase in dimension of lead battery plates caused by oxidation of the metallic lead grids to lead dioxide which consumes more volume.

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High-Rate Charge

Charge at a rate equal to or greater than 1C.

High-Rate Discharge

Discharge at a rate greater than 5C.

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Method of providing electrical linkage between cells in a battery. May be external in the case of single cells or internal in the case of monobloc batteries.

Internal Resistance

Apparent resistance value calculated from the cell voltage difference between high rate and low rate discharge.

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Josephson effect

The flow of electric current through nonconductive material when placed between two superconductors. Used to detect very weak magnetic fields.


A measure of work, energy or cell capacity. For electrical energy, one Joule is one Amp at one Volt for one Second, or one Watt Second. 1 Wh = 3.6kJ. For mechanical energy one Joule is a force of one Newton acting over one metre i.e. One newton metre.

Joule heating

The I2R loss or heating effect of a current I flowing through a resistance R.

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Kilovolt – Ampere (KVA)

One thousand volt-amperes. The output of a UPS is typically rated in volt-amperes.

K-ion battery

A potassium-ion battery or K-ion battery (abbreviated as KIB) is a type of battery and analogue to lithium-ion batteries, using potassium ions for charge transfer instead of lithium ions.

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Lead Dioxide

Active material for the positive electrode in a sealed-lead battery.

Lead Sulfate

Chemical compound formed at both positive and negative plates of a lead battery when the battery is discharged.


Duration of satisfactory performance, measured in years (float life) or in the number of charge/discharge cycles (cycle life).

Loaded Storage

Harmful condition of storing a battery under load, a non-open circuit storage condition.

Low-Rate Charge

Charging at a rate that is slightly higher that the self-discharge losses.

Low-Rate Discharge

Less than 0.1C.

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Maintenance-Free Battery

Battery that does not require addition of water. Often applied to sealed recombining cells, but also used for flooded batteries carrying excess electrolyte so that water addition is not required over the course of life.

Maximum-Power Discharge Current, Imp

Discharge rate at which maximum power is transferred tot he external load. Normally this is the discharge rate when the discharge voltage is approximately equal to one-half of Eo.


Misnomer for voltage depression, referring to apparent loss of capacity on extended overcharge which can be reversed by reconditioning.

Midpoint Voltage

Battery voltage when 50 per cent of the actual capacity has been delivered.

Monobloc Battery

Sealed-lead battery constructed as an integrated unit rather than assembled from single cells.

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Negative Electrode

Electrode which has an electrical potential below that of the other electrode during normal cell operation. The electrode impregnated with cadmium salts is the negative electrode which undergoes chemical oxidation when a nickel-cadmium cell is discharged. The sponge-lead electrode is the negative electrode for the sealed-lead battery.

Net Charge Acceptance

Sometimes used to describe charging efficiency. Refers o the amount of discharge capacity that can be delivered as the result of a charging input.

Nominal Capacity

Typical capacity; greater than the rated capacity which is a minimum value.

Nominal Voltage

Midpoint voltage observed across battery during discharge at a selected rate, usually at the 0.2C or 0.1C rate.

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Open-Circuit Voltage

Voltage of a battery with no load applied to it.

Operating Voltage

Voltage between the two terminals when a battery is subjected to a load. usually it is expressed by the voltage of the battery at the 50 per cent discharge point.


Normal application of charge current after the battery has reached full charge.

Overcharge Current

Charging current flowing to the battery after all the active material has been converted to a charged state.


Continuous charge after a battery has reached full capacity. In a sealed cell, a result will be increased cell temperature. In a flooded or vented cell, the result will be venting of gases evolved at the electrodes.


Discharge past the point where the full capacity of the cell has been obtained.


Release of electrons by the cell's active material tot he external circuit. During discharge, cadmium at the negative electrode of the nickel-cadmium call and sponge lead at the negative electrode of the sealed-lead cell are oxidised.

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Electrical term used to describe the interconnection of batteries in which all like terminals are connected together.


Mixture of various compounds that are applied to positive and negative grids of lead batteries. These pastes are then converted to positive and negative active materials.


Common term for electrodes.


Electrical term used to denote the relative voltage relationship between two electrodes.

Positive Electrode

Electrode which has an electrical potential higher than that of the other electrode during normal cell operation. The electrode impregnated with nickel salts is the positive electrode which undergoes chemical reduction during discharge of a nickel-cadmium cell while the electrode for the sealed-lead battery.

Primary Cell

Cell designed to be used only once, then discarded. It is not capable of being returned to its original charged state by the application of current. Both carbon-zinc and alkaline cells are primary cells.

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Quick Charge

Rate of charging which returns full capacity reasonably quickly, but may be sustained into overcharge with a specially designed cell.

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Amount of current, either charge or discharge current, frequently expressed as a fraction or multiple of the specification rate, C.

Rated Capacity

Capacity value shown in the specification sheet. The minimum expected capacity when a new, but fully formed, cell is measured under standard conditions. The basis of the C rate. Note that rated capacities depend on the standard conditions used which may vary from battery to battery.

Residual Capacity

Capacity remaining after discharge.

Retained Capacity

Capacity remaining in a cell after an open circuit rest period. The result of self- discharge.


Return of electrical energy to a battery.


Chemical reaction of gases at the electrodes to form a non-gaseous product.


Charge/discharge cycling regime to eliminate voltage depression or temporary loss of capacity.

Required Application Voltage

Lowest battery voltage that will produce acceptable end-product function.

Reversible Reaction

Chemical change which takes place in either direction, as in the reactions for charging or discharging a secondary battery.

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Sealed Cell

Cell where the internal environment is controlled and isolated from the external atmosphere, often by use of some form of vent valve. Sealed cells often operate (charge) at above-ambient conditions to promote recombination. Since it is sealed, it is free from routine maintenance and can be operated without regard to position. A sealed cell minimises release of reactants outside the container.

Secondary Battery

Battery system which is capable of repeated use through employing chemical reactions that a re reversible; i.e., its discharge capability may be restored by supplying electrical current to recharge the cell.


Spontaneous decomposition of battery material from charged to discharged states.


Material which provides physical separation and electrical insulation between plates of opposite polarity. In some cells the separator may also be used to absorb excess electrolyte.


Electrical term used to describe the interconnection of cells or batteries in such a manner that the positive terminal of one cell is connected to the negative terminal of the next cell.


Condition in a battery where two plates of opposite polarity make electrical contact with each other. Shorts may be of two forms: hard or intermittent.

Hard Short

Short where the current path between the plates is firmly established and the cell is rendered useless.

Intermittent Short

Condition where the short path is unstable such as contact between two plates when the cell is moved. An intermittent short can be either low or high resistance shorts may sometimes be burned off by charging at high charge currents; the process is called zapping the cell.

Split-Rate Charge

Charging method in which the battery is charged at a high rate and then automatically reduced to a lower charge rate as the battery approaches full charge.

Sponge Lead

Porous form of metallic lead that serves as the active material at the negative electrode of a lead battery.

Standard Conditions

Laboratory conditions of rates, times, voltages, and temperatures during charge, rest and discharge.


Non-cyclic use of a battery such as in backup power application (float duty).

State of Charge

Residual capacity expressed as a per cent of fully-charged capacity.


Process occurring in lead batteries that have been stored and allowed to self-discharge for extended periods of time. Large crystals of lead sulfate grow that interfere with function of the active materials.

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One form of battery terminal; a flat metal strip protruding from the cell, often containing a hole for wire connection. Routinely used in smaller sealed-lead cells.


Ambient Temperature Average temperature of the battery's surroundings.

Cell Temperature

Average temperature of the battery's components.

Temperature Cut-off (TCO)

Method of switching the charge current flowing to a battery from fast charge to topping charge by a control circuit in the charger that is activated by battery temperature.


Location on the cell or battery exterior that is electrically connected to wither the positive and negative electrode. May be either a discrete site, such as the positive and negative tabs on sealed-lead cells, or a more generalised location, such as the positive cover and negative can on sealed nickel-cadmium cells.

Topping Charge

Reduced rate charge that completes (tops) the charge on a cell and can be continued in overcharge without damaging the cell.

Trickle Charge

Low-rate charge used to keep battery fully charged after charging at higher rate.

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UN 38.3

UN 38.3 presents a combination of significant environmental, mechanical, and electrical stresses designed to assess lithium batteries' ability to withstand the anticipated rigors incurred during transport.

Universal Serial Bus (USB)

Bi-directional data port featuring a 5-volt supply and two data lines to accommodate auxiliary devices and to charge batteries.

UL 94:V0

UL 94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing. UL 94 rating V-0 definition means burning stops within 10 seconds on a vertical part allowing for drops of plastic that are not inflames.

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Valve-regulated lead acid (VRLA)

Maintenance-free lead acid battery recombines oxygen (positive plate) with hydrogen (negative plate) on charge; valve regulates pressure by release of excess gases. Repeated venting will lead to dry out.

Voltage (V)

Electric energy potential per unit charge. 1V = 1J/Coulomb. (1,000 joules = 0.277Wh).

Voltage delay

During prolonged storage, some battery systems develop a passivation layer. This results in a momentarily lower voltage until the film is dissipated through discharge.

Voltage limit

Battery thresholds on charge and discharge.

Voltage-limiting charger

Current is allowed to fluctuate in saturation mode while the voltage is capped (lead acid and Li ion charging).

Volumetric energy density

Also known as energy density; specifies energy storage in volume (Wh/l).

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Watt (W)

Unit of power; ampere (A) times volt (V) equals watts (W).

Watt-hour (Wh)

Unit of electrical energy equivalent to a power consumption of one watt for one hour (One watt-hour = 3600 Joules). Multiplying a battery voltage (V) by the rated capacity (Ah) gives the battery energy in Wh. Example: 14.4V x 2.5 Ah = 36 Wh.

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