Battery Glossary: 122 Terms Explained

Plain-language definitions of the terms used across the battery value chain — cell chemistry, degradation, manufacturing, BESS, EVs and EU regulation.

122 terms

A

Accelerating rate calorimetry(ARC)

Accelerating rate calorimetry is a test method that holds a cell under adiabatic conditions and measures its self-heating rate, identifying the temperature at which exothermic reactions become self-sustaining.

Active material

Active material is the component of an electrode that stores and releases lithium during charge and discharge. Cathode active material and anode active material are usually abbreviated CAM and AAM.

Anode

The anode is the negative electrode of a cell during discharge, holding lithium when the cell is charged. In commercial lithium-ion cells it is almost always graphite, sometimes blended with silicon.

See also: Cathode, Graphite, Silicon anode

Arbitrage

Arbitrage is the practice of charging a storage asset when electricity prices are low and discharging when they are high, earning the spread minus round-trip losses and degradation cost.

Areal loading

Areal loading is the amount of active material coated per unit area of electrode, usually given in milligrams per square centimetre. Higher loading raises energy density and reduces rate capability.

Augmentation

Augmentation is the planned addition of battery capacity during a storage asset's life to offset degradation and maintain contracted energy. It is normally done at rack or block level rather than by mixing aged and new cells.

Battery management system(BMS)

A battery management system is the electronics that measure cell voltages, currents and temperatures, estimate state of charge and state of health, enforce safe operating limits, balance cells and report to the host system.

Battery passport

The battery passport is a digital record required under the EU Battery Regulation from 18 February 2027 for electric vehicle, light means of transport and industrial batteries above 2 kWh, accessible via a QR code and carrying composition, carbon footprint, performance, durability and state of health data.

BESS

A battery energy storage system is a grid-connected installation combining battery racks, a bidirectional power conversion system, transformers, thermal management and controls behind a single point of interconnection.

Binder

Binder is the polymer that holds active material and conductive additive together and bonds the coating to the current collector. Cathodes commonly use PVDF, anodes commonly use water-based CMC and SBR.

Black mass

Black mass is the powdered mixture of electrode materials left after battery cells are shredded and casings, foils and separator are removed. It is the traded intermediate feeding metallurgical recovery.

Buffer

A buffer is capacity reserved at the top and bottom of a pack's range and hidden from the user, protecting calendar life and preventing over-discharge. It is the difference between gross and usable capacity.

Busbar

A busbar is a rigid conductor carrying high current between cells, modules or system components. Its cross-section is sized against continuous current, and it is often a binding constraint on pack-level C-rate.

C

C-rate

C-rate expresses current relative to rated capacity, so 1C is the current that would discharge the rated capacity in one hour. Duration is its reciprocal, making a four-hour system a 0.25C system.

Calendar ageing

Calendar ageing is capacity and power loss that occurs with time regardless of use, driven by temperature and by the state of charge at which a cell is stored. It continues even in an unused cell.

Calendering

Calendering is the step in electrode manufacturing where the coated foil passes through rollers to a target thickness and porosity. It physically sets the trade-off between energy density and rate capability.

Capacity

Capacity is the charge a cell can deliver, measured in ampere hours. It depends on discharge rate and temperature, so a capacity figure without its test conditions cannot be compared with another.

Capacity market

A capacity market is a mechanism paying generation and storage assets to be available at times of system stress, rather than for energy delivered. Storage earning here is often held idle at high state of charge.

Carbon footprint declaration

A carbon footprint declaration is the calculated lifecycle emissions of a battery model from a specific plant, expressed as kilograms of CO2 equivalent per kWh delivered over service life. Offsets cannot reduce the declared figure.

See also: EU Battery Regulation

Cathode

The cathode is the positive electrode of a cell during discharge. It determines most of the trade-off between energy density, cycle life, thermal stability and cost, which is why chemistries are named after it.

Cathode electrolyte interphase(CEI)

The cathode electrolyte interphase is a passivating layer forming on the cathode surface from electrolyte oxidation. It grows faster at high upper cut-off voltage and contributes to resistance rise.

Cell balancing

Cell balancing is the equalisation of cells in a series string so the pack is not limited by a single outlier at either end of its range. It may be passive, dissipating charge, or active, transferring it.

Cell-to-chassis(CTC)

Cell-to-chassis is a design approach integrating cells directly into the vehicle structure, removing both module and separate pack enclosure to save mass and volume at the cost of serviceability.

Cell-to-pack(CTP)

Cell-to-pack is a design approach that omits the module level and mounts cells directly into the pack enclosure, raising volumetric efficiency and reducing part count.

Coating

Coating is the application of electrode slurry onto metal foil, usually through a slot die at high line speed. Uniformity of coating weight determines uniformity of current density in the finished cell.

Contactor

A contactor is a high-current switch that connects or isolates a battery pack from its load. Welded-contactor detection matters because a welded contactor leaves a pack apparently disconnected while it is not.

Coulombic efficiency

Coulombic efficiency is the ratio of charge extracted during discharge to charge supplied during the preceding charge. Small shortfalls indicate side reactions, so it is a sensitive early degradation indicator.

Current collector

A current collector is the metal foil carrying the electrode coating and conducting electrons to the tabs. Lithium-ion cells use aluminium on the cathode and copper on the anode; sodium-ion can use aluminium on both.

Cut-off voltage

Cut-off voltage is the terminal voltage at which charging or discharging stops. Raising the upper cut-off buys capacity and costs life, since electrolyte oxidation accelerates with cathode potential.

Cycle ageing

Cycle ageing is degradation caused by use, scaling with charge throughput, depth of discharge, current and temperature. It runs alongside calendar ageing rather than instead of it.

DCIR

Direct current internal resistance is voltage change divided by current step over a defined pulse duration. The value depends on that duration, so a one-second and a ten-second figure legitimately differ.

Dead lithium

Dead lithium is plated lithium metal that has lost electrical contact with the anode and can no longer participate in cycling. It is a permanent loss of lithium inventory.

Degassing

Degassing is the removal of gas generated during formation. In pouch cells the gas pocket is cut away and the cell resealed before ageing and final testing.

Dendrite

A dendrite is a needle-like metallic lithium structure that can grow from plated deposits and eventually penetrate the separator, creating an internal short circuit.

Depth of discharge(DoD)

Depth of discharge is the proportion of usable capacity removed during a cycle. A cycle from 90 to 20 per cent state of charge has a depth of discharge of 70 per cent.

Dew point

Dew point is the temperature at which air becomes saturated with water vapour, used as the humidity specification for battery dry rooms. Cell assembly areas run at very low dew points to keep moisture away from electrolyte.

Differential voltage analysis(DVA)

Differential voltage analysis differentiates voltage with respect to capacity during a slow cycle to identify degradation modes. It is generally more workable than incremental capacity analysis on flat-curve chemistries such as LFP.

Direct recycling

Direct recycling recovers cathode material as an intact compound and relithiates it, rather than breaking it down to base metals. It preserves the most value but demands feedstock sorted by chemistry.

Dry electrode processing

Dry electrode processing forms an electrode coating without liquid solvent, removing the drying oven and solvent recovery plant. It cuts energy use, floor space and capital cost relative to the conventional route.

Dry room

A dry room is a humidity-controlled manufacturing area used for cell assembly. It exists because water reacts with fluorinated electrolyte salt to form hydrofluoric acid, which attacks both electrodes.

Due diligence

Under the EU Battery Regulation, due diligence is the obligation to operate a supply chain policy covering cobalt, lithium, natural graphite and nickel. Its application was postponed to 18 August 2027.

See also: EU Battery Regulation

Duration

Duration is a storage system's energy rating divided by its power rating, giving the hours it can discharge at full power. It is the reciprocal of C-rate and defines which markets an asset can serve.

Electrochemical impedance spectroscopy(EIS)

Electrochemical impedance spectroscopy applies a small alternating signal across a frequency sweep and measures the response, separating ohmic, interfacial and diffusion contributions that a single resistance value combines.

Electrolyte

The electrolyte is the ionically conductive, electronically insulating medium carrying lithium ions between electrodes. Conventional formulations combine a lithium salt with organic carbonate solvents and additives.

Energy

Energy is the work a cell can deliver, measured in watt hours, and equals capacity multiplied by average voltage. Energy rather than capacity is the meaningful basis for comparing cells of different voltages.

Energy density

Energy density is energy per unit volume, given in watt hours per litre. It is distinct from specific energy, which is per unit mass, and the two are often confused.

Equivalent full cycle(EFC)

An equivalent full cycle normalises partial cycles by dividing total charge throughput by rated capacity, so four cycles at 25 per cent depth of discharge count as one.

EU Battery Regulation

Regulation (EU) 2023/1542 governs batteries placed on the EU market, covering carbon footprint, due diligence, recycled content, recycling targets and the battery passport. It has applied since 18 February 2024.

Extended producer responsibility(EPR)

Extended producer responsibility places the cost of collection and end-of-life treatment on the party placing a product on the market, which over time creates pressure to design for disassembly and recycling.

See also: EU Battery Regulation

Fast charging

Fast charging is charging at a high rate, limited by the anode rather than the charger, since lithium that cannot intercalate into graphite quickly enough plates as metal instead.

Formation

Formation is the first controlled charge of a finished cell, during which the solid electrolyte interphase forms on the anode. It is slow and capital intensive and occupies a large share of a cell plant.

Frequency containment reserve(FCR)

Frequency containment reserve is a fast-responding ancillary service that arrests grid frequency deviations. Batteries serving it perform frequent shallow bidirectional cycles around a target state of charge.

Gigafactory

Gigafactory is an informal term for a cell plant with annual capacity measured in gigawatt hours. Its economics are dominated by utilisation and yield during ramp rather than by materials procurement.

Graphite

Graphite is the standard anode active material in lithium-ion cells, storing lithium by intercalation between its layers. It expands roughly ten per cent on full lithiation.

Grid-forming inverter

A grid-forming inverter establishes voltage and frequency itself rather than synchronising to an existing waveform, allowing it to provide synthetic inertia and fault current contribution as synchronous generation retires.

Gross capacity

Gross capacity is the total nameplate capacity of the cells in a pack, before any buffer is reserved. It is larger than the usable capacity available to the application.

H

Hybrid pulse power characterisation(HPPC)

Hybrid pulse power characterisation applies current pulses in both directions at a series of states of charge, deriving resistance and available power across the operating window rather than energy capacity.

Hydrometallurgy

Hydrometallurgy recovers battery metals by leaching black mass in acid and separating them through solvent extraction and precipitation. It recovers lithium as well as nickel and cobalt, unlike most pyrometallurgical routes.

IEC 62619

IEC 62619 sets safety requirements for secondary lithium cells and batteries used in industrial applications, including stationary storage. It sits alongside UL standards depending on the market.

IEC 62660

IEC 62660 is the standard series covering performance, reliability and abuse, and safety testing for lithium-ion cells used in electric road vehicle propulsion, including defined pulse conditions for resistance measurement.

Impedance

Impedance is the frequency-dependent opposition to alternating current in a cell, combining ohmic, charge transfer and diffusion contributions that respond on different timescales.

Incremental capacity analysis(ICA)

Incremental capacity analysis differentiates capacity with respect to voltage during a slow cycle, producing peaks whose movement and shrinkage indicate which degradation mode is active.

Internal resistance

Internal resistance is the opposition to current within a cell, arising from conductors, electrolyte, electrode interfaces and diffusion. It rises with age and falls as temperature rises.

Internal short circuit

An internal short circuit is unintended electrical contact between electrodes inside a cell, caused by separator failure, mechanical damage or metal particle contamination. It is a principal initiation route for thermal runaway.

ISO 26262

ISO 26262 is the functional safety standard for road vehicle electrical and electronic systems. It assigns hazards an ASIL rating, which determines the architecture and independence a battery management system requires.

Jelly roll

A jelly roll is the wound assembly of electrode and separator layers used in cylindrical and some prismatic cells. Winding is faster than stacking but gives less uniform thermal behaviour.

K

Knee point

The knee point is where gradual capacity fade turns into rapid fade, usually because a mechanism has crossed a threshold. Extrapolating linear early fade past an unobserved knee overestimates life.

LCO

Lithium cobalt oxide is a high energy density cathode chemistry used mainly in consumer electronics. Its cobalt content and modest thermal stability have kept it out of large-format applications.

Levelised cost of storage(LCOS)

Levelised cost of storage is the discounted lifetime cost of a storage asset divided by the discounted energy it discharges over its life, giving a cost per MWh that allows comparison across technologies and durations.

LFP

Lithium iron phosphate is a cathode chemistry with no nickel or cobalt, high cycle life and a high thermal decomposition onset. Its flat voltage curve makes state of charge estimation harder.

Lithium plating

Lithium plating is the deposition of metallic lithium on the anode during charging when ions cannot intercalate quickly enough. It occurs at low temperature, high charge rate and high state of charge.

LMFP

Lithium manganese iron phosphate substitutes manganese into the LFP structure to raise the voltage plateau and therefore energy density, while keeping much of the phosphate family's thermal advantage.

Loss of active material(LAM)

Loss of active material is a degradation mode in which electrode material stops participating, through particle fracture, contact loss or structural transformation. It is tracked separately for each electrode.

Loss of lithium inventory(LLI)

Loss of lithium inventory is a degradation mode in which cyclable lithium is consumed by side reactions while the electrodes remain intact. It is the dominant mode in cells that spend most of their life at rest.

LTO

Lithium titanate is an anode material offering very high rate capability and long cycle life at the cost of low cell voltage and energy density. It is used in high-power and extreme-temperature niches.

M

Module

A module is an assembly of cells with its own mechanical structure, sense wiring and often local electronics. Cell-to-pack designs omit this level to save volume and part count.

N

N/P ratio

The N/P ratio is the ratio of negative to positive electrode capacity in a cell. It is set above one to give the anode margin and reduce plating risk, at some cost in energy density.

NCA

Lithium nickel cobalt aluminium oxide is a high specific energy cathode chemistry used in premium electric vehicles. It has the narrowest thermal margin of the common layered oxides.

NFPA 855

NFPA 855 is the installation standard for stationary energy storage systems, covering spacing, separation, ventilation and fire protection. Local authorities having jurisdiction may impose additional requirements.

NMC

Lithium nickel manganese cobalt oxide is a layered oxide cathode family named by its metal ratio, so NMC 811 is eight parts nickel to one each of manganese and cobalt. Higher nickel raises energy and lowers thermal stability.

NMP

N-methyl-2-pyrrolidone is the solvent conventionally used to dissolve PVDF binder in cathode slurry. It is costly and subject to regulatory attention, so it must be recovered rather than released.

Nominal voltage

Nominal voltage is the representative average voltage of a cell over a discharge, used for rating and system design. It is roughly 3.2 volts for LFP and 3.6 to 3.7 volts for nickel-rich chemistries.

Nyquist plot

A Nyquist plot displays impedance data as negative imaginary against real impedance. Its ohmic intercept, semicircles and low-frequency tail correspond to distinct physical processes inside the cell.

Off-gassing

Off-gassing is the release of gas from a cell before or during thermal runaway, containing flammable and toxic components. Detecting it gives earlier warning than temperature sensing.

Open circuit voltage(OCV)

Open circuit voltage is the terminal voltage of a rested cell with no current flowing. Mapping it to state of charge works well on sloped chemistries and poorly on flat-curve chemistries such as LFP.

Over-discharge

Over-discharge is discharging a cell below its lower voltage limit, which can dissolve copper from the anode current collector and later redeposit it as metallic bridges.

Overcharge

Overcharge is charging a cell beyond its upper voltage limit, driving electrolyte oxidation, gas generation and lithium plating. It is a recognised initiation route for thermal runaway.

Overpotential

Overpotential is the voltage difference between a cell under load and at equilibrium, arising from ohmic, charge transfer and diffusion losses. It is what makes terminal voltage sag on discharge.

P

Pack

A pack is the complete assembled battery, including cells or modules, the battery management system, wiring, contactors, thermal management and enclosure. Pack-level specific energy sits well below cell level, because enclosure, electronics and thermal hardware add mass without adding energy.

Point of interconnection(POI)

The point of interconnection is the metered boundary between a storage or generation asset and the grid. Commercial guarantees for round-trip efficiency and output are measured here rather than at the battery terminals.

Porosity

Porosity is the void fraction of an electrode coating, filled by electrolyte. Lower porosity raises energy density and restricts ionic transport, so it is set deliberately at the calendering step.

Power conversion system(PCS)

The power conversion system is the bidirectional inverter connecting a battery's direct current side to the alternating current grid, and implementing the grid-support behaviour the connection agreement requires.

Propagation

Propagation is the spread of thermal runaway from one cell to its neighbours. Preventing it, rather than preventing initiation, is where most modern pack safety engineering effort goes.

PVDF

Polyvinylidene fluoride is the binder conventionally used in cathodes, dissolved in NMP solvent. Water-based binder systems are standard on the anode side.

Pyrometallurgy

Pyrometallurgy recovers battery metals by high-temperature smelting, producing a nickel, cobalt and copper alloy. It tolerates mixed feedstock but commonly loses lithium to slag and consumes considerable energy.

Rate capacity effect

The rate capacity effect is the fall in delivered capacity as discharge current rises, caused by concentration gradients and voltage losses bringing the cell to its cut-off sooner. It is described by Peukert's equation for lead-acid.

Recycled content

Recycled content is the share of a battery's cobalt, lead, lithium or nickel originating from recycled sources. The EU Battery Regulation sets minimum shares from 18 August 2031, rising in 2036.

See also: EU Battery Regulation

Round-trip efficiency(RTE)

Round-trip efficiency is energy out divided by energy in over a full cycle. For a storage system it must be quoted with its measurement boundary, since conversion and auxiliary loads sit between cells and the grid.

Second life

Second life is the redeployment of a battery retired from its original application into a less demanding one. Its economics depend on diagnostics cost, disassembly labour and the falling price of new cells.

Self-discharge

Self-discharge is the slow loss of stored charge in a cell at rest, from internal side reactions. Abnormally high self-discharge indicates an internal defect, which is why it is screened after formation.

Separator

The separator is a porous membrane keeping electrodes physically apart while allowing ions through. Its melting behaviour is a key stage in thermal runaway, so many are ceramic coated for stability.

Silicon anode

A silicon anode uses silicon in place of or blended with graphite to raise capacity. Silicon expands roughly 300 per cent on full lithiation at the particle level, against about ten per cent for graphite, causing mechanical fatigue and continuous interphase reformation.

Slitting

Slitting is cutting coated electrode web to its final width. Burrs left at the cut edge are a direct route to internal short circuits, so edge quality is closely inspected.

Slurry

Slurry is the mixture of active material, conductive additive, binder and solvent applied to current collector foil during coating. Dispersion quality set here propagates into every downstream defect.

Sodium-ion

Sodium-ion is a battery chemistry using sodium rather than lithium as the charge carrier, allowing aluminium current collectors on both electrodes. It offers strong low-temperature performance at lower specific energy.

Solid electrolyte interphase(SEI)

The solid electrolyte interphase is a passivating film forming on the anode during first charge from decomposed electrolyte. It is necessary for the cell to work, and its continued slow growth consumes lithium.

Solid-state battery

A solid-state battery replaces the liquid electrolyte with a solid ionic conductor, removing the flammable solvent and enabling lithium metal anodes. Manufacturing readiness lags laboratory performance considerably.

Specific energy

Specific energy is energy per unit mass, in watt hours per kilogram. It is the figure usually meant when people say energy density, though energy density strictly refers to volume.

Stacking

Stacking assembles a cell from individual cut electrode and separator sheets rather than winding a continuous web. It gives better space use and thermal behaviour, and it is slower.

State of charge(SoC)

State of charge is the charge currently available divided by the charge the cell can hold today, as a percentage. It is estimated rather than measured, since no sensor reports it directly.

State of health(SoH)

State of health compares a cell's present condition with its beginning-of-life condition, expressed either as capacity retention or as resistance rise. The two definitions can diverge sharply in the same cell.

State of power(SoP)

State of power is the current or power a cell can accept or deliver right now without violating a voltage, current or temperature limit. It depends on state of charge, temperature and age.

Tab

A tab is the conductive extension welded to the current collector that carries current out of the electrode stack to the terminal. Weld quality determines both resistance and durability under vibration.

Thermal runaway

Thermal runaway is a self-sustaining chain of exothermic reactions inside a cell, where heat released outpaces heat removed. Once started, external cooling cannot stop it in the affected cell, only limit propagation to its neighbours.

Tolling agreement

A tolling agreement is a contract in which an offtaker pays a fixed fee for the right to dispatch a storage asset, transferring market price risk from the owner to the offtaker.

Tortuosity

Tortuosity describes how indirect the ionic path through an electrode's pore structure is. Higher tortuosity, typically from denser calendering, limits rate capability and fast-charge acceptance.

Transition metal dissolution

Transition metal dissolution is the release of metals, particularly manganese, from the cathode into the electrolyte. Dissolved metals migrate to the anode and catalyse further electrolyte reduction there.

UL 9540A

UL 9540A is a test method characterising thermal runaway fire propagation in battery energy storage systems at cell, module and unit level. The full report matters more than the certificate.

UN 38.3

UN 38.3 is the transport safety test series that lithium cells and batteries must pass before shipment, covering altitude, thermal cycling, vibration, shock, external short circuit, impact, overcharge and forced discharge.

Usable capacity

Usable capacity is the capacity an application may actually draw, after buffers are reserved at the top and bottom of the range. It is what a displayed percentage maps to, not gross capacity.

W

Winding

Winding assembles a cell by rolling continuous electrode and separator webs into a jelly roll. It is faster and mechanically simpler than stacking and is standard for cylindrical cells.

Y

Yield

Yield is the proportion of started cells that pass end-of-line testing. Because a scrapped cell carries the full cost of every prior step, yield during ramp dominates new plant economics.