This glossary defines common battery, electrical, electrochemical, charging, testing, and energy-storage terms. Entries are grouped alphabetically and cover chemistry, construction, electrical behavior, safety, and measurement. A cell is one electrochemical unit; a battery contains one or more cells and may include connections, monitoring, protection, and thermal components.
A–C: Core Electrical, Electrochemical, and Battery Terms
AC, alternating current: Current that periodically changes direction, such as household mains power.
Acid: A substance involved in electrochemical reactions by donating hydrogen ions or accepting electron density. Lead-acid batteries use sulfuric-acid electrolyte.
AGM, absorbent glass mat: A valve-regulated lead-acid design in which porous glass mat holds electrolyte. It is not immune to overcharge or venting.
Allotrope: A structural form of an element. Graphite and diamond are carbon allotropes.
Ampere-hour, Ah: A unit of charge equal to one ampere flowing for one hour, or 3,600 coulombs. Ratings require specified discharge conditions and cutoff voltage.
Anode: The electrode where oxidation occurs. It is normally negative during discharge, but terminology depends on operating direction during charging.
Antimony: An alloying metal used in some lead-acid grids. It can improve strength and casting but may increase gassing and water loss.
Absolute state of charge, ASoC: Estimated charge relative to a defined reference capacity or full condition. Battery-management systems may define it differently.
Absolute state of health, ASoH: Estimated condition relative to a reference or new state, using indicators such as capacity, resistance, or power capability. It is not a universal measurement.
Battery: An energy-storage device containing one or more electrochemical cells, electrical connections, and sometimes monitoring, protection, or thermal components. Cells may connect in series, parallel, or both.
Battery cycle: A defined charge-and-discharge operation. Partial cycles may count as fractions of a full equivalent cycle, depending on the method.
BESS, battery energy-storage system: A complete stationary installation that may include cells, modules, racks, a BMS, power conversion, thermal management, protection, controls, and enclosure equipment.
BMS, battery-management system: Electronics and software that monitor and control a battery. Functions may include voltage, current, and temperature monitoring; limits; balancing; fault detection; contactor control; and communication.
Bluetooth: Short-range wireless communication used by some battery monitors, chargers, and instruments. Range depends on radio class, antenna, obstacles, interference, and implementation.
Boolean bit: A binary value represented as 0 or 1. Battery electronics use bits and bit fields for status, alarms, and configuration.
Button cell: A small, usually coin-shaped cell that may be primary or rechargeable and may use different chemistries.
Calcium: An alloying element used in some lead-acid grids. It can reduce water loss, but charging behavior depends on the complete design.
Candela, cd: The SI base unit of luminous intensity. It may appear in equipment specifications but is not a battery unit.
Capacitance: The ability to store electric charge per unit voltage, measured in farads.
Capacitor: A component that stores energy in an electric field. It is not an electrochemical battery.
Capacity: The charge a battery can deliver under specified conditions, usually in ampere-hours. It depends on current, temperature, cutoff voltage, age, and test procedure.
Capacity offset: A correction for capacity differences caused by discharge rate, temperature, reference conditions, or other test factors.
Carbon dioxide, CO2: A gas produced by combustion, respiration, and decomposition. It is distinct from carbon monoxide.
Cathode: The electrode where reduction occurs. It is normally positive during discharge; polarity must be interpreted in relation to charging.
C-code or C-rate: A current expressed relative to rated capacity. At 1C, current numerically equals the rated ampere-hour value in amperes under stated conditions.
Cell mismatch: Differences among cells in capacity, resistance, voltage, temperature behavior, or state of charge. In series packs, mismatch can reduce performance and increase balancing needs.
Cell reversal: A weak or discharged cell driven below normal polarity by other cells in a series string. It can cause damage, heating, or gas generation.
Chemical battery: A battery that stores and releases energy primarily through chemical reactions rather than electrostatic charge storage.
Cobalt: A transition metal used in some lithium-ion cathode materials. Its role and quantity vary by chemistry and cell design.
Co-generation: Simultaneous production of useful heat and electricity from one energy source. It is not regenerative battery charging.
Coke: A carbon-rich material produced by heating coal or other carbonaceous feedstocks, used in metallurgy and industrial materials.
Coulomb: The SI unit of charge; one coulomb is transferred by one ampere in one second.
Coulombic efficiency: The ratio of charge removed to charge previously supplied under defined conditions. It measures charge efficiency, not total energy efficiency.
Current-limiting charger: A charger that restricts output current to a maximum, often alongside voltage regulation, tapering, or termination control.
Cycle: A defined battery operation, such as discharge followed by recharge. Depth of discharge and test limits must be stated for comparisons.
Cycle life: The number of specified cycles before a defined end-of-life condition. Results depend on chemistry, temperature, rates, depth of discharge, rest periods, and the selected capacity, resistance, or power criterion.
Cylindrical cell: A cell in a cylindrical metal can, commonly using wound electrodes and separators. Format does not determine chemistry.
D–F: Current, Conversion, Discharge, Energy, and Charging Concepts
DC, direct current: Current flowing in one direction. Battery voltage can vary with load, temperature, and state of charge.
DC-to-DC converter: Power electronics that convert one DC voltage to another. They may step voltage up or down, regulate, isolate, or control bidirectional flow.
dT/dt: Temperature change over time. Some nickel-based chargers use rapid temperature rise, with other limits, to help terminate charging.
Digital battery: A battery system with electronics that measure, calculate, store, or communicate information. It is not a separate chemistry.
DIN: Deutsches Institut für Normung, the German Institute for Standardization. The applicable document, edition, and test type must be identified.
IEC: International Electrotechnical Commission, which publishes international electrical and electronic standards. An IEC number alone does not identify scope or applicability.
Depth of discharge, DoD: The fraction or percentage of defined rated or usable capacity removed. A 70% DoD means approximately 70% of that reference capacity was discharged.
Double-layer capacitor: An electrochemical capacitor storing charge at the electrode-electrolyte interface. Pseudocapacitors use a different storage mechanism.
Driving range: Distance an electric vehicle can travel under defined conditions. It depends on usable energy, efficiency, speed, terrain, temperature, auxiliary loads, driving style, and test method.
Dumb battery: A battery with little or no embedded communication or monitoring. External equipment may still monitor or control it.
Electrochemical impedance spectroscopy, EIS: A method applying a small AC signal over multiple frequencies and analyzing impedance. It can investigate resistance, interfaces, charge transfer, diffusion, and aging, but requires suitable models and conditions.
Electrode: A conductive component through which current enters or leaves a cell. Anode and cathode are defined by reaction, not permanently by position or external polarity.
Electrolyte: An ionically conductive medium between electrodes. It may be liquid, gel, polymer, or solid; ions move internally while electrons use the external circuit.
Electrolyte oxidation: An unwanted reaction promoted by high potential, heat, overcharge, or unsuitable conditions. No universal lithium-ion oxidation voltage applies to every formulation and test.
Energy: The ability to do work. Battery energy is expressed in watt-hours or joules and is approximately voltage multiplied by charge, subject to variation and losses.
Energy cell: A cell optimized to store substantial energy, often with less maximum power capability than a power-oriented design.
Energy density: Stored energy per unit mass or volume. It does not alone determine runtime because load, efficiency, usable state-of-charge range, and temperature also matter.
Energy Star: A voluntary energy-efficiency program administered by the U.S. Environmental Protection Agency and, for some products, the U.S. Department of Energy. Requirements depend on product category.
Exercise: A controlled charge-discharge operation used to refresh, characterize, or verify a battery. It is chemistry- and application-dependent and cannot restore every aged battery.
Farad, F: The SI unit of capacitance; one farad equals one coulomb per volt.
Fast charge: Charging at a relatively high rate compared with normal procedure. Meaning depends on chemistry, thermal limits, charger design, and specifications.
Float charge: Regulated charging that keeps a battery near full while supplying a load and compensating for self-discharge. Voltage is chemistry- and temperature-dependent.
Flow battery: A rechargeable battery in which liquid electrolytes circulate from external tanks through an electrochemical cell. Stack size mainly determines power; electrolyte quantity and concentration mainly determine energy.
Frequency: The number of periodic cycles per second, measured in hertz. It is important in AC systems and impedance testing.
Fuel cell: An electrochemical device that converts continuously supplied fuel and oxidant into electricity, with heat and reaction products. It generally requires ongoing reactant supply rather than recharge.
Fuel gauge: Hardware and software estimating state of charge, remaining capacity, runtime, or related values. It may be part of a BMS without providing all BMS functions.
Fuzzy logic: A control method using graded rules rather than only binary decisions. Results depend on its model, inputs, and validation.
G–L: Materials, Measurement Units, Battery Types, and Lithium-Ion Terms
Graphene: A one- or few-layer carbon structure with a hexagonal lattice. Calling a material graphene does not automatically make it a complete or high-performance battery electrode.
Graphite: A layered carbon allotrope widely used as a lithium-ion anode. It is related to, but not interchangeable with, graphene.
Gravimetric energy density: Energy stored per unit mass, commonly watt-hours per kilogram.
Halon: A group of halogenated fire-suppression agents. Environmental restrictions apply, and suitable battery incident response depends on chemistry, installation, rules, and emergency guidance.
Hertz, Hz: The SI unit of frequency, equal to one cycle per second.
Hydrogen: The lightest element. It can fuel fuel-cell systems, but a fuel cell is not itself a rechargeable battery.
Hydrometer: An instrument measuring liquid density or specific gravity. In flooded lead-acid batteries, electrolyte specific gravity can indicate state of charge; sealed designs differ.
Joule, J: The SI unit of energy; one joule equals one watt-second.
Lithium-ion: A rechargeable battery family in which lithium ions move between host materials. Chemistries differ in voltage, energy, power, life, and safety characteristics.
Lithium iron phosphate, LFP: A lithium-ion chemistry using lithium iron phosphate as cathode material. It requires charging and protection suited to the manufacturer’s specifications.
Lithium polymer: A descriptive term for lithium-based cells using a polymer or polymer-containing electrolyte system, often in pouch construction. It does not specify cathode chemistry or safety behavior.
Lithium titanate, LTO: A lithium-based chemistry using lithium titanate as anode material. Its voltage, power, charging behavior, and energy density differ from graphite-anode cells.
Nominal voltage: A representative voltage assigned for classification and system design, not the exact voltage under all conditions.
Open-circuit voltage, OCV: Voltage measured with no significant external current, normally after a defined rest. Its relationship to state of charge depends on chemistry, temperature, history, and relaxation.
Internal resistance: Effective resistance inside a cell or battery that causes voltage drop and heat under load. The measured value depends on current, timescale, temperature, state of charge, and method.
Impedance: Opposition to alternating or time-varying current, including resistance and reactive behavior. It depends on frequency.
M–R: Materials, Power, Resistance, and Rechargeable-Battery Performance
Memory effect: A change in apparent usable capacity or voltage behavior associated with repeated limited-range operation. It is most associated with some nickel-cadmium patterns and is not a general explanation for battery aging.
Polarization: Voltage deviation caused by reaction kinetics, ion transport, concentration gradients, and related processes during current flow.
Power: The rate of delivering or absorbing energy, measured in watts. In DC systems, it is approximately voltage multiplied by current.
Power density: Power per unit mass or volume. It describes a different design priority from energy density.
Rechargeable battery: A battery designed to accept electrical energy and restore chemical energy through controlled reactions. Rechargeability is limited by chemistry, conditions, and aging.
Reserve capacity: A defined time a battery supplies a specified current before reaching a specified cutoff voltage. Both current and cutoff must be stated.
Self-discharge: Loss of stored charge while disconnected from an external load. Rate depends on chemistry, temperature, state of charge, age, and construction.
Separator: A porous or ion-conductive insulating layer between electrodes. It prevents direct electronic contact while allowing ion transport; damage can contribute to internal shorts.
Short circuit: An unintended low-resistance path between different voltages. Battery shorts can cause very high current, heating, arcing, fire, or damage.
State of charge, SoC: Estimated remaining charge relative to defined usable or rated capacity, usually as a percentage. It is affected by current measurement, voltage, temperature, aging, and model accuracy.
State of health, SoH: Estimated condition relative to a reference state. Capacity, resistance, power, self-discharge, and fault history may all matter.
Thermal runaway: A self-reinforcing temperature rise in which heat generation exceeds heat removal. It can cause venting, fire, propagation, or hazardous gases in some chemistries.
Trickle charge: Low-rate charging intended to offset self-discharge. It is suitable only when permitted for the chemistry; excessive current can cause overcharge.
Thermistor: A resistor whose resistance changes with temperature. Packs use thermistors for measurement, charge control, and protection.
S–Z: Safety, Standards, Testing, Charging, and Connectivity
Safety: Control of electrical, chemical, thermal, mechanical, and fire hazards during manufacture, transport, charging, use, storage, and disposal. It depends on the complete system, not chemistry alone.
Smart battery: A battery with embedded measurement, control, memory, and communication. It may report limits and state information; a BMS may additionally control contactors, balancing, and protection.
Capacity test: A controlled discharge measuring delivered charge or energy to a specified cutoff. Current, temperature, rest periods, starting condition, and cutoff voltage should be documented.
Impedance measurement: A test evaluating resistance or frequency-dependent impedance. It can reveal aging or defects, but results from different methods are not automatically comparable.
Cycle-life test: Repeated charge-discharge testing under defined conditions until a specified end-of-life criterion, such as capacity, resistance, power, or safety.
Constant-current charging: Charging at a controlled current. Voltage generally rises, and a later control stage may be required.
Constant-voltage charging: Charging at a controlled voltage while current changes as the battery approaches that voltage. Permitted voltage depends strongly on chemistry and temperature.
Maintenance charge: Charging intended to offset self-discharge during storage or standby. It is not automatically equivalent to continuous float charging.
Titanate: A material containing titanium and oxygen in a titanate structure. Lithium titanate is used as an anode material in some rechargeable cells.
UL 1642: A UL standard addressing safety testing for lithium batteries. Applicability depends on product type and edition; it is not universal approval for every battery system.
UN 38.3: A United Nations transport-testing framework for lithium cells and batteries. Applicable testing supports shipment compliance but is not complete product-safety certification.
Universal Serial Bus, USB: A wired interface carrying power and bidirectional data. Charging depends on the applicable specification, negotiated power, cable, host, and battery electronics; USB does not itself define the charging algorithm.
Voltage: Electrical potential difference, measured in volts. Battery voltage varies with chemistry, state of charge, current, temperature, and aging.
Watt: The SI unit of power; one watt equals one joule per second.
Watt-hour, Wh: A unit of energy equal to one watt delivered for one hour. Watt-hours are generally more useful than ampere-hours for comparing batteries with different voltages.
Working temperature: The permitted temperature range for operation, charging, or discharging. Charge and discharge limits may differ, so specified ranges should be followed.
Zero-voltage condition: A measured voltage near zero that may indicate deep discharge, an open connection, protection circuitry, or cell failure. Further testing is needed to determine the internal condition.
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