Battery Terms, Definitions and Glossary

A battery glossary is an alphabetical reference that defines the technical vocabulary of batteries - the units, components, ratings and processes used to specify, test and maintain them. This page collects the essential battery terms and definitions for lead acid batteries, from Acid and Ampere-Hour to VRLA and Watt-Hour, in plain English.

Use it to decode spec sheets and product labels: ratings such as CCA and Reserve Capacity describe starting power and endurance, terms such as AGM, Gel and VRLA identify battery construction, and parts such as grids, plates, separators and terminals explain how a battery is built. If you are new to the subject, start with our guide What Is a Battery? and the other articles in our battery basics library.

Jump to: A  B  C  D  E  F  G  H  I  L  M  N  O  P  R  S  T  V  W

A

Acid

A chemical compound that can release hydrogen ions when mixed with water. In a lead-acid battery, diluted sulfuric acid serves as the electrolyte.

Active Material

The porous structure of lead compounds that produces and stores electrical energy within a lead-acid battery. The active material in the positive plates is lead dioxide, and in the negative plates it is metallic sponge lead. During charging and discharging, these materials react with sulfuric acid according to the reaction: PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O.

AGM (Absorbent Glass Mat)

A type of non-woven separator material composed almost entirely of glass microfibers. AGM absorbs and retains the electrolyte, leaving no free liquid to spill, and VRLA batteries built with this material are commonly called AGM batteries.

Ampere (Amp, A)

The unit of measure of the electron flow rate, or current, through a circuit.

Ampere-Hour (Ah)

A unit of measure for a battery's electrical storage capacity, obtained by multiplying the current in amperes by the discharge time in hours. Example: a battery that delivers 5 amperes for 20 hours provides 5 x 20 = 100 amp-hours of capacity.

B

Battery

One or more galvanic (electrochemical) cells electrically connected into a single unit and equipped with terminals for external electrical connections.

Battery Charger

A device that supplies electrical energy to a battery to restore its charge.

Boost Charge

A short, high-current charging process that ensures the cells and plates within a battery are charged sufficiently to perform their intended function.

BCI Group Number

A number assigned by the Battery Council International (BCI) that fingerprints a battery by its physical characteristics: (a) dimensions (L x W x H), (b) voltage (6V or 12V), (c) polarity (right-hand front positive, left-hand front positive, etc.) and (d) terminal type (top, side, L, etc.). The BCI group number does not designate capacity; it defines the listed physical characteristics only.

C

Capacity

The number of amp-hours a battery will deliver at a specific discharge rate and temperature. Capacity is not a constant value: it decreases as the discharge rate increases, and it is affected by factors such as active material weight and density, adhesion of the active material to the grid, plate number, design and spacing, separator design, electrolyte specific gravity and quantity, grid alloys, final limiting voltage, discharge rate, temperature, internal and external resistance, and the age and life history of the battery.

Cell

The basic electrochemical current-producing unit in a battery, consisting of a set of positive plates, negative plates, separators, electrolyte and casing. A lead-acid cell has an open-circuit voltage of approximately 2 volts, so a 12-volt lead-acid battery contains six cells connected in series.

Charge Acceptance

The quantity of current, expressed in ampere-hours, that a battery in a defined state of charge can accept at a specified temperature and charge voltage within a defined period.

Circuit

The path followed by a flow of electrons. A closed circuit is a complete path; an open circuit has a broken or disconnected path.

Circuit (Parallel)

A circuit that provides more than one path for the flow of current. In a parallel arrangement, batteries of like voltage have all positive terminals connected to one conductor and all negative terminals connected to another. Two 12-volt 50 amp-hour batteries connected in parallel deliver 12 volts and 100 amp-hours.

Circuit (Series)

A circuit with only one path for the flow of current. Batteries arranged in series are connected from the negative of the first to the positive of the second, and so on: two 12-volt 50 amp-hour batteries in series deliver 24 volts and 50 amp-hours.

CCA (Cold Cranking Amps)

An industry rating that defines a battery's ability to start an engine in cold temperatures. The rating is the number of amperes a new, fully charged 12-volt battery can deliver at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts. The higher the CCA rating, the greater the starting power of the battery. As a battery ages, it may no longer meet its original CCA rating.

Conductance

The ability of a circuit or battery to transmit current.

Container and Cover

The reservoir and lid that contain the battery plates and electrolyte, made from impact- and acid-resistant material such as polypropylene.

Corrosion

The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces deterioration of the material and its properties. The positive lead grids in a battery gradually corrode in service, often leading to battery failure, and terminals corrode if they are not properly maintained.

Current

The rate of flow of electricity, or the movement of electrons along a conductor, measured in amperes. It is comparable to the flow of water in a stream.

Current (Alternating, AC)

A current that varies periodically in magnitude and direction. A battery does not deliver alternating current.

Current (Direct, DC)

An electrical current flowing in one direction only. A secondary battery delivers direct current and must be recharged with direct current applied in the opposite direction of the discharge.

Cycle

In a battery, one discharge plus one recharge equals one cycle.

D

Deep Discharge

A state in which a cell is fully discharged at low current so that its voltage falls below the final discharging voltage.

Deep-Cycle Battery

A battery designed to provide a steady amount of current over a long period of time, deliver a surge when needed, and withstand being deeply discharged over and over again.

Discharging

When a battery is delivering current to an external circuit, it is said to be discharging.

E

Electrolyte

In a lead-acid battery, sulfuric acid diluted with water. The electrolyte is a conductor that supplies water and sulfate for the electrochemical reaction: PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O.

Electronic Tester

An electronic device that assesses the condition of a battery through an ohmic measurement such as resistance or conductance, typically without applying large current loads.

Element

A set of positive and negative plates assembled with separators.

Equalization Charge

A controlled charge, with specified current, time and upper voltage limits, that brings all cells and plates within a battery to full charge and makes the electrolyte uniform and free of stratification.

F

Formation

In battery manufacturing, the process of charging the battery for the first time. Electrochemically, formation converts the lead oxide paste on the positive grids into lead dioxide, and the lead oxide paste on the negative grids into metallic sponge lead.

G

Gel

Electrolyte that has been immobilized by the addition of a chemical agent, normally fine silica, to prevent spillage. Batteries made with gelled electrolyte are commonly called gel batteries and are one of the two typical types of VRLA battery.

Grid

A lead alloy framework that supports the active material of a battery plate and conducts current.

Ground

The reference potential of a circuit. In automotive use, one battery cable is attached to the body or frame of the vehicle, which serves as a path for completing the circuit instead of a direct wire from the component. Today, over 99 percent of automotive and light-truck applications use the negative terminal of the battery as the ground.

Group Size

The size designation assigned by the Battery Council International (BCI) for common battery types, covering maximum container size, terminal location and type, and special container features. See BCI Group Number above.

H

Hydrometer

A device used to measure the strength of the electrolyte - the concentration of sulfuric acid in it - through the specific gravity of the electrolyte.

I

Intercell Connectors

Lead structures that connect adjoining cells in series within a battery, from the positive of one cell to the negative of the next.

L

Lead-Acid Battery

A battery made up of plates of lead and lead oxide, with various other elements used to change density, hardness and porosity, immersed in an electrolyte of about 35 percent sulfuric acid and 65 percent water. The electrochemical reaction between the plates and the electrolyte produces electrons.

Load Tester

An instrument that draws current from a battery through an electrical load while measuring voltage, to determine the battery's ability to perform under actual discharge conditions.

Low Water-Loss Battery

A battery that does not require periodic water addition under normal driving conditions; also referred to as a maintenance-free battery.

M

MCA (Marine Cranking Amps)

An industry rating defining a marine battery's ability to deliver a large amount of amperage for a short period of time. Because marine batteries are typically never used at temperatures below freezing, MCA is measured at 32°F rather than 0°F: the rating is the number of amperes a new, fully charged 12-volt marine battery can deliver at 32°F for 30 seconds while maintaining a voltage of at least 7.2 volts. The higher the MCA rating, the greater the starting power of the marine battery.

Maintenance-Free

A battery that normally requires no service watering during its lifetime of use.

N

Negative

Designating, or pertaining to, electrical potential. The negative battery terminal is the point from which electrons flow during discharge; it is the opposite of the positive terminal.

O

Ohm

The unit for measuring electrical resistance or impedance within an electrical circuit.

Ohm's Law

The relationship between volts (V), amperes (I) and resistance (R) in an electrical circuit, expressed as V = IR. If any two of the three values are known, the third value can be calculated using the equation.

Open-Circuit Voltage

The voltage of a battery when it is not delivering or receiving power.

P

Plates

Thin, flat structures composed of a grid and active material. The grid supports the active material and conducts electrons out of the cell; plates are positive or negative depending on the active material they hold.

Positive

Designating, or pertaining to, electrical potential: a point or terminal on a battery having higher relative electrical potential. The positive battery terminal is the point to which electrons flow during discharge; it is the opposite of the negative terminal.

Primary Battery

A battery that can store and deliver electrical energy but cannot be recharged. A lead-acid battery is not a primary battery.

R

Reserve Capacity Rating

The time in minutes that a new, fully charged battery at 27°C (80°F) will deliver 25 amperes while maintaining a terminal voltage of at least 1.75 volts per cell. This rating represents how long the battery will continue to operate essential accessories if the alternator or generator of a vehicle fails.

Resistance

The opposition to the free flow of current in a circuit or battery, commonly measured in ohms.

S

Sealed Battery

A common name for a VRLA battery, whose immobilized electrolyte and one-way pressure relief valves eliminate free liquid and the need for regular watering. See VRLA below.

Secondary Battery

A battery that can store and deliver electrical energy and can be recharged by passing direct current through it in a direction opposite to that of discharge. A lead-acid battery is a secondary battery.

Separator

A porous divider between the positive and negative plates in a cell that allows ionic current to pass through, but not electrical current. Separators are made from numerous materials, such as polyethylene, polyvinyl chloride, rubber, glass fiber and cellulose.

Short Circuit

An unintended current bypass in an electric device or wiring. Outside the battery, a short circuit occurs when a conductive path is established between the two terminals. Inside a battery, contact between the positive and negative plates discharges the cell and can render the battery useless.

Specific Gravity (Sp. Gr. or SG)

A measure of the electrolyte concentration in a battery, based on the density of the electrolyte compared with the density of water and typically determined with a hydrometer. By definition, the specific gravity of water is 1.00, and the electrolyte in a typical fully charged battery has a specific gravity of 1.265-1.285. Specific gravity readings are used to determine whether a battery is fully charged or has a bad cell.

Splash Barrel

Part of a battery's vent system. Its purpose is to keep acid out of the vents when the battery is upright, as acid is splashed around inside the cell by motion and vibration.

SLI (Starting, Lighting, Ignition) Battery

A rechargeable battery that supplies electric energy to an automobile to power the starter motor, the lights and the ignition system of the vehicle's engine.

State of Charge (SoC)

The amount of deliverable low-rate electrical energy stored in a battery at a given time, expressed as a percentage of the energy when fully charged and measured under the same discharge conditions. A fully charged battery is at 100 percent state of charge.

Stratification

The unequal concentration of electrolyte caused by density gradients from the bottom to the top of a cell. This condition occurs most often in batteries recharged from a deep discharge at constant voltage with little gassing. Continued deep cycling of a stratified battery softens the bottoms of the positive plates; equalization charging is a way to avoid acid stratification.

Sulfation

The generation or conversion of the lead sulfate discharge product in the plates to a state that resists normal recharging. Sulfation often develops when a battery is stored or cycled in a partially discharged state at warm temperatures.

T

Terminals

The electrical contacts on a battery to which the external circuit is connected. Batteries typically have top terminals (posts), side terminals, or both (dual terminal).

V

Vents

Mechanisms that allow gases to escape from the battery while retaining the electrolyte within the case. Flame-arresting vents contain porous disks that reduce the probability of an internal explosion caused by an external spark. Vents come in permanently fixed and removable designs.

Volt

The unit of measure for electrical potential or voltage.

Voltage Drop

The net difference in electrical potential (voltage) measured across a resistance or impedance. Its relationship to current is described by Ohm's law.

Voltmeter

An electronic device used to measure voltage, normally in digital format.

VRLA (Valve-Regulated Lead-Acid) Battery

A lead-acid battery with immobilized electrolyte - AGM and gel are the two types - that operates on the oxygen recombination cycle to minimize water loss. VRLA batteries feature one-way, low-pressure relief valves (below 3 psi) that prohibit air ingress while permitting gases to vent when necessary. The slight internal pressure facilitates the recombination of oxygen generated at the positive plates back into water.

W

Watt

The unit for measuring electrical power - the rate of doing work in moving electrons by, or against, an electrical potential. Watts = amperes x volts.

Watt-Hour (Wh)

The unit of measure for electrical energy, expressed as watts multiplied by hours.