Acid: A type of chemical that can release hydrogen ions when mixed with water. Sulfuric acid is used in a lead-acid battery.

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 that in the negative is metallic sponge lead. When an electrical circuit is created, these materials react with sulfuric acid during charging and discharging according to the following chemical reaction: PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O.

AGM (Absorbent Glass Mat):A type of non-woven separator material composed almost entirely of glass microfibers that absorbs and retains the electrolyte, leaving no free electrolyte in the cell to spill. VRLA batteries made with this material are often referred to as "AGM" batteries.

Ampere (Amp, A): The unit of measure of the electron flow rate, or current, through a circuit.

Ampere-Hour (Amp-Hrs, Ah): A unit of measure for a battery’s electrical storage capacity, obtained by multiplying the current in amperes by the time in hours of discharge. (Example: A battery that delivers 5 amperes for 20 hours delivers 5 amperes x 20 hours = 100 amp-hrs of capacity.)

Battery: One or more galvanic (electrochemical) cells electrically coupled into a single unit and equipped with attachments for external electrical connections.

Battery Charger: Device supplying electrical energy to a battery.

Boost Charge: The process of ensuring that the cells and plates within a battery are charged sufficiently for the battery to perform its desired function. Boost charging is typically done for a short duration at high current.

BCI Group: The Battery Council International (BCI) Group Number "fingerprints" a battery with the following characteristics: (a) dimensions (L x W x H), (b) voltage (6V or 12V), (c) polarity (right-hand front positive, left-hand front positive, etc.), (d) type terminals (top, side, "L", etc.). The BCI Group Number does not designate a battery’s capacity; it merely defines the above-listed physical characteristics.

Capacity: The capacity of a battery is specified as the number of amp-hrs that the battery will deliver at a specific discharge rate and temperature. The capacity of a battery is not a constant value and is seen to decrease with increasing discharge rate. The capacity of a battery is affected by a number of factors such as active material weight, density of the active material, adhesion of the active material to the grid, number, design and dimensions of plates, plate spacing, design of separators, specific gravity and quantity of available electrolyte, grid alloys, final limiting voltage, discharge rate, temperature, internal and external resistance, 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, electrolyte, separators and casing. In a lead-acid battery, the cell has an open-circuit voltage of approximately 2 volts. There are six cells in a 12-volt lead-acid battery.

Charge Acceptance:The quantity of current in ampere-hours which a battery in a defined charge state can accept at a specified temperature and charge voltage within a defined period.

Circuit: An electrical circuit is 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. A parallel arrangement of batteries (usually of like voltages and capacities) has all positive terminals connected to a conductor and all negative terminals connected to another conductor. If two 12-volt batteries of 50 ampere-hour capacity each are connected in parallel, the circuit voltage is 12 volts, and the ampere-hour capacity of the combination is 100 ampere-hours.

Circuit (Series): A circuit that has only one path for the flow of current. Batteries arranged in series are connected with negative of the first to positive of the second, negative of the second to positive of the third, etc. If two 12-volt batteries of 50 ampere-hours capacity each are connected in series, the circuit voltage is equal to the sum of the two battery voltages, or 24 volts, and the ampere-hour capacity of the combination is 50 ampere-hours.

CCA: Cold Cranking Amps is a rating used in the battery industry to define a battery’s ability to start an engine in cold temperatures. A great amount of amperes is needed to start the engine, but only for a short time.The actual rating is the number of amps that can be removed from a new fully charged battery at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts (for a 12-volt battery).As a battery ages with use, it may no longer be able to meet its original CCA rating. The higher the CCA rating, the greater the starting power of the battery.

Conductance: The ability to transmit current in a circuit or battery.

Container and Cover: The reservoir and lid containing the battery parts 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 a battery failure. Battery terminals are subject to corrosion if they are not properly maintained.

Current: The rate of flow of electricity, or the movement of electrons along a conductor. It is comparable to the flow of a stream of water. The unit of measure for current is the ampere.

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

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

Deep Discharge: State in which a cell is fully discharged using low current, so that the voltage falls below the final discharging voltage.

Deep-Cycle Battery: Battery that provides a steady amount of current over a long period of time, provides a surge when needed and is designed to be deeply discharged over and over again.

Discharging:When a battery is delivering current, it is said to be discharging.

Electrolyte: In a lead-acid battery, the electrolyte is sulfuric acid diluted with water. It 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 drawing large current loads.

Element:A set of positive and negative plates assembled with separators.

Equalization Charge: The process of ensuring that the cells and plates within a battery are all at full charge and that the electrolyte is uniform and free of stratification. This is normally done by charging the battery under controlled conditions (charge current, time and upper voltage limits are usually specified).

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

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 often referred to as gel batteries. Gel batteries are one typical type 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, the result of attaching one battery cable to the body or frame of a vehicle that is used as a path for completing a circuit in lieu of a direct wire from a component. Today, over 99 percent of automotive and LTV applications use the negative terminal of the battery as the ground.

Group Size: The Battery Council International (BCI) assigns numbers and letters for common battery types. There are standards for maximum container size, location and type of terminal and special container features.

Hydrometer: A device used to measure the strength (i.e., the concentration of sulfuric acid in the electrolyte) of the electrolyte through specific gravity of the electrolyte.

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

Lead-Acid Battery: Battery made up of plates, lead and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35 percent sulfuric acid and 65 percent water solution. This solution is called electrolyte, which causes a chemical reaction that produces electrons.

Load Tester: An instrument that draws current (discharges) from a battery using an electrical load while measuring voltage. It determines 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.

MCA (Marine): MCA is an industry rating defining a marine battery’s ability to deliver a large amount of amperage for a short period of time. Since marine batteries are typically never used at temperatures below freezing, marine cranking amps are measured at 32°F as opposed to 0°F for cold cranking amps. The rating is the number of amps that can be removed from a marine battery at 32°F for 30 seconds while maintaining a voltage of at least 7.2 volts for a 12-volt battery. 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.

Negative: Designating, or pertaining to, electrical potential. The negative battery terminal is the point from which electrons flow during discharge.

Ohm: A unit for measuring electrical resistance or impedance within an electrical circuit.

Ohm’s Law: Expresses the relationship between volts (V) and amperes (A) in an electrical circuit with resistance (R). It can be expressed as follows: V=IR. Volts (V) = amperes (I) x ohms (R). If any two of the three values are known, the third value can be calculated using the above equation.

Open-Circuit Voltage:The voltage of a battery when it is not delivering or receiving power.

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 either positive or negative, depending on the active material they hold.

Positive:Designating, or pertaining to, a kind of electrical potential; opposite of negative. 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.

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

Reserve Capacity Rating: The time in minutes that a new, fully charged battery will deliver 25 amperes at 27°C (80°F) and maintain a terminal voltage equal to, or higher than, 1.75 volts per cell. This rating represents the time 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. It is commonly measured in ohms.

Sealed Battery:See VRLA.

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 the flow of ionic current to pass through it, 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 is established when a conductive path is established between the two terminals of a battery. Inside a battery, a cell short circuit is the result of contact between the positive and negative plates and will cause a cell to discharge and render the battery useless.

Specific Gravity (Sp. Gr. or SG):Specific gravity is a measure of the electrolyte concentration in a battery. This measurement is based on the density of the electrolyte compared to the density of water and is typically determined by the use of a hydrometer (see Hydrometer). By definition, the specific gravity of water is 1.00 and the specific gravity of the sulfuric acid electrolyte in a typical fully charged battery is 1.265-1.285. Specific gravity measurements are typically used to determine if the battery is fully charged or if the battery has a bad cell.

Splash Barrel:A splash barrel is part of the battery's vent system. Its purpose is to keep acid out of the vents when the battery is upright as acid is splashed around in the cell from motion and vibration.

Starting, Lighting, Ignition (SLI) Battery:Rechargeable battery that supplies electric energy to an automobile to power the starter motor, the lights and the ignition system of a vehicle’s engine.

State of Charge (or State of Health): 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. If the battery is fully charged, the state of charge is said to be 100 percent.

Stratification: The unequal concentration of electrolyte due to density gradients from the bottom to the top of a cell. This condition is encountered most often in batteries recharged from a deep discharge at constant voltage without a great deal of gassing. Continued deep cycling of a stratified battery will soften 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 in the plates to a state that resists normal recharge. Sulfation often develops when a battery is stored or cycled in a partially discharged state at warm temperatures.

Terminals: The electrical structures on the battery to which the external circuit is connected. Typically, batteries have either top terminals (posts) or side terminals. Some batteries have both types of terminals (dual terminal).

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

Volt: The unit of measure for electrical potential or voltage.

Voltage Drop:The net difference in the electrical potential (voltage) when measured across resistance or impedance (ohms). Its relationship to current is described in Ohm’s law.

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

VRLA: Valve-regulated lead-acid battery. AGM and gel are the two types of VRLA batteries. These batteries have no “free” liquid electrolyte and in the cell operate on the oxygen recombination cycle, which is designed to minimize water loss. VRLA batteries feature vents that are one-way burp valves. These low-pressure burp valves prohibit air ingress to the cell while permitting gases to vent from the cell if necessary. The pressure maintained in the battery, though only very slight ('< 3-psi') is required to facilitate the oxygen generated at the positive plates back into water.

Watt: The unit for measuring electrical power, i.e., the rate of doing work, in moving electrons by, or against, an electrical potential. Formula: watts = amperes x volts.

Watt-Hour (Watt-Hrs, WH): The unit of measure for electrical energy expressed as watts x hours.