Battery Definitions and Terms

Active material - Constituents of a cell that participate in the electrochemical charge/discharge reaction.

Battery - Two or more cells electrically connected to form a unit. Under common usage, the term "battery" also applies to a single cell.

Capacity - Number of ampere-hours (Ah) a fully charged cell or battery can deliver under specified conditions of discharge.

Cell - Basic electrochemical unit used to store electrical energy.

Current - Flow of electrons equal to one coulomb of charge per second, usually expressed in amperes (A).

Cutoff voltage - Cell or battery voltage at which the discharge is terminated. The cutoff voltage is specified by the manufacturer and is a function of discharge rate and temperature.

Cycle - The discharge and subsequent charge of a secondary battery such that it is restored to its fully charged state.

Duty cycle - Operating parameters of a cell or battery including factors such as charge and discharge rates, depth of discharge, cycle length, and length of time in the standby mode.

Electrode - Electrical conductor and the associated active materials at which an electrochemical reaction occurs. Also referred to as the positive and negative plates in a secondary cell.

Electrolysis - Chemical dissociation of water into hydrogen and oxygen gas caused by passage of an electrical current.

Electrolyte - Medium which provides the ion transport function between the positive and negative electrodes of a cell.

Equalizing charge - Charge applied to a battery which is greater than the normal float charge and is used to completely restore the active materials in the cell, bringing the cell float voltage and the specific gravity of the individual cells back to equal values.

Float charge - Method of charging in which a secondary cell is continuously connected to a constant-voltage supply that maintains the cell in a fully charged condition.

Gassing - Evolution of gas from one or more electrodes resulting from electrolysis of water during charge or from self-discharge. Significant gassing occurs when the battery is nearing the fully charged state while recharging or when the battery is on equalizing charge.

Potential difference - Work which must be done against electrical forces to move a unit charge from one point to the other, also known as electromotive force (EMF).

Primary cell or battery - Cell or battery which is not intended to be recharged and is discarded when the cell or battery has delivered its useful capacity.

Secondary battery - A battery that after discharge may be restored to its charged state by passage of an electrical current through the cell in the opposite direction to that of discharge. (Also called storage or rechargeable.)

Separator - Electrically insulating layer of material which physically separates electrodes of opposite polarity. Separators must be permeable to ions in the electrolyte and may also have the function of storing or immobilizing the electrolyte.

Specific gravity - Ratio of the weight of a solution to an equal volume of water at a specified temperature. Used as an indicator of the state of charge of a cell or battery.

Sulfation - Formation of lead sulfate crystals on the plates of a lead-acid battery.

Terminal - External electric connections of a cell or battery, also referred to as "terminal post" or "post."

Thermal runaway - A condition that occurs in a battery (especially valve-regulated types) when charging energy results in heat generation within the battery greater than the heat dissipated, causing an uncontrolled rise in battery temperature. This can cause failure through cell dry-out, shortened life, and/or melting of the battery.

Trickle charge - Method of charging in which a secondary cell is either continuously or intermittently connected to a constant current supply in order to maintain the cell in fully or nearly fully charged condition.

Voltage - Electromotive force or potential difference, expressed in volts (V).