Aircraft Battery Glossary

Ampere- Unit of electrical current.

Ampere-hour (Ah)- The capacity of a storage battery is measured in ampere-hours. One ampere hour is defined as a current flow of one ampere for a period of one hour. Five ampere-hours means a current flow of one ampere for five hours, a current flow of 2 1/2 ampere for 2 hours, or any multiple of current and time that will result in five. This relationship can be expressed as follows: Capacity (In ampere hours)= I*T. Where I is the current (in amperes) and T is the time (in hours). The capacity of a storage battery is usually based on a given discharge rate, since the capacity will vary with the rate of discharge. The capacity of an aircraft battery is usually based on a one hour discharge rate. A 17 ampere hour battery will supply a current of approximately 17 amperes for a period of one hour. A 34 ampere battery will deliver twice that amount of current over the same period of time. If a very heavy load is applied to the battery, it may become discharged in a few minutes.

Battery- A combination of two or more chemical cells electronically connected together to produce electric energy.(Common usage permits this designation to be applied also to a single cell used independently.)

Boost Charge- A charge applied to a battery which is already near a state of full charge, usually of short duration.

C rate- Discharge or current rate in amperes; numerically equal to rated capacity of a cell in ampere-hours.

Capacity- The quantity of electricity delivered by a battery under specified conditions, usually expressed in ampere-hours.

Capacity, rated- See nominal capacity.

Cell- An electrochemical device composed of positive and negative plates, separator, and electrolyte which is capable of storing electrical energy.

Cell reversal- Reversing of polarity within a cell in a multi-cell battery due to over discharge.

Charge- The conversion of electrical energy from an external source, into chemical energy within a cell or battery.

Charge Rate- The rate at which current is applied to a secondary cell or battery to restore its capacity.

Charge retention- The tendency of a charged cell or battery to resist self-discharge.

Concavo-concave- RG batteries have one-way cell vent valves designed to relieve excess positive internal pressure. Occasionally, when the atmospheric pressure is greater than the internal pressure of the battery, caused by rapid decrease in altitude, the battery case may become temporarily concave.

Constant Potential (CP) charge- Charging technique where the output voltage of the charge source is held constant and the current is limited only by the resistance of the battery.

Constant Current (CI) charge- Charging technique where the output current of the charge source is held constant.

Counter EMF- Voltage or a cell or battery opposing the voltage of the charging source.

Current- The rate of flow of electricity. The movement of electrons along a conductor. It is comparable to the flow of a stream of water. The unit of measurement is an ampere.

Cut off voltage- Battery voltage reached at the termination of a discharge. Also known as end point voltage or EPV.

Deep discharge- Withdrawal of 50% or more of the rated capacity of a cell or battery.

Depth of discharge- The portion of the nominal capacity from a cell or battery taken out during each discharge cycle, expressed in percent. Shallow depth of discharge is considered as 10% or less, deep depth of discharge is considered 50% or more.

Discharge- The conversion of the chemical energy of a cell or battery into electrical energy and withdrawal of the electrical energy into a load.

Discharge rate- The rate of current flow from a cell or battery.

Distilled Water- Water which has been freed of minerals by a process or vaporization and subsequent condensation.

Dry charge- Process by which the electrodes are formed and assembled in a charged state. The cell or battery is activated when the electrolyte is added.

Effective internal resistance Re- The apparent opposition to current within a battery that manifests itself as a drop in battery voltage proportional to the discharge current. Its value is dependent upon battery design, state-of-charge, temperature, and age.

Electrolyte- In a lead-acid battery, the electrolyte is sulfuric acid diluted with water. It is a conductor and is also a supplier of hydrogen and sulfate ions for the reaction.

Electromotive Force (EMF)- Potential causing electricity to flow in a closed circuit.

Electron- That part of an atom having a negative charge.

End-of-discharge voltage- The voltage of the battery at the termination of a discharge but before the discharge is stopped. See Cut off voltage.

End-of-life- The stage at which the battery or cell meet specific failure criteria.

End-point voltage- Cell or battery voltage at which point the rated discharge capacity had been delivered at a specified rate-of-discharge. Also used to specify the cell or battery voltage below which the connected equipment will not operate or below which operation is not recommended. Sometimes called cutoff voltage or voltage end point.

Entrainment- The process whereby gases generated in the cell carry electrolyte through the vent cap.

Fast charging- Rapid return of energy to a battery at the C rate or more.

Float charge- A method of maintaining a cell or battery in a charged condition by continuous, long-term, constant voltage charging at level sufficient to balance self-discharge.

Flooded cell- A cell design which incorporates an excess amount of electrolyte, also see Vented Cell.

Gassing- The evolution of gas from one or more of the electrodes in a cell. Gassing commonly results from local action (self discharge) or from the electrolysis of water in the electrolyte during charging.

Ground- In aircraft use, the result of attaching one battery cable to the body or airframe which is used as a path for completing a circuit in lieu of a direct wire from a component.

Hydrometer- A float type instrument used to determine the state-of-charge of a battery by measuring the specific gravity of the electrolyte (i.e. the amount of sulfuric acid in the electrolyte).

Internal impedance- The opposition to the flow of an alternating current at a particular frequency in a cell or battery at a specified state-of-charge and temperature.

Internal Resistance- The opposition or resistance to the flow of a direct electric current within a cell or battery; the sum of the ionic and electronic resistance of the cell components. Its value may vary with the current, state-of-charge, temperature, and age. With an extremely heavy load, such as an engine starter, the cell voltage may drop to approximately 1.6. This voltage drop is due to the internal resistance of the cell. A cell that is partly discharged has a higher internal resistance than a fully charged cell, hence it will have a greater voltage drop under the same load. This internal resistance is due to the accumulation of lead sulfate on the plates. The lead sulfate reduces the amount of active material exposed to the electrolyte, hence it deters the chemical action and interferes with the current flow.

Ion- Part of a molecule or group of atoms, positively or negatively charged, which transports electricity through the electrolyte.

Joules- Unit of energy, equal to a watt/second (newton/meter).

Lead-acid- Terms used in conjunction with a cell or battery that utilizes lead and lead peroxide as the active plate materials in a diluted electrolyte solution of sulfuric acid and water. Nominal cell voltage about 2.1 volts.

Lead dioxide- A higher oxide of lead present in charged positive plates and frequently referred to as lead peroxide.

Lead sulfate- A lead salt formed by the action of sulfuric acid on lead oxide during paste mixing and formation. It is also formed electrochemically when a battery is discharged.

Load tester- An instrument which measures the battery voltage with an electrical load on the battery to determine its overall condition and its ability to perform under engine starting conditions or essential power requirements.

Nominal capacity- A designation by the battery manufacturer which helps identify a particular cell model and also provides an approximation of capacity; usually expressed in ampere-hours at a given discharge current.

Nominal voltage- Voltage of a fully charged cell or battery when delivering rated capacity at a specified discharge rate.

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

Overcharge- The forcing of current through a cell after all the active material has been converted to the charged state. In other words, charging continued after 100% state-of-charge is achieved. The result will be the decomposition of water in the electrolyte into hydrogen and oxygen gas.

Oxygen recombination- The process by which oxygen generated at the positive plate during charge reacts with the pure lead material of the negative plate and in the presence of sulfuric acid and reforms water.

Parallel connection- A circuit in which battery poles of like polarity are connected to a common conductor.

Polarity- The electrical term used to denote the voltage relationship to a reference potential (+).

Power- Rate at which energy is released or consumed (expressed in watts).

Rated Capacity- The number of Ahs a battery can deliver under specific conditions (rate of discharge, end voltage, temperature).

Recombination- State in which the hydrogen and oxygen gases normally formed within the battery cell during charging are recombined to form water.

Re-sealable- In a cell, pertains to a safety vent valve which is capable of closing after each pressure release, in contrast to the non-resealable vent cap.

Sealed cell- Cells that are free from routine maintenance and can be operated without regard to position.

Self discharge- The decrease in the state-of-charge of a cell or a battery, over a period of time, due to the internal electro-chemical losses.

Separator- A porous, insulating material placed between plates of opposite polarities to prevent internal short circuits.

Specific Gravity (S.G.)- The weight of the electrolyte is compared to the weight of an equal volume of pure water, used to measure the strength or percentage of sulfuric acid in the electrolyte.

Starved cell- A cell containing little or no free fluid electrolyte solution; this enables gases to reach electrode surfaces readily, and permits relative high rates of recombination.

State-of-charge (SOC)- The available ampere-hours in a battery at any given time. State-of-charge is determined by the amount of sulfuric acid remaining in the electrolyte (S.G.) at the time of testing or by the stabilized open-circuit voltage.

Sulfation- In its common usage, the term refers to the formation of lead sulfate of such physical properties that it is extremely difficult, if not impossible, to reconvert it to active material.

Swelling- RG Battery cases swell or bulge when the cell vent valves maintain an internal pressure that is greater than the outer (atmospheric) pressure.

Trickle charge- A continuous, low rate charge, the rate being just about sufficient to compensate for self-discharge losses.

Vent valve- A normally sealed mechanism which allows the controlled escape of gases from within a cell.

Vent cap- The plug on top of a cell. It can be removed to allow for electrolyte level adjustments.

Venting- A release of gas either controlled (through a vent) or accidental.

Volt- Unit of electromotive force, voltage or potential. The volt is the voltage between two points of a conductor carrying a constant current of one ampere, when the power dissipated between these points is one watt.