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Because sometimes understanding the terms we use isn’t easy, here is a very comprehensive collection of definitions and explanations of our vocabulary. You can scroll through the following pages or download a pdf version by (eventually) clicking here (440KB).   This isn't a text book or set of specifications so we have simplified some content in order to convey the concepts and ideas briefly and clearly.



  • Common name for a battery technology developed by the UScompany A123Systems, using Lithium Iron Phosphate (LiFe) material (
  • The cell is cylindrical in an aluminum canister and is very robust.  It has a nominal voltage of 3.3V and a charge voltage of 3.6V.
  • These batteries have a very fast charge time.  They can be charged to full capacity in 15 minutes or less with a charger capable of providing the input amperage and voltage.  Charging at these high rates seems to have little effect on the life of the pack.

see illustration under Battery 


  • Alternating Current.  An electric current that reverses direction in a circuit at regular intervals.
  • Brushless motors require an AC supply (produced by an ESC).


  • A device for measuring current,
  • Multimeters generally include an ammeter function, but are only suitable for low current values. 
  • Wattmeters and Emeters include ammeter functions that cover the range of most electric flight applications.
  • Ammeters are connected so that current flows through the meter (except for clamp ammeters).
  • Clamp ammeters are a specialty type used for high current applications.


see Current

Balance Connectors

  • A balance connector is one that provides electrical access to the individual cells in a battery pack.  This allows each cell to be individually monitored or its voltage adjusted.
  • There are no standards for balance connectors so the plug configurations vary between manufacturers. Thunder Power (TP), JST-XH and Hyperion (Polyquest) are three common and incompatible forms.



  • Balancing is the process that adjusts the fully charged voltage of each individual cell in a pack to the same value.
  • A ‘balancing charger’ will incorporate the process in its charge phase while a non-balancing charger will stop once pack voltage is achieved without paying any attention to individual cells.  ‘Balancers’ are available to perform the task independently of charging.
  • The balancing process is needed because cells have different manufacturing tolerances and behave differently during charge and discharge.  Balancing can extend the service life of a pack and improve its performance.


  • A connected group of electrochemical cells that store electric charges and generate direct current.  Cells are usually connected in series.  Sometimes a single cell e.g. a typical watch battery is a single lithium-manganese dioxide cell.
  • Battery Symbols used in circuit diagrams g3a

Battery Cycling. 

The process of fully charging and discharging a battery.

Battery Eliminator Circuitry (BEC)

  • A device that eliminates the need for a separate battery for receivers and servos (sometimes called the flight pack).
  • Usually part of an ESC but is completely separate from the circuitry which controls the speed of an electric motor. 
  • Draws from the motor battery to supply power to the receiver and servos,
  • Does not cut power to the motor when the battery voltage drops below a set level.  Low Voltage Cut-Off circuitry within the ESC controls this function.
  • A Linear BEC incorporate a linear voltage regulator so output to the externally fed devices (like Rx and servos) will remain constant as the supply battery voltage drops, until the device’s drop out voltage is reached.  A linear power supply usually dissipates excess power to regulate the output.
  • A Switched BEC switches very quickly between full-on and full-off states, which minimises wasted energy.  Voltage regulation is provided by varying the ratio of ‘on’ to ‘off’ time.
  • see also UBEC

Note that the ‘Best Practice’ section has information on using BECs.


  • A programmable feature built into many ESCs.
  • When power is removed from an electric motor, it could continue spinning a propeller, creating unwanted drag.  A brake works by electrically shorting the motor so that it stops.  A folding propeller can then fold into the air stream.

Brushed Motor

  • Traditional type of DC motor where carbon brushes make contact between the rotor (rotating part) and the stator (stationary part).
  • Very inefficient, typically in the region of 50-65%.
  • Only a brushed ESC can be used with a brushed motor.
  • Direction of rotation can be changed by swapping the two wires between the motor and ESC.


Brushless Motor

  • A 3 phase AC motor where there is no contact between the rotor and the stator.
  • A complex controller is required to make the motor spin with the correct timing.
  • More efficient than brushed motors, generally in the range of 85-92%, which means more battery energy is converted to useful form and less to heat.
  • Come in two types, inrunners and outrunners (see those entries for more details).
  • Direction of rotation can be changed by swapping any two of the three wires between the motor and ESC.g4b

Bullet Connector

  • Metal, cylindrical and either male or female.
  • Available in a range of sizes to accommodate different current ranges and are described by diameter with common sizes being 3.5, 4 and 6mm.
  • Typically used between motor and ESC but in high current applications may be used for battery connections.  g5


  • The maximum amount of energy a battery can store.  Expressed in Amp Hours (Ah), or milliAmp Hours (mAh), it defines the current that can be supplied from a battery for one hour.  C=I*t
  • For example, 3000mAh battery capacity would supply 3000mA (3amps) for one hour (60 minutes).  For a given battery, the current*time relationship is constant so the same battery would also allow 30A for 6 minutes, i.e. ten times the current in one tenth the time, or 15 A for 12 minutes.
  • In IC terms it is equivalent to the quantity of fuel in the tank. 

From top to bottom, illustration shows three LiPo packs, 4S, 3S and 2S; C rating of 18C, 20-30C and 40C; capacity of 2500, 2200 and 1050 mAh respectively.


C Rating

  • Specifies how quickly a battery can be charged or discharged.  C is the rate of current flow (amps) that will fully charge or discharge a cell in 1 hour
  • It's the same number as the cell capacity but expressed in mA not mAh. 
  • For LiPo, normal charge is 1C, fast charge would be 2*C or higher, and slow charge C/10, or trickle C/100.
  • For example, for the pictured 1050mAh battery, rated at 40C, charge current at 1C would be 1050mA (1.05 amps), and maximum discharge current would be 42 amps (40*1.05).


  • A compete unit capable of generating electricity.  1.5V AA and AAA ‘batteries’ are examples of NiCd or NiMH cells.
  • Often assembled into a battery pack such as in an RC Transmitter where 8 Nixx cells (each nominally 1.2V) are combined to give 9.6V pack voltage.
  • LiPo packs are provided in different configurations described by their cell count and connectivity.  For example, a 3S LiPo has three cells connected together in series, so the voltage is cumulative (three lots of 3.7V gives pack voltage of 11.1V).  They can also be connected in parallel so the capacity is cumulative.  For example, two cells of 2200mAh capacity in a 2P arrangement would give a pack capacity of 4400mAh.  If these cells were connected as 3S2P, pack voltage would be 11.1V and capacity 4400mAh.

Clamp Ammeter

A device for measuring current without interrupting the circuit.


  • Describes the movement of electrons through a wire or other conductor.
  • Measured in Amperes, often abbreviated to ‘Amps’ or ‘A’.
  • Symbol is ‘I’.
  • Some explanatory texts draw an analogy between electrical current and the flow of water through a pipe e.g.
  • Is synonymous with fuel flow rate in IC engines


  • Direct Current   An electric current flowing in one direction only.
  • Batteries produce Direct Current – see also ‘AC’.

Deans Connector

  • Deans Ultra Plug™ is a small, light, and efficient high-power two wire connector.
  • T configuration eliminates the possibility of reverse polarity connections.
  • Provides a secure connection for battery and ESC connections.
  • By convention female connectors are used on a battery’s supply side and male on the ESC.


Discharge Curve

  • Displays how cell voltage reduces over time, for a nominated discharge current.
  • Illustration shows typical curve for LiPo at different rates of discharge (expressed in terms of C)


Electronic Speed Control (ESC)

  • Electronic devices that function as a throttle for an electric motor.  They work by turning the power to the motor on and off at a high rate.  The motor ‘sees’ an average voltage based on the percentage of the time the power is on.
  • Come in two types, brushed and brushless.  A brushed ESC (two motor wires) can only be used with a brushed motor and a brushless ESC (three motor wires) can only be used with a brushless motor.
  • Additional circuitry in the ESC will cut power to the motor when the battery voltage drops to a pre-set level, to stop damaging the battery (Low Voltage Cut-Off).
  • May or may not have a BEC built in.



  • A purpose designed multifunction device that amongst other things, will measure and record current, voltage, power, and RPM data on the ground
  • Produced only by Hyperion.


Energy Density


Electric Power

Field Charger

A fast battery charger designed to work from a 12-volt power source, such as a car battery. 

Flight Pack

All of the radio equipment installed in the airplane, i.e., Receiver, Servos, Battery, Switch Harness. 

Folding Propeller

  • Designed so the blades will fold into the air stream when not under power
  • Under power, centrifugal force will deploy the blades to their operating position.
  • Blades are hinged on a hub whose size can be varied to match the width of the aircraft fuselage


  • Essentially slang, abbreviation for ‘Internal Combustion’, and used to categorise aircraft powered by internal combustion engines (as distinct from electric powered).
  • Also referred to as ‘wet fuel’

Inrunner Motors

  • A type of brushless motor.
  • The outer case is stationary and the central shaft spins.
  • Commonly used in high speed low torque applications like high speed aircraft, Electric Ducted Fans (EDFs), geared applications and helicopters.
  • May be used in conjunction with a reduction gearbox to trade off RPM for an increase in torque.



  • Kv (capital K, subscript v) expressed in RPM/Volt is an important motor constant that relates a motor’s RPM to the voltage of its power supply.
  • Mathematically, RPM= Kv*V
  • For example a 1000Kv motor can be expected to produce roughly 11,100 rpm with a 3S battery supply (11.1V).  A 500Kv motor would produce the same revs with a 6S battery (22.2V).
  • Also (wrongly) appearing as kV or KV.


  • Lithium-ion, chemistry used in rechargeable batteries.
  • Lighter in weight than Nixx.
  • Nominal voltage is 3.6 Volts per cell.  Maximum charged voltage is 4.1 Volts per cell. 
  • Generally used in portable devises like phones and laptops which don’t have high discharge rates.


  • Lithium Polymer, chemistry used in rechargeable batteries, derived from Lithium Ion batteries.
  • Compact and light, with the ability to offer 50% more power in less space than comparable Nixx packs.
  • Must be used with a LiPo-compatible charger.
  • Nominal voltage is 3.7 Volts per cell.  Maximum voltage is 4.2 Volts per cell.  Minimum voltage per cell before serious damage results is 3.0 Volts per cell under no load.
  • Can burst into fire if abused when charging.
  • Do not suffer any memory effect unlike NiCds.

Memory Effect

The capacity of a NiCd battery may reduce in a way that appears as if it remembers its last discharge level.


  • Prefix in the metric system denoting a factor of one thousandth.
  • Symbol ‘m’.
  • For example, 5000mAh is equal to 5Ah.

NiCd / NiCad

  • Nickel Cadmium, chemistry used in many commercial rechargeable batteries.
  • Nominal voltage of 1.2V per cell.
  • Heavy, have a low energy storage density compared to LiPo cells and suffer from memory effect.
  • No longer manufactured within Europe due to the poisonous effect of Cadmium, and have been replaced by either NiMH, Li-ion or LiPo cells/batteries.


Abbreviation for ‘both NiCd and NiMH’.


  • Nickel Metal Hydride, a chemistry used in rechargeable batteries, derived from Lithium Ion batteries.
  • Nominal voltage of 1.2V per cell.
  • Heavy, but have two to three times the energy storage density of NiCds.
  • Don’t suffer from memory effect.
  • More environmentally friendly than NiCds.

Ohms Law

Opto or Optoisolator 

  • Technology used in some ESCs for the connection between the receiver and the ESC.
  • Uses light instead of current to stop electrical noise from the motor flowing back into the receiver and causing interference.
  • Cannot be used in conjunction with a BEC.

Outrunner Motors

  • Design allows the outer body and shaft to rotate around the stationary (stator) windings. 
  • Particularly suited to high torque lower RPM applications, park fliers, general sports and scale models, they can efficiently turn large propellers at low RPMs.g12


  • When two or more electrical components are connected so that there is more than one path for current through the components.
  • For two similar cells connected in parallel, pack voltage is the same as for a single cell but pack capacity is the sum of the two cell capacities.
  • For example, two 1.2V 450mAh cells in parallel produces a pack voltage of 1.2V and pack capacity of 900mAh.


Peak Detection

  • Usually refers to a type of battery charger that automatically shuts off when a battery is fully charged.
  • NiCd cells, and to a lesser extent, NiMH cells, have a characteristic when being charged that their voltage rises rapidly when the cell is almost charged, and then starts to drop.  A peak detection charger detects this voltage peak, and stops charging.
  • LiPOs do not show the same characteristic and may be damaged if charged using this technique.


  • The rate of doing work. 
  • In electric flight, power is measured in watts.
  • Symbol is ‘P’.
  • The product of voltage (volts) and current (amps),
  • Mathematically, P=I*R.
  • In some applications, e.g. generators or transformers, power is rated in KVA, standing for Kilo Volt Amps.
  • In the IC domain, engine power is normally expressed in horsepower (HP), and 1HP is equivalent to about 750 Watts.

Prop Constant

(Propeller Constant)

  • Values which can be used to better relate motor output power to the prop rpm achieved.
  • A general rule for the power needed to drive a prop is:
    Watts = Const * rpm^3.0 * diameter^4 * pitch’.
  • A more accurate equation is:
    Watts = Prop Const * rpm ^ Power Factor
    where Prop Constant and Power Factor are specific to a particular propeller from a particular manufacturer.

Prop Adapter

  • The device used to attach a propeller to a motor.
  • Two types secure adapters to a motor shaft, either collet style or those using grub screws.  A third type is bolted to an outrunner motor’s body.g14g15



  • Electrically, the opposition to current.
  • Measured in Ohms, abbreviated to the Greek letter ‘Ω’
  • Symbol is ‘R’
  • Using a water analogy, it is a restriction in a water pipe.


Revs Per Minute per Volt.  See Kv.




  • An arrangement of the parts in an electric circuit whereby the whole current passes through each part without branching.
  • For two similar cells connected in series, pack voltage is sum of the two cell’s voltage but pack capacity is the same as for a single cell capacities.
  • For example, two 1.2V 450mAh cells in series produces a pack voltage of 2.4V and pack capacity of 450mAh


Speed Control

See ESC.

Suppression Capacitors

  • Capacitors fitted on brushed motors to reduce electrical noise generated by the carbon brushes’ contact with the motor commutator.
  • Not needed on brushless motors.

Switching BEC (SBEC)

See BEC.



Universal BEC (UBEC)

  • A separate device to an ESC which provides a BEC function.
  • Can be used in conjunction with an opto ESC (which has no BEC capability).
  • Provides BEC function when expected requirements may exceed an ESC’s internal BEC current or voltage rating
  • Also see BEC.


  • A device for measuring voltage.
  • Multimeters, Wattmeters and Emeters include a voltmeter function.
  • Voltmeters are connected ‘across’ a circuit to measure the difference in potential between two points.


  • In simple terms, voltage is a short name for the electrical force between two points that would drive an electric current between those points.
  • Measured in ‘Volts’.
  • Symbol is ‘V’.
  • Can be considered as the electrical equivalent of water pressure.  Without pressure, no water will flow, and hence, there will be no current.


(or Power Meter)

  • A device for measuring power by simultaneously measuring current and voltage.
  • Wattmeters are connected so that current flows through the meter in the same way as for clamp ammeters.



Unit of measurement for ‘Power’. 

Wire Gauge

  • Measure of the cross-sectional area of a wire.
  • The measure is important because ‘resistance’ of a wire is inversely proportional to its cross section (smaller wire, higher resistance).
  • A commonly used wire gauge system is American Wire Gauge (AWG).  In this system, the higher the gauge, the thinner the wire and low its current rating.
  • For example AWG 16 will handle 10-15 amps while AWG 12 is better in the 25-60 amp range.
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