Electric Flight

Electric Motor Powered Model Aircraft

The current generation of electric motor powered model aircraft have similar flying performances to I.C. engine, diesel, glowplug and petrol powered aircraft. The electric  motors currently available, and suitable  transistorised controllers and Lithium Polymer batteries were simply not available a few years ago.  The main benefit of electric powered models is their quietness and cleanliness in operation when compared to I.C. engine powered model aircraft.

“TECH SPECS” of  Member’s Electric Power Projects

The club has many members who fly electric powered “trainer” planes at Wrayford field. Some models have been converted from .40 sized glow engine power by installing powerful “outrunner” electric motors, and Lithium Polymer batteries to supply the electric power.

Nigel Rollason converted an old Boomerang Mk1 trainer, which was originally fitted with an SC 40 engine. This was an ebay “loft find” which had a 35 Mhz receiver, a nicad battery and servos installed. The fin and tail surfaces were damaged, along with a few holes in the wing covering. A little used outrunner motor and speed controller were also acquired quite cheaply from ebay.
A separate receiver battery was fitted alongside the Lipo battery in the nose to achieve the correct centre of gravity, instead of adding lead weight. This is preferred, because if the speed controller battery eliminator fails, (BEC), the receiver and servos still  receive independent power to maintain flying control to land the plane as a Glider…… The Multiplex radio control equipment and Overlander Lithium Polymer batteries were purchased from Phoenix Model Products

Radio Control Equipment used: 

Transmitter      : Multiplex 2.4 Ghz Royal Pro, 7 channel 2.4 Ghz with telemetry

Receiver          : Multiplex 7 channel dual receiver with telemetry

GPS module    : Multiplex programmable GPS telemetry unit which plugs into the receiver.

Rx Power        : Overlander Eneloop 4.8 volt (4x AA ) battery for receiver & servo power.

The battery access hatch is fitted into the top of the former fuel tank cavity behind the engine mounting bulkhead. Another mounting for the 42/40-10 Overlander motor was fitted 50mm further forwards.

The JP EnerG Pro 60 amp speed controller is attached to the right side with Velcro and the Overlander Eneloop 4.8 volt NiMh battery for the receiver and servos is mounted on the left side.

The Overlander 4250 Mah Lithium Polymer motor battery is mounted in between,  on a 6mm thick balsa plate with Velcro fastenings. The dual Multiplex 2.4 Ghz aerials are slid up plastic tubes to keep them in the optimum transmit and recieve orientation.

Electric Power Specification:

Electric Outrunner Motor: Overlander 42/40-10, 890Kv

Speed Controller              : JP Ener G pro 60 Amp (BEC not used)

Lithium Polymer Battery : 1x Overlander 4250 Mah, 350C 4s (4cell, 14.8 Volts)

Propeller                             : APC 12” x 6”E

Alternative:  Electric flight wooden  12” x 7” E prop

Lipo Batteries have different weights according to their capacity and “C” rating. The higher the “C” rating the heavier the battery and the grater the Mah, the larger is the physical size. The original pair of 60C 3200Mah 2s batteries in series, to give 14.8 volts weighed 415 grammes,  You will notice 115 grammes of lead weights stuck to the lighter 3200Mah 30C 4s, 14.8 volt Lipo battery to make it heavier at 416 grammes. The blue Overlander 4250 Mah 30C, 4s 14.8 volt battery weighs 419 grammes. This is so the Centre of Gravity and all trim adjustments will remain the same when different batteries are used.

Martin Jones converted an Irvine Tutor Mk1 trainer to electric power, which is the same size of model as the Boomerang and almost identical, but with different graphics on the heat shrink covering. Both airframes may have been made on the same production line ?

Martin converted his model from the original tricycle undercarriage to a taildragger. He created a battery access hatch on the underside of the former fuel tank cavity.  As Martin has used the battery eliminator cable (BEC) instead of fitting a separate receiver battery, he had to add lead in the nose to achieve the correct centre of gravity.

However,  He must, NEVER overload the speed controller or the BEC, because if these fail, all power will be lost to the receiver and servos, with predictable results when contact is made with Terra Firma.

The complete “powertrain” comprising the Outrunner electric motor, speed controller, Lithium Polymer batteries, special charger and propeller, was recommended and supplied by Phoenix Model Products.  

Radio Control Equipment used:

Transmitter      : Spektrum DX6i 2.4 Ghz

Receiver          : Spektrum AR500

Rx Power        : Speed Controller with BEC. (Battery Eliminator Cable)

Electric Power Specification:

Electric Outrunner Motor        : Overlander 35/48-05, 900Kv

Speed Controller                     : Overlander 60 Amp with BEC

Lithium Polymer Battery      : 1x Overlander 3250Mah 25C 4s (4cell, 14.8 volts) (Good performance)

Ditto——————-            : 1x Overlander 2200Mah 25C 3s (3cell, 11.1 volts) (Poor performance)

Propeller                                  : APC 12” x 8”E

Flying Performance:

Both Trainer planes fly very well and are extremely quiet, Take offs are easy with quick acceleration on our grass flying patch as the motors achieve maximum revs almost instantly when the throttle control is fully advanced.

Martin Jones’s Irvine Tutor is capable of a good aerobatic performance with the 4s, 14.8 volt LiPo and uphill take-offs are no problem, even with little or no wind.

Nigel Rollason’s electric powered Boomerang has more power and a better flying performance than his “Arising Star” trainer powered by an RMX 40 I.C. glow engine, which has been in use for over six years as a primary flying trainer for novices, using a buddy box dual control system.

All of these robust “trainer” planes are similar in size, and weigh about 3 Kilogrammes ( 6.5 to 7 lbs)

The 42/40 Overlander motor draws about 50 amps at full power, with an APC 12”x 6”E propeller fitted and the model is restrained. On a fully charged 4s battery at 16 volts this equates to 800 watts of power, (16 x 50= 800) which is just over one horsepower !

This explains why the Boomerang flies as if it has a .60 (10cc) sized glow engine fitted up front. It can perform the full BMFA “B” aerobatic schedule as well as and most other aerobatic manoeuvres, But Quietly!

The only downside is the short flying duration of about six minutes when flying aerobatics. Gentle cruising around will give about 8 minutes of flying time with sufficient reserve for safe landings, including any “go-arounds”.

Both models are similar in this respect. However, there are two things we can both do to improve the flight durations of our models.

1: Improve our energy management techniques when flying aerobatics.

2: Fit the larger Overlander 4250 Mah 35C, 4s Lipo which fits in the space available, and is only 5 grammes heavier than a pair of Overlander 3200 Mah 60C, 2s Lipo batteries connected in series. This provides another couple of minutes of useful flying time.

Martin may then be able to remove the lead weights in the nose, and/or, fit a separate receiver battery.  Nevertheless, we are both very pleased with the results of our electrical calculations and the flying performance of our “experimental”models.   

To help you understand modern Electric Motor Technology, here are a series of excellent articles written by Stan Yeo, of Phoenix Model Products.  


Definition of Terms
Ohms Law
Power (watts)
Selecting the Battery, Speed Controller and Motor
Brushless Motors
Electronic Speed Controller (ESC)
Electric Flight Packs
Converting IC Model to Electric
Useful Tools
Noise / Interference Suppression

For a number of modellers electrics is a black art that induces a mental block when the word is mentioned. This is unfortunate as the basics are simple and easy to understand if presented in a digestible form. This is the challenge of this article but before starting an apology to the purists as some of the terminology may not be academically pure.

Definition of Terms (Back to top)

Volts (V) This is a measurement of electrical pressure the equivalent of air pressure in a car tyre.

Amps (I or A) This is a measure of the amount of electricity (current) flowing in a circuit. In non electrical terms gallons or litres per minute.

Resistance (R or Ohms) This is a measure of electrical resistance measured in Ohms. This is equivalent to drag on a model or the force required to push/pull an object on a flat surface.

Impedance (Ohms) This is a measure of resistance in an alternating current( AC) circuit.

Watts (W) This is a measure of electrical power, i.e. the equivalent of the horse power developed by an engine. 748 watts = 1 horsepower. The metric equivalent of horsepower is PS or Pferdestarke. 1HP = 1.0139 PS. For the purposes of this article 1 PS = 1 HP = 750 watts.

Ohms Law (Back to top)

The basics of Ohms law is that if you increase the voltage (pressure) then more electricity (amps) will flow in the circuit. A practical example of this is if you are watering the garden and turn the tap on more i.e. increase the pressure more water flows out the end of the hose. A practical modelling example is using a 5 cell (6v) receiver battery pack instead of the usual 4 cell (4.8v) pack will not only increase servo torque but increase current consumption by 25%, which is why a larger capacity battery should always be used.

Ohms law is very easy to use. Just draw a triangle, divide it horizontally in two then divide the bottom half vertically in two. In the top half write ‘V’. In the bottom half write ‘I’ and ‘R’.

If you know two of the three variables block out the third and what is left is the formula for the unknown variable i.e. R = V/I, V = I x R and I = V / R

Power (watts) (Back to top)

The formula for power is even simpler it is just

Watts = Volts x Amps.

Selecting the Battery, Speed Controller and Motor (Back to top)

Selecting the right battery, speed controller (ESC) and motor combination are very critical when compared to choosing the equivalent IC motor. There are a number of factors to be taken into account which have a big impact on performance i.e. propeller, motor windings, ESC rating and battery type / capacity / current rating. Each will be discussed in turn but before we start there are two basic rules for the novice electric flyer. One, if you know the current required i.e. current being drawn by the motor, multiply it by 1.5 when selecting the battery / ESC. Secondly if you know the current rating of the ESC / battery divide by 1.5 to select a safe operating current.

Brushless Motors (Back to top)

Currently there is very little in common in the way brushless motors are classified but most manufacturers supply three important pieces of information. These are RPM per volt (kV), maximum output power in watts and maximum current draw. All three should be used in selecting the optimum propeller. The RPM per volt (kV) is useful in determining the NO load motor RPM, the actual motor RPM acheived will vary with propellor loading. Just multiply the kV by the battery voltage under load to achieve the motor revs per minute (RPM). Unless the motor is to power a ducted fan model or drive an electric helicopter avoid high kV motors. A typical kV range for non-ducted fan, fixed wing models is 800 to 1200. Unlike IC motors, the efficiency / power output graph of an electric motor is very narrow. To small a propeller = less thrust than the motor is capable of. Too large a propeller = less thrust and increased current draw / shorter flight times with the added danger of damaging the motor, ESC and battery. An indication of how hard the ‘electrics’ are working is how hot they get, assuming they are adequately ventilated. They should get warm but not hot. This means choosing the right propeller is critical for optimum performance and may require trying a selection of propellers before finding the one most suitable for that particular model.

If you increase the battery voltage i.e. cell count and the motor is operating at maximum recommended power then the propeller must be changed for one with less pitch and or reduced diameter. Remember power (watts) is the multiple of Voltage x Amps so increasing the voltage means that you must reduce the current (Amps) to stay within the maximum permitted power (Watts) to avoid damaging the motor. One reason why modellers increase battery voltage is to reduce current consumption to either increase flight times or overcome a battery’s inability to deliver the current required to produce maximum power. Very occasionally motor information is available showing thrust per watt for a range of battery / propeller combinations to assist in choosing the right one.

To assist in choosing a suitable propellor we have produced a propellor loading chart for a wide range of pitches and diameters. Use the propellor recommended for initial flights. Obtain the loading for that propellor then select a suitable propellor to either increase or reduce the load as required.

A final point for consideration when selecting a motor is the that maximum power rating (watts) is usually only attainable on the maximum battery voltage i.e. max cell count due to the current considerations mentioned above. This means that when selecting your motor you must multiply the max. current rating of the motor by the voltage of the battery you intend using to determine the power it will produce with the correct propellor. As an example, if the max. power of a motor is 500w, max current is 40A and max voltage is 14v (4 cell LiPo) it can only produce 500w at 14.8v. On a 3 cell LiPo pack (11.1v) it will only produce 444w due to the current limit of 40A (11.1 x 40). On a 2 cell pack (7.4v) it will on be 296w (7.4 x 40).

Electronic Speed Controller (ESC) (Back to top)

There are two types of speed controller, one for brushless motors and one for brushed motors. The brushed ESC has two wires to connect to the motor whilst the brushless speed controller has three. The reason for this is that for the motor to work electrical power to the motor windings have to be switched on and off in sequence. In the brushed motor this is done mechanically by the commutater and the brushes. With a brushless motor the switching is done electronically by the speed controller hence the need for three wires. As an aside if a brushless motor does not work or runs in reverse when switched on try swopping the position of the three wires before panicking! Most ESCs have a BEC (Battery Eliminator Circuit) to power the receiver and servos. Often there are restrictions on the use of the BEC i.e. the number of servos it can drive and the battery voltage (number of cells) it can be used with. If the BEC current is not enough to drive the number of servos fitted in the model then we recommend the use of a UBEC or using a separate receiver battery. The UBEC reduces the flight pack battery voltage to 5v and is connected directly to the battery.

If using more than one speed controller i.e. in a multi motor setup then the positive lead on all but one speed controller must be disconnected to prevent them ‘fighting’ each other to supply the Rx with electrical power. The same applies if using a UBEC or separate Rx battery. Also, if possible, to minimise motor to motor interference on a multi motor setup it is advisable not to use a ‘Y’ lead to link two ESCs together but use a spare channel and link the channels using a mixer in the transmitter.

As well as controlling the speed of the motor ESCs are also responsible for switching the motor off when the battery voltage is low. This is to prevent the battery being discharged below a safe level (particularly important when using LiPo batteries) and to reserve sufficient energy in the battery to allow the model to be landed safely. To do this there are two types of motor cutoff systems, one that works on a percentage of the battery start voltage and one the that cuts the motor off at a preset voltage. The latter is the better system as with the percentage cutoff system you must always start with a fully charged battery, particularly if using LiPos, otherwise there is a danger of discharging the battery below safe limits and ruining the battery / crashing the model due to radio failure. Most current ESCs either allow you to preset the cutoff voltage or automatically determine the number of cells present and set the cutoff voltage accordingly. Read the instructions!

When selecting the ESC use the 1.5 rule to determine the current rating i.e. if the max current draw on the motor is 30 Amps use a 45 Amp ESC. The harder the ESC works the less efficient it is.

Electric Flight Packs (Back to top)

There are basically two types of electric flight packs on offer currently although more are under development. These are the now dated nickel cadmium / nickel metal hydride batteries and the class commonly known as Lithium Polymer (LiPos). LiPos. These are approximately 40% lighter than Nicad / NimH batteries and require careful handling. LiPo battery packs from reputable sources normally come with an extensive list of does and don’ts and I strongly recommend that these are read. The important points to remember are:

1. Always charge the batteries on a concrete floor or large ceramic tile. Avoid charging if possible inside domestic accommodation, charge in the garage or outdoors. If something does go wrong during the charge and the batteries self ignite they can easily cause a major fire.

2. Regularly check the batteries when on charge. A typical from flat charge time is approximately 1.5 hours.

3. Always charge through a dedicated LiPo charger and cell balancer. Balance charging is where each cell is charged individually. Failure to regularly balance the cells can result in overcharging some cells and over discharging others. In either case the battery will be irreparably damaged.

4. Regularly check the battery for damage and puffing out. Any signs of either discard the battery.

4. DO NOT charge LiPo batteries from the car battery with the engine running. Electrical spikes from the alternator, particularly when starting the engine, can damage the charger or at the very least befuddle the micro processor in the charger resulting in the charger malfuctioning and damaging the battery.

As with the speed controller select the battery capacity using the 1.5 rule i.e. if the max motor current is 30A then select a battery capable of delivering a constant current of 45A. To determine the max. constant current draw of a LiPo battery, multiply the battery capacity by the battery’s ‘C’ rating. If a recommended max. current draw is printed on the battery label then use this. To determine the battery capacity simply divide the battery max. current by the max. motor current. Most current LiPos have a ‘C’ rating of 20C. In the example above the minimum battery capacity would be 45A (30Ax1.5) / 20C = 2250mAhr. The reason for using the 1.5 rule with LiPos is that there is a direct relationship between current draw and the number of battery life cycles. The higher the current draw the lower the number of battery life cycles. If the battery does not have a ‘C’ rating printed on the label as is the case with some direct imports please make an effort the find the information. If it is not available, for safety reasons, assume a value of 50% of the current norm i.e. 10C. It could be old stock!

One final point, as a general rule the higher the ‘C’ rating of a battery for a given capacity the longer the flight time due to the battery’s better performance under load. Remember the higher the load the lower the battery voltage, the sooner the speed controller cuts the motor. The battery could only be half discharged but if the battery voltage under load drops below the cut-off voltage then the ESC will cut the motor.

Nicad and NiMh batteries should have a recommended maximum discharge current on the label. Again select the battery using the 1.5 rule. The problem with these batteries is that the higher the discharge current then the greater the internal voltage drop due to the internal resistance of the battery. In other words the harder the battery is driven the less efficient it is.

Converting IC Model to Electric (Back to top)

The key to converting an IC model to electric flight is knowing the power the I.C. engine is developing in flight. This is best done measuring the RPM of the motor at full throttle and reading the power output off the RPM power curve for the recorded RPM. This not easy as the RPM Power output curves are not always readily available. If this is the case then an educated guess must be made based on the published maximum power output of the engine. This, in reality, is a hypothetical figure as the RPM at which it is produced is unrealistically high necessitating the use of a very small propeller that would make a lot of noise! In reality the power the engine is developing is probably only 60% of its max. quoted power. If we take a 0.25cu in engine with a max power output of 0.66HP then the equivalent electrical power produced would be 0.66 x 0.6 = 0.396hp i.e. 296 watts (0.396 x 748). Using an 8 cell 9.6 volt sub C battery back this would equate to a maximum current of 296 / 9.6 = 30.8 amps. This of course is subject to an optimum propeller / motor combination as previously discussed.

We have recently converted one of our EPP Peppi Trainers to electric. Originally it was fitted with and OS 25FP and climbed at about 60 – 70 degrees under full power. It is now fitted with a Twister 09, which is equivalent to a 0.25cu in. IC motor, and a Tornado 40 amp ESC. The climb under full power is similar. For initial flights an 11 x 6in propeller was fitted but the speed controller got very hot (it melted the heatshrink) so we fitted a 10 x 6 in propeller instead. With the 10 x 6 propeller the ESC and battery just got comfortably warm, flight times increased by more than 50% and there was a marginal increase in performance, thus emphasising the need for good propeller / motor selection. Dividing the battery capacity by estimated full power duration time (the model was not flown on full power all the time) would suggest a maximum current draw of between 30 and 35 amps which is conveniently in line with our rough calculations. They have also been tested on other models and with similar results.

Useful Tools (Back to top)

The two most useful readily available tools for electric flyers are a Tachometer and a Watt Meter. The tachometer is used to measure propeller RPM allowing it to be compared with the expected RPM achieved by multiplying the motor kV (revs per volt) by battery voltage whilst the watt meter measures the power being consumed. A third useful piece of equipment is some means of measuring the thrust produced. This could be a sophisticated rig on which the motor is mounted with a high capacity battery used as a stable power supply or something as simple as a tethered spring balance attached to rear of the model. Once the thrust is known this can be used to calculate the thrust per watt for a given propeller. The thrust is measured using a variety of propellers to determine the one with the best thrust per watt ratio for that model.

Noise / Interference Suppression (Back to top)

All electric motors and electronic switching devices generate electrical noise, some more than others. In radio control systems this often results ‘glitching’ i.e. un-commanded servo movement.

To minimise this risk there are a number of precautions that can be taken.

1. Keep the leads from the speed controller (ESC) to the motor to less than 100mm (4 inches).

2. If using a brushed motor fit additional noise suppression capacitors. There should be three, one from each terminal to earth (case) and a third across the two terminals. The largest value capacitor is the one fitted across the two terminals.

3. The lead from the ESC to the receiver should be fitted with a torrodial or noise suppression choke. The lead, with the plastic end, removed is wound around the choke at least 4 times close to the receiver end. The more turns the better the noise suppression.

4. Is using a UBEC instead of the ESC onboard BEC (battery eliminator circuit) fit noise suppression chokes as above and carry out a range check. Some makes have been known to significantly reduce range without a choke being fitted.

5. Where possible install the receiver as far away as practical from the motor / ESC.

6. If you are experiencing ‘glitching’ in flight then we recommend changing the receiver to one of a higher specification. If single conversion try a dual conversion one. If dual conversion try either a PCM Rx or an Rx with IPD. Both have built in signal verification systems i.e. they check the information the Rx has received has not been corrupted before it is passed on to the servos.

7. If 6 fails then try a different make speed controller or try one with an higher current rating.

8. If 7 fails then try a separate Rx battery but remeber to disable the BEC by to removing the positive lead from the ESC.

9. Finally if after trying all the above you still have problems try changing the battery, possibly for one of a higher capacity, and by a process of elimination try and identify the noisy component.

Summary (Back to top)

I hope you have found this article useful and informative. I have erred on the safe side, kept it as simple as possible and made a number of assumptions but if you understand the basics then it should be easier to put the information to practical use.

Propeller Loading Comparison ChartThis link is very useful when comparing different poropellers. Use a larger diameter with less pitch for a better take-off and climb performance. Use more pitch with a smaller diameter to fly faster with a similar engine loading. Remember that maximum static revs on the ground may not give you the best flying performance. Prop for maximum torque rpm on the ground and the motor will unload and give more rpm when in flight.


 First On Last Off 

Nothing to do with the working environment but the sequence of switching the transmitter and receiver on and off with particular reference to electric models. Since discussing the topic in an earlier newsletter we have heard of a number of similar incidents. Fortunately none so serious as the one discussed, even so it is worth re-iterating the message.  The transmitter must always be switched on first, after checking the throttle is in the low throttle position, and only switched off AFTER the receiver / flight pack has been switched off or disconnected. However, during the building phase care must be taken when installing the electrics and setting up the model. We recommend the control surfaces are set up using an external receiver battery without the motor electrics (motor plus ESC) being connected. We also recommend that the throttle fail-safe is also set at the same time using a servo in the throttle channel to check that it is functioning correctly. When the time comes to connect up the power train it is advisable to check throttle / motor operation minus the propeller. If motor the motor sits there and does nothing or runs in reverse then the most probable cause, assuming the motor and ESC are serviceable, is either a bad connection in the three wires connecting the motor to the ESC or they are in the wrong order. Check there is good contact in the bullet connectors between the ESC and the motor. If the motor is running in reverse swap any 2 motor connections. Lastly, always remember to stand behind a rotating prop and take note of what is in front and behind the model that could be easily blown away or sucked into the propeller!

 Receiver Aerial Positioning

On 35Mhz we never worried too much about the positioning of the receiver aerial in our models except that it was not curled up! We did however encounter interference problems with carbon fuselages and electric power trains. The situation however is different with the positioning of 2.4Ghz Rx aerials. The transmitted signal on 2.4Ghz is line of sight and is easily blocked by obstacles in its path. This includes fellow modellers standing front of the transmitter being used controlling a model in flight. It is all about the presentation of the receiver aerial(s) to the Tx so please follow the instructions for your receiver. We suggest the Rx aerials are kept as far away as possible in the model from solid lumps such as flight packs and motors that are likely to block the Tx signal. If your telemetry system has the facility to send back transmitted signal strength use this to optimise your Rx aerial(s) position by doing 360 degree turns at varying distances and noting the TX signal reading.

  Small LiPo Battery Charging

A useful tip for charging small LiPo batteries. One of our customers was experiencing problems with his single cell LiPos. Despite charging the cells as per the instructions his flight times were virtually none existent (recommended charge rate was 1.4C on a 12C battery!). Research on the forums suggested he charge the batteries at 0.6C. This he did and the result was normal flight times. Small batteries with low ‘C’ ratings have a high internal resistance. This results in a higher terminal voltage on a constant current charge. This fools the charger into thinking the battery is fully charged so the charger terminates the charge (LiPo chargers cutoff when cell voltage reaches 4.2v).

 Soldering Tips

Occasionally customers seek our advice on a modelling problem. One regularly discussed is soldering. Changes in the material used for the manufacture of metal cable adapters etc. and new health and safety regulations concerning the use of lead are often the cause of the problem. Certain nickel alloys are difficult to solder, particularly if using lead free solder so here are a few tips. Firstly in addition to the reasons given above there are other possible causes for a poor solder joint. One is dirt / grease contamination another is lack of heat i.e. using too small a soldering problems plus poor technique. Possible solutions are:

Use a lead alloy solder. It is still available!

  1. Use an ‘active’ flux such as FRY Power Flux available from plumbers merchants.
  2. Ensure the joint to be made is scrupulously clean and free from grease.
  3. Use a larger wattage soldering iron or larger solder iron bit.
  4. Freshly clean and ‘tin’ the soldering iron tip, leaving a small puddle on solder on the tip to aid heat transfer.

When soldering Deans connectors we recommend that they are soldered as an assembled pair, using pliers and rubber band hold them in place whilst the joint is made to avoid the heat distorting the plug. When soldering plugs to Flight Packs great care must be taken to avoid them shorting out. With gold connectors solder the female half of the plug first and shroud with heatshrink before soldering the male connection.

Stan Yeo (Phoenix Model Products)   http://www.phoenixmp.com/

The ‘C’ Rating & Time to Charge | Back to Article Index

By Stan Yeo


The maximum recommended continuous discharge current of a Lithium Polymer battery is the Battery Capacity (mAhr) x the ‘C’ Rating, i.e. a 1000mAhr pack with a 25C rating would have a maximum continuous discharge rating of 1000 x 25 = 25,000ma (25Amps). The ‘C’ rating is a reflection of the internal resistance of the battery and its ability to satisfy high current demands. Unfortunately not all batteries live up to the ‘C’ rating on the label. An indication of this is how hot the battery gets at high currents loads but within the maximum current recommended on the label. Remember the harder (hotter) the battery works the less life cycles! The ‘C’ rating also has an impact on how long it takes to charge the battery particularly with auto detect chargers. There are four main factors affecting the charge time other than the residual charge in the battery.

They are:


  1. The internal resistance of the battery (‘C’ Rating).
  2. The degree of charge imbalance between the individual cells.
  3. The type and output capability of the charger.
  4. Charger set-up.



1. Internal Resistance

Just as the internal resistance of a cell determines it’s current delivery capability it also affects it’s charge performance in that a higher voltage will be required to deliver the same charge current (Ohms Law). Where this becomes apparent is when using chargers that automatically detect the battery capacity and number of cells. The smaller the physical size of a battery the higher the internal resistance is likely to be so a ‘large’ capacity battery with a low ‘C’ rating can mimic a smaller capacity battery with a high ‘C’ rating. This can result in the ‘large’ battery taking longer to charge and the smaller battery being charged at too higher rate. Even in cases where the batteries are of the same capacity but different ‘C’ ratings there is often a noticeable difference in the charge times and power output (lower propellor / rotor head RPM). Customers sometimes comment on this when discussing the performance of different battery brands.

2. Cell Imbalance

If the individual cells are out of balance with each other this will effect the charge time due to cell balancing bleeding off charge from cells in a higher charge state. If the imbalance is too great then the pack may have to be partially charged independent of the balancer, cell balanced and then balanced charged. If this does not work then try charging each cell individually.

3. Charger Output Limitations

All chargers have limits on the output current (amps), output voltage (number of cells to be charged) and output power (watts). Operating at any of these limits can affect the time it takes to charge the ‘pack’. As an example a 3 cell 4000mAhr pack charging at 4 Amps would require at least 48 watts (12v x 4A = 48W) of power. A similar 6 cell pack would require double this i.e. 96W (2 x 48W) but if the charger could only deliver 72W then the charge current would only be 3A (72w/24v) extending the charge time by 25%. Unfortunately high output chargers are quite expensive. A less costly alternative would be to use 2 x 3 cell packs in series and have two less expensive chargers. The 2 packs in series option also has other benefits i.e. if a single cell goes down then only 3 cells are scrapped instead of 6 and a single 3 cell pack can be used in other applications such as a small helicopter or small fixed wing model.

4. Charger Set-up

The recommended charge current for most Lithium Polymer batteries is 1C i.e. the capacity of the pack in Amps (3200mA = 3.2A). Obviously selecting a lower charge current will result in a longer charge time. Setting a higher charge current will charge the battery quicker but does run the risk of permanently damaging the pack. Our advice is stick to 1C.


I hope you have found this article useful along with the other related articles on this site (Simple Electrics and Causes of Lipo Battery Failure). I try to present technical information in an easily digestible form so sometimes a little licence is used in the terminology and ‘maths’ but it is the concepts that are important rather than precise detail. If you can understand the concepts then it is easy to transfer the knowledge from set of circumstances to another.

One final point, I have just checked the LiPo batteries I am going flying with today. Both batteries were fully charged after the last flying session as per recommendations. One would not accept a charge because it was still fully charged whilst the other needed topping up. LiPos are not supposed to self discharge but they do, albeit no where near as rapidly as Nicads or /NimHs so our advice is that if the batteries have not been used for a number of weeks give them a top-up charge to avoid them discharging below the dreaded three volts per cell.


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Causes of Li Poly Battery Failure

By Stan Yeo

High Discharge Currents
Discharging below the Minimum Cell Voltage

Foreward (Back to top)

Lithium Polymer batteries supplied by us, in common with other retailers / distributors, do not carry the same guarantees normally associated with other electronic products. The cell voltages of all batteries supplied by us are checked by ourselves and our suppliers before dispatch (except when bubble packed). This is to ensure that you receive the batteries in good condition as once the batteries are taken out of their packets and charged / discharged we can no longer accept warrantee claims because we have no control on charging conditions or how the battery is used. In view of this we have written the following as a guide to good practice when using Li Poly batteries.

We strongly recommend that:

1. You check individual cell voltages with a digital multi-meter within 24hrs of receipt of your battery, if possible without removing it from the packing and certainly before fitting the battery connectors. If, in the unlikely event an individual cell is below 3v contact us immediately.

2. You balance charge your battery as soon as possible after cell checking your Li Poly battery and before attempting to use it no matter how short the period. If the difference in cell voltages prevents balance charging as it sometimes can, partially charge the battery without a balancer, then using a balancer independant of the charger balance the cells before completing the charge with the balancer in circuit.

Charging (Back to top)

It is important that Lithium Polymer batteries are charged using a Li Poly compatible charger in conjunction with a suitable Li Poly cell balancer and the charger is correctly set up for the number of cells being charged and their capacity /correct charge current. This is normally 1’C’ i.e. the milliAmp hour capacity of the battery. If using a separate cell balancer then we recommend the battery is partially charged, removed from the charger and the cells balanced before completing the charge.

Cell balancing is important because no two cells behave in the same way and during use a variation develops in the charge state of the individual cells. This is a double cause for concern as cells that hold more charge than others run the risk being damaged during charging by being over-charged and whilst the remaining undercharged cell(s) risk being ‘killed’ by being discharged below their low voltage threshold.

High Discharge Currents (Back to top)

Exceeding the recommended maximum discharge current can irreparably damage the battery as can continually operating at the maximum continuous discharge current. Very high discharge currents can cause the battery to ‘balloon’ whilst the latter will significantly shorten the life of the battery.

As a gauge to safe currents loads on Li Poly batteries, if the battery gets hot then it is in danger of being overloaded and sustaining damage. Our recommendation is, with adequate ventilation, the battery should only get warm i.e. be operating at 2/3rds to 3/4 of the max. continuous current. If the battery gets hot operating in this range then there is either inadequate ventilation, the battery has been damaged or the ‘C’ rating is incorrect. There is a correlation between operating current and the number of usable life cycles. The higher the current the fewer the life usable life cycles.

Discharging below the Minimum Cell Voltage (Back to top)

Once a Li Poly battery is discharged below a nominal 3 volts per cell it is unlikely that it can be recharged. This is the most common cause of Li Poly battery failure. There are a number of reasons for this, the most common being the speed controller failing to cut-off at the required voltage. This could be due to incorrect set-up, using the wrong type of speed controller i.e. one that is not Li Poly compatible or failing to recharge the battery between flights. The latter is particularly important if using a speed controller that cuts off at a percentage of the start voltage. If the speed controller cuts off at 75% of the start voltage this would be 12.6v x 0.75 = 9.45v for a fully charged 3 cell pack. If after an initial flight the start voltage on a subsequent flight is 11.4v then the cut-off voltage would be 8.55v i.e. 2.85 volts – below the recommended 3 volts per cell. This could be further compounded if cell voltages are not equal i.e. unbalanced, leading to damage to at least one cell.

Recommendations (Back to top)

We recommend that initial flights are timed and deliberately kept short and a note made of the energy (milliwatts) supplied by the charger to recharge the battery. This will give an indication as to how much of the batteries energy has been used. From this a usable flight time can be judged. We suggest up to 75% of what is on the label. Remember that the higher the discharge current the less that can be taken out due to internal losses and a higher voltage drop under load which means the battery will reach the cut-off / minimum cell voltage sooner.

We strongly recommend a top-up charge between flights and a charger is used that gives a readout of current supplied. We also recommend using an integrated charger / balancer system rather than balancing the cells independently after charging.

Finally, the most common cause of LiPo battery failure is failing to disconnect the battery immediately after landing from the ESC (speed controller) and UBEC (if fitted). If left connected then the speed controller / UBEC will, over a very short period of time, discharge the battery below the critical 3 volts per cell threshhold.

August, 2008