Cairo University

Faculty of Engineering

Aeronautics and Aerospace department

Quadrotor graduation project

Quadrotor power system documentation

Prepared by : Mostafa Hasan Abd-el hay

Abstract:

This document first describes the main aspects of quad rotor power system components

including batteries and motors then explains how to choose them to achieve the longest fly

time (endurance) with help of well-prepared data base of batteries and motors in Hobby king

store .

Power system properties:

Lipo batteries properties:

voltage (number of cells)

LiPo battery cells are rated at 3.7 volts per cell. The cells are hooked up in series to provide

higher voltages.

3.7 volt battery = 1 cell x 3.7 volts (1S)

7.4 volt battery = 2 cells x 3.7 volts (2S)

11.1 volt battery = 3 cells x 3.7 volts (3S)

14.8 volt battery = 4 cells x 3.7 volts (4S)

18.5 volt battery = 5 cells x 3.7 volts (5S)

22.2 volt battery = 6 cells x 3.7 volts (6S)

29.6 volt battery = 8 cells x 3.7 volts (8S)

37.0 volt battery = 10 cells x 3.7 volts (10S)

44.4 volt battery = 12 cells x 3.7 volts (12S)

Note: you may run across packs or cells hooked up in parallel to increase the capacity.

This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two

celled series packs hooked up in parallel to double the capacity.

Figure 1 (2 cells battery)

The importance of this property is that every electric motor have specific number of cells to

work with .

CAPACITY

Capacity indicates how much power the battery pack can hold and is indicated in milliamp

hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps)

can be put on the battery for 1 hour at which time the battery will be fully discharged.

For example a RC LiPo battery that is rated at 1000 mAh would be completely discharged in

one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load

placed on it, it would take 2 hours to drain down.

Then the relation between capacity and drained current would be :

Time(hours)=

DISCHARGE RATE

Discharge rate is simply how fast a battery can be discharged safely. In the RC LiPo battery

world it is called the “C” rating.

A battery with a discharge rating of 10C would mean you could theoretically & safely discharge

it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C

pack = 20 times more, and so on.

Using our 1000 mAh battery as an example; if it has a 20C discharge rating, that would mean

you could pull a maximum sustained load up to 20,000 milliamps or 20 amps off that battery

(20 x 1000 milliamps = 20,000 milliamps or 20 amps). From a purely theoretical time stand

point, this equals 333 mAh of draw per minute so the 1000 mAh pack would be completely

exhausted in about 3 minutes if it's exposed to the maximum rated 20C discharge rate the

entire time.

Max current drained safely(in amp) =

There is also ( burst c rating) indicates the battery discharge rate for short bursts of extended

power.

The higher the C rating, usually the more expensive the battery. The most important thing is

you can't go with too low a discharge C rating or you will damage your battery and possibly

your ESC (electronic speed control).

HEAT

taking a temperature reading of your packs after running them is another good way to gauge if

you're using a high enough C rating. Using low c rating leads to warmer battery after running

them.

Important note

‘”OVER DISCHARGING - THE NUMBER ONE KILLER OF LIPO'S!!!”

A very good rule to follow here is the "80% rule". This simply means that you should never

discharge a LiPo pack down past 80% of its capacity to be safe. For example, if you have a 2000

mAh LiPo pack, you should never draw more than 1600 mAh out of the pack (80% x 2000). This

is assuming a healthy pack as well that has the full 2000 mAh capacity (as packs age, their

capacity drops).

INTERNAL RESISTANCE

This one is verifiable and one of the best ways to monitor your RC LiPo batteries condition.

Most decent higher capacity and higher discharge rated LiPo cells will have roughly 2 to 6

milliohms (0.002 to 0.006 ohms) of internal resistance when brand new. To calculate the total

internal resistance of a series wired pack, you would then add these numbers together so a 4S

pack with each cell having 4 milliohms of resistance will show a total internal resistance of

about 16 milliohms (0.016 ohms).

As packs age, the internal resistance goes up and the warmer they run. Lower discharge rated

packs and small capacity packs will generally have higher internal resistance readings.

Motors properties

kV Ratings

The letters kV stand for the the RPM of your motor per volt with no load. For example if you

own a brushless motor with a kV rating of 4600 and 12V. Take the 4600, multiply by 12 to get

55,200 RPMs. This is the max RPMs that this motor can reach under no load. Once you get it

inside your vehicle, this will come down due to friction.

A motor with a higher kV will have more top end speed, but not as much

acceleration/torque.

A motor with a lower kV will not be as fast, but will accelerate faster (more torque).

Motor Turns

Motor Turns is the same for brushed motors and brushless motors. The word turns stands for

the amount of wire windings around each of the motor's rotor poles.

The higher the number of wirings/turns means less top speed, but higher

acceleration/torque.

The lower the number of turns equals higher top end speed and lower

torque/acceleration.

A motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than

a motor with a 12 turn rating.

Current Rating – Amps

the max current rating is the maximum amount of current that a motor is able to handle

safely. This current is measured in Amps. The continuous current rating of a motor is the Amps

that a motor can handle safely over a long period of time.

The estimated current rating of a motor is usually on the factory specs sheet. However other

factors affect the actual current that a Brushless motor will draw. Things like the kV rating,

battery voltage, how heavy the RC vehicle is, and gear ratio or prop size. The harder a motor

needs to work to reach its top speed, the higher the Amp draw is.

It is a great idea to find an ESC that has a current rating that is higher than your motors by at

least 20%. It will be a good safety cushion to make sure that you don’t burn up your brushless

power plant.

Power (watts)

Watts are the power rating or the horsepower equivalent of your brushless RC Motor. The

math here is Amps x Volts = Watts. You will see a watt rating in the brushless motor specs.

Your brushless motor should have a watt rating on its spec sheet, something like "180W". This

is the amount of "horse power" that it should produce safely. Running anything over this

rating could damage your motor, especially over a long period of time.

Motor Efficiency

The efficiency of a motor determines its quality. Higher efficiency means better design and

high quality components. The higher the efficiency of the motor the more power it can

produce before it overheats. A 70% efficient motor produces 70% power and 30% heat. A 85%

efficient motor produces 85% power and 15% heat. If your battery is sending the ESC 180

watts, your motor will produce 153 watts (85%), the rest is gold ole heat. 27 Watts of heat will

melt solder with some soldering irons, so, that is a lot of wasted watts!

A cooler running motor will give you much less trouble. To reduce heat you can change your

gearing or prop size, use a more efficient motor, reduce your voltage or amps, or try a motor

heat sink and motor fan. Keeping the heat down on your motor allows it to run longer, and

give you the power it needs.

Propeller properties

Usually the propellers are determined by 2 numbers for example

14" x 12" propeller the first number relates to the diameter, in our example 14".The second

number 12" in our example, refers to the pitch. Pitch is the blade angle . The more pitch angle

more lift you get from the propeller but cost you more drag so need more torque to run the

propeller.

As the diameter of propeller increases the aspect ratio increases which increase lift / drag ratio

but needs wider frame which cost more weight to the quad rotor.

Considerations

General guiding points:

(Battery capacity/weight )ratio and c rating the most important factors in battery.

Use 0.8 of battery capacity to be safe.

Higher motor kv rate provides higher propeller speed but low torque so if the wind

speed is high or flying outside (wind gusts) it will be inefficient. Should use small

propeller diameter.

Lower kv rate provides slow propeller speed but high torque. Should use large propeller

diameter.

The more the motor drains current the less the flight time, so compromise should be

done when using large propellers because it’s more draggy than small propellers.

Try to avoid battery heating or motor heating because this reduces the efficiency for

battery check c rating well, for motors try to get max thrust with less current drained to

improve endurance and motor efficiency.

Choosing power system

Choosing battery

We will choose the best 2 cells battery and best 3 cells battery from our point of view.

Choosing the best (Battery capacity/weight) with max c rating is the goal.

To achieve that a graph will be made containing c rating at x axes and (BC/w) at y axes and we

will choose the upper right point on the graph. From the data base of hobby king batteries.

2s batteries

The shown graph in figure (2) indicates the distribution of 2s batteries.

The red point is the upper right point indicates the largest c rating and largest BC/w ratio

hence it is our battery here. After choosing the motors ,a check must be done we should make

the same graph but replacing the C rating with max drained current to ensure that this battery

can handle the motors their needs of current this graph is shown in figure (3) .

Figure 2 2s batteries C rating vs (BC/w)

Figure 3 max drained current vs (BC/w)

The red point indicates Turnigy nano-tech A-SPEC 6600mah 2S 65~130C Lipo

Weight: 274

3s batteries

For 3s batteries the graphs are shown in figures 4,5

Figure 4 C rating vs (BC/w)

Figure 5 max drained current vs (Bc/w)

In figure 4 there is a point which has larger bc/w than the red point but it will be neglected

because it has very low c rating. The red point indicates

Turnigy nano-tech A-SPEC 4500mah 3S 65~130C Lipo

Weight: 287 gm.

Choosing engine

Choosing engine to maximize endurance is more complicated than choosing battery. the ratio

(max thrust/weight ) is not very important here as in battery case because the difference in

motors weights is not very high enough to consider it a figure of merit . the most important

criteria is generating required thrust with less current drained to maximize flight time .

Of course the drained current differs by changing propellers and flight conditions but we will

depend on the already tested motors with known propellers in hobby king site .

Selection procedure will be as follows:

Weight is approximated to average value .

Calculate required thrust of each motor .

Determine a small range of thrust around thrust from last step .

We will include all engines that have thrust test’s data in this range of thrust .

Choose the max (thrust/drained current at this thrust) .

Weight estimation

Wtotal =4*(Wmotor+Wspeed-controller)+Wbattery+Wframe+Wsensors and control board+Wpayload(camera + video transmitter

+battery ).

Wmotor≈85 gm.

Wspeed-controller≈60 gm.

Wbattery≈290 gm. (2s or 3s )

Wframe≈500 gm.

Wsensors and control board≈300 gm. (including gps ,IMU, battery, power distribution board, arduino

chip, wireless communication module).

Wpayload≈500 gm.

Wtotal≈2170 gm≈2200 gm.

Then required thrust at hovering is 2200 gm so we need 550 gm of thrust from each motor at

hovering condition.

Taking the range of thrust from 400gm to 800 gm , we want to get the best motor to generate

thrust with less drained current and with max power at this range (without considering

propellers for now ) .

Engines that works in 2s batteries

2 tested engines only generate thrust at our interval, those engines are :

Turnigy Park450 Brushless Outrunner 1200kv

Tested thrust=640gm @ 12 A (drained current)

Ratio=53.33.

D3128-1550 Brushless Bell Motor

Tested thrust=550gm @15.5 A

Ratio=35.48.

It’s clear that Turnigy Park450 is the best to use with 2s motors

To calculate approximated endurance :

Assume linearity between (640 thrust) point and (550 thrust)point

Approximate drained current=

=10.3 A

Approximated Time in minutes=

*60=9.6 mins .

This value with one battery, Later the effect of adding another battery will be discussed.

Engines that works in 3s batteries

The highest 2 engines that give the highest (thrust/drained current)ratio in our interval were:

Turnigy Park480 Brushless Outrunner 850kv.

Wight: 80 gm.

Max current : 28 A.

Required: 30A~35A ESC.

Generate 650gm of thrust @ 8.6 A with properller 9*5 (ratio=75.58).

Price: 19.84 $.

Max power=250 watt

Figure 6 • Turnigy Park480 Brushless Outrunner 850kv.

Turnigy L3010C-1300kv (420w)

Wight: 87 gm.

Max current : 40 A.

Required: 50A~60A ESC.

Generate 502gm of thrust @ 7.1 A with properller 10*6 (ratio=70.7).

Price: 11.31 $.

Max power = 420 watt

Comments about these 2 engines:

The advantages of turnigy L3010C are :

Can handle power up to 420 watt which means that it can handle high current without

being damaged (can operate in tough condition).

While operating at 550 gm. thrust will be far away from max current so no heat will be

generated.

Cheaper than turnigy park by 8.53 $ for each motor.

Disadvantages of turnigy L3010C are :

Need to operate with (50 A -60 A) ESC which are heavier and more expensive than

(30A-35A) in case of working with Turnigy Park480 motor and this disadvantage

destroys the advantages of the motor price.

Max thrust is 1650 gm . which need special consideration in frame structure .

The advantages of turnigy park480 are :

The highest (thrust/current)ratio.

Low weight .

Operate with low weight ESC.

Final determination of motors

In case of working with turnigy L3010C the speed controller will be :

Hobby King 60A ESC 4A UBEC

Weight: 61 gm price:17.78$

Then the total weight of motors and speed controllers will be :

Wmotors+ESCs=4*(61+87)=592gm.

Price=4*(11.31+17.78)=116.36$.

In case of working with turnigy park480 the speed controller will be:

Hobby King 30A ESC 3A UBEC

Weight=32gm. Price=10$.

Then the total weight of motors and speed controllers will be :

Wmotors+ESCs=4*(80+32)=448gm.

Price=4*(10+19.84)=119.36$.

The difference between 2 motors with ESCs is 144 gm .

It’s clear that turnigy park480 is the best motor.

Calculating flying time approximation

Assume linearity between (650 thrust) point and (550 thrust)point

Approximate drained current=

=7.27 A

Approximated Time in minutes=

*60=9.28 mins .

Effect of adding another battery

Due to the low weight of batteries chosen and if the approximated endurance wasn’t

satisfactory we can add another battery to improve this .

For the 2s motor and battery, the total weight will be =2200+274=2474gm.

Hovering thrust=618.5 gm.

Approximated drained current==

=11.58A

Approximated time =

*60=17 mins

For the 3s motor and battery , the total weight will be =2200+287=2487gm.

Hovering thrust=621.75 gm.

Approximated drained current=

=8.22 A

Approximated time =

*60=16.42 mins

Tabulated final results

2s Battery: Turnigy nano-tech A-SPEC 6600mah 2S 65~130C Lipo

Motor: Turnigy Park450

Time : 9.6 minutes(1 battery) 17 minutes (2 batteries)

3s Battery: Turnigy nano-tech A-SPEC 4500mah 3S 65~130C Lipo

Motor: Turnigy Park480

Time: 9.28 minutes(1 battery) 16.4minutes(2 batteries)

References

http://www.hotslots132.com/understanding-rc-brushless-motor-ratings-a-263.html

http://www.rctoys.com/pr/2006/12/11/choosing-the-right-electronic-speed-control-

esc-for-your-electric-rc-airplane/

http://www.rchelicopterfun.com/rc-lipo-batteries.html

http://www.hobbyking.com/hobbyking/store/index.asp