LiPo battery confussion with motor !!! Please Help

Hello friends,

I am going to buy 4 motors which are having following specifications :

Outrunner Brushless DC Motor
Dual sealed ball bearings
Weight: 78gms
KV: 1500 RPM/V
Thrust: 1200gms max.
Voltage: 7V to 16.8V
Current: 18Amp continuous, 20Amp max.


& related motor controller has following specs.

• Output: 20A continuous; 25Amps for 10 seconds
• Input voltage: 2-4 cells Lithium Polymer / Lithium Ion battery or 5-12 cells NiMH / NiCd
• BEC: 5V, 3Amp for external receiver and servos
• Max Speed: 2 Pole: 210,000rpm; 6 Pole: 70,000rpm; 12 Pole: 35,000rpm
• Weight: 32gms
• Size: 55mm x 26mm x 13mm

& I am willing to purchase a LiPo battery with following Specs. to drive all these stuff :

Battery type: Lithium Polymer
Battery Voltage: 18.5V
Battery capacity: 5000mAh
Number of cells: 5
Maximum constant discharge current: 150 Amps (30C)
Maximum charging current: 10Amps (2C)
Recommended battery cut of voltage: 16.3V.
Battery weight: 583gms
Dimensions: Length-16cm, Width-4.6cm, Hight-4.5cm


My Queries :

1) what Ah will my motor & motor controllers will draw ?

--> It is mentioned that 1 motor will draw 18 Amps continuous so does it means that 4 motors will draw total
72 Amps !!!!

--> 1 motor controller (ESC) is drawing 3A so my total consumption for whole system(4 motors + 4 ESC) will be :
72 Amps + 3x4 Amps = 84 Amps
my additional circuit will hardly consume 1A so can I say that my system is consuming 85Ah ???

Please tell me whether this calculations are correct or not? if not then please help me to find out How much current my system will draw ?


2) LiPo Battery Confusion !!!

--> battery is delivering 5000mAh with max. 30C so I can draw max. 150 Amps within 2 min. (for full load)

--> Suppose my system will draw 85Ah then for how much time will it last? I've got a calculation as
follow & don't know whether it is right or not? so please help me.



5000mAh x 30 C = 150 Ah will last for 60min/30C = 2 min.
so, 5000mAh x 17C = 85 Ah will last for 60min/17C = 3.53 min ???



Is it correct ???

if yes then is there any way to increase my capacity ?
Can I increase the time by adding one more battery with parallel to another ?
And does motor draw 18Amps !!! CONTINUOUSLY as it says in it's specs. ??

Please help me regarding to find out the net time for which my battery will last & about how to increase my battery life ??

Thanks a lot for your time.

hello. those esc are 3a ? if they are 3a they can drive up to 3a the motors,then you fry the esc and buy new more powerful. also consider buying some quality esc , because i have wasted much money on cheap ones to realise that what you get is what you pay.

the bitter truth is that 70grams motor can drink 20A yes... . buy lipo, 30c discharge rate. but buy 2 or 3 batteries, not just one as it will not last long and it will short its life.(and also if 1 battery dies(or gets on fire) you will have 2 left. if you have only 1 5A battery when it dies you lose more money and the only battery)

also you can buy 3x 20c batteries wich are cheaper than the 30c but in parallel it will give around the same discharge rate. Using protection diodes when paralleling batteries is very very recommended in my opinion because you are protected from shortcircuit. and believe me you dont want to short a 2700mah lipo in your room when doing your tests.... the only drawback of diodes is the voltage drop (around 1v more or less)

your calculation i gues is pretty ,uch right.
if you want much constant power , and dont want to use it for rc model, buy a computer power supply, it can deliver easily continuous 250w at 12v
also take extra care of lipo and charger because you can burn the whole house

what is the purpose of this project? maybe i can help, i have wasted lots of money time and research on rc!
at 11.1v without load on shaft you will have approx 16.5k rpm but it will significant drop even with a small load ( to maintain high rpm you need 50% more amperes rated esc , and of course many many battery capacitance, or psu output power)

Jigar 4 Electronics said:

It is mentioned that 1 motor will draw 18 Amps continuous...

Click to expand...

No. When there is no load on the motor it will draw very little current. The greater the mechanical load it has to drive, the higher the current it will draw. 18A is the maximum current the motor can draw continuously without burning.

It looks like your battery voltage is too high for the controller and the motor.

thebadtall said:

those esc are 3a ? if they are 3a they can drive up to 3a the motors,then you fry the esc and buy new more powerful.

Click to expand...

Actually that is BEC (Battery Eliminator Circuit) ratings.
It can give output upto 20 A (as I mentioned above) and I've to fed it with 7 to 14.8 V to drive a motor, while it's BEC requirements is 5 V and 3 A to drive it's own body.

Though my motor can have max. 16.8V This ESC is feeding max. 14.8V with 20 A Max. current.
motor will provide me 1500 RPM/V and so for 14.8V it'll provide me 22.2k RPM, And I think it's enough for me.

thebadtall said:

Using protection diodes when paralleling batteries is very very recommended in my opinion because you are protected from shortcircuit. and believe me you dont want to short a 2700mah lipo in your room when doing your tests.... the only drawback of diodes is the voltage drop (around 1v more or less)

Click to expand...

Thanks for your recommendation sir. Can you Please provide me that circuit (I wanna see just an Example of it)

thebadtall said:

what is the purpose of this project? maybe i can help, i have wasted lots of money time and research on rc!
at 11.1v without load on shaft you will have approx 16.5k rpm but it will significant drop even with a small load ( to maintain high rpm you need 50% more amperes rated esc , and of course many many battery capacitance, or psu output power)

Click to expand...

Well Actually I am designing a Quadropter using ARM 7.
Can you tell me sir whether all this selection is better for me or not ?
I've 4 fixed pitch propellers (2 Pusher & 2 Puller). They've a weight of 12 gms each.
And the total Weight (if I'll use 3 batteries) will be near about 2.5 kg
So can you guide me regarding to whether my selections is proper or not?
Thanks for your Time.

- - - Updated - - -

godfreyl said:

No. When there is no load on the motor it will draw very little current. The greater the mechanical load it has to drive, the higher the current it will draw. 18A is the maximum current the motor can draw continuously without burning.

Click to expand...

Thanks for the reply.
I've to attach 12 gm propellers to each motor, can you tell me how can I calculate the current ratings for this load?

godfreyl said:

It looks like your battery voltage is too high for the controller and the motor.

Click to expand...

How you've conclude that the battery ratings is high?
Which Type of battery should I use ?
Thanks for your time.

here is a fast schematic, as about the diodes im not sure if they can stand those watts, however they can handle 3A each



i have never deal with quadropter, but, i guess you should need more equipment such as 3axis gyroscopes, would you use arduino ?
do not know about arm7


Also in the schematic IGNORE the parallel pwm connection, i didnt knew it was about the quadropter

godfreyl said:

No. When there is no load on the motor it will draw very little current. The greater the mechanical load it has to drive, the higher the current it will draw.

Click to expand...

Can I say that a Propeller is the only load for the motor.
Or the whole body weight should be in the account ?
What exactly my load (for motors) will be ?
And how can I count the Amps drawn by my motor for x amount of load ?

- - - Updated - - -

thebadtall said:

i have never deal with quadropter, but, i guess you should need more equipment such as 3axis gyroscopes, would you use arduino ?
do not know about arm7


Also in the schematic IGNORE the parallel pwm connection, i didnt knew it was about the quadropter

Click to expand...

Yes I am gonna use Gyro, Accelerometer and some other sensors will also be there.
As godfreyl said that the motor will not draw 18 A continuously then do I need more than 1 battery ?
I think I should first try with just a 1 battery, May be it'll prove enough for me ?!

Jigar 4 Electronics said:

Can I say that a Propeller is the only load for the motor.
Or the whole body weight should be in the account ?
What exactly my load (for motors) will be ?
And how can I count the Amps drawn by my motor for x amount of load ?

Click to expand...

the whole thing hovering is the load=load for motors

to calculate the amps, you must get the static thrust of the motors+fans,
for instance if it is 1000gr static thrust and max consumption is 20A (1kgr/20A) then
you get the total weight of plane , say 2000gr , means 500gr per motor means 10A/motor = 40A total consumption
(i hope i am right!)

Well If that will be the load of motor then the max. thrust which the motor can give is 1200 gm and for that it is drawing 18 Amps.

The propeller is the only load for the motor and so the motor with propeller of 12 gm will draw 0.18 A means 180 mA current.

So now my current requirement is,

motors (180 x 4) + motor controller (1000 x 4) = 4720 mA if I take it approx. 5000 mA then also my battery can work for 1 hour.

Is it correct ???


Well in my above posts, The BEC (Battery Eliminator circuit) is an O/P terminals which provides me 5 V, 2 A o/p to drive my other circuit.
Ans so I've take 1A as a current requirement of motor controller in above calculation.
And all other circuitry part consist Ultra low power ICs drives on 3.3 V , 100-150 mA so (5000 - 4720) 280 mA is enough for that.

So can I say now that this is what my final requirement regarding with my battery ?

Because if it's not then may be I've to buy 2 or 3, 20 C battery.
So I am confuse whether I should buy this 5000 mAh, 30 C Discharge battery or not.