### What battery? [SOLVED]

I have 4 hobbypower motors at 1000kV that draw 4-10 amps. I have a KKmulticopter flight board V5.5, and hobbypower 30A ESCs. I am just curious. If i am thinking correctly, I believe I need at least a battery for a 4000 maH for the motors alone.Correct? I figure a 2200maH would be insufficient, if even able to turn the motors at all. If not, please help!

#### Replies

• yes, the motors are A2212/T13 1000kV

• I am overwhelmed by the responses. Thank you all very much. I have more than enough info to get the right battery. Thank you very much.
• Don't worry about what the ESC are rated for, as long as they're stronger than the motors they will not be an issue.

A 4000mAh batter on 4 motors rated for a max of 10A sounds like a very big battery, but without knowing the motor specs and propellers you plan to use it's difficult to say for certain.

You can play with this tool:

http://www.ecalc.ch/xcoptercalc.php?ecalc&lang=en

You might have to make some guesses, but it will allow you to see roughly how batteries will effect your flight time.

• Here's the short answer: The current going into the speed control is the current that goes into the motor (ignoring some minor losses). We don't have to perfectly characterize the signals going into the motor to know what is happening here.

I'm short of time at the moment, but I'll try to go into more detail on why this is the case later.
• Please do elaborate if you get time. I don't see how what you say is true, not without setting fire to motors, but I may be missing something.

• Moderator
Philip is correct, battery current = motor current + esc losses . You are confusing two different motor types. A brushless motor is an AC motor and relies on an induced voltage in the motor to generate the rotational power . this induced voltage is proportional to the slip frequency ( esc phase Esc hz - actual motor speed) also consider that the motor current is the phase sum of the 3 individual phases, that's why our motors have 3 wires , keep it simple our battery - esc- motor circuit is a series circuit. In a series circuit the current is the same in all sections or devices
• I'm not confusing it. Your explanation doesn't convince me (not saying i'm right just trying to understand why/if i'm wrong). if it were an induction motor wired to mains running at proper speed (not stalled)I would believe you.

I know brushless is an ac motor really and why it has three wires.

I know it is in series, but that doesn't mean the current in the battery and the motor are the same.

As I see it the esc must pwm the voltage down (I know escs use pwm, mine have selectable frequency as an option)  in the motor other wise at zero and low speeds the battery would effectively be short circuited across the windings. 11 odd volts across miliohms equals a lot of current!

During PWM one half of the fets being used are switch on and off, when off the battery current is zero. therefore battery current is on average lower than the peak current. In the motor though the current isn't zero, the inductance keeps it flowing through  fet diodes (fets inherently have a diode built in), a freewheel current, the current decays but isn't zero (pwm freq to high to let it be zero), so the motor current is higher than the average battery current. and by the nature than the motor volts is pwmed, the motor average voltage is lower than battery voltage.

if the esc was only using frequency control will a ful voltage wave form, I struggle to see at lower speeds how the motor emf will be high enough to not result in very very high motor currents.

•   In any type of energy system, there must be a way to account for the movement/conversion/storage of the energy.  For example, if you look at the engine of a car, you can see energy going in (fuel) and energy going out (waste heat, drive shaft torque, un-burn fuel, etc...).  All of these inputs and outputs MUST balance in a steady state system.

The same principle holds true in the electrical systems driving these aircraft.  The energy going into the ESC must be equal to the energy leaving the ESC.  If we observe power flowing into the ESC at 11v and 3A, then that much power is going to the motor (minus minor heat losses).  Power can't magically disappear in the ESC.  If the the ESC was somehow reducing the voltage to 5.5v, then the current flowing from the ESC to the motor would have to DOUBLE to keep the sum of the power in and out of the ESC zero.

The PWM effect that the ESC has is to control how much time the motor is pulling current (whiles also keeping the timing correct).  The ESC supplies the full voltage to the motor coils, but by reducing the amount of time that voltage is applied effectively controls the amount of power the motor is allowed to consume.  At zero throttle, the pulse has zero length, at full throttle the pulse is "long".

As a side note, motors don't even care about amps and volts directly.  They really only care about waste heat.  If a motor can stay cool, then the amps and volts going into don't matter.  If the motor fails to stay cool, then it burns up and dies.  You can run a cheap motor rated for 2s on a 6s battery, but you have to keep the load low enough that the motor doesn't over heat.  It's really that simple.

I hope this helps.

Phillip

•   "In any type of energy system, there must be a way to account for the movement/conversion/storage of the energy.  For example, if you look at the engine of a car, you can see energy going in (fuel) and energy going out (waste heat, drive shaft torque, un-burn fuel, etc...).  All of these inputs and outputs MUST balance in a steady state system."

agreed, i never said anything to say power mystically disappeared or was lost.

"The same principle holds true in the electrical systems driving these aircraft.  The energy going into the ESC must be equal to the energy leaving the ESC.  If we observe power flowing into the ESC at 11v and 3A, then that much power is going to the motor (minus minor heat losses).  Power can't magically disappear in the ESC.  If the the ESC was somehow reducing the voltage to 5.5v, then the current flowing from the ESC to the motor would have to DOUBLE to keep the sum of the power in and out of the ESC zero.

"

agreed...I already said as much...did you misusnderstand or was my power transformation statement mis interpreted...that is what I meant, watts are constant, volts are traded for amps...ignoring any losses.

"The PWM effect that the ESC has is to control how much time the motor is pulling current (whiles also keeping the timing correct).  The ESC supplies the full voltage to the motor coils, but by reducing the amount of time that voltage is applied effectively controls the amount of power the motor is allowed to consume.  At zero throttle, the pulse has zero length, at full throttle the pulse is "long".

"

agreed! I already talked about PWM, I already talked about average voltage, average voltage being peak voltage (battery) x PWM duty cycle.

" As a side note, motors don't even care about amps and volts directly.  They really only care about waste heat.  If a motor can stay cool, then the amps and volts going into don't matter.  If the motor fails to stay cool, then it burns up and dies.  You can run a cheap motor rated for 2s on a 6s battery, but you have to keep the load low enough that the motor doesn't over heat.  It's really that simple. "

agreed, I have been looking at motor heating on a project I picked up at work.

so it seems we were trying to say the same thing....but you tal in peak without mentiong duty cycle and am talking averages which combines peak and duty cycle in one number.

• "If the the ESC was somehow reducing the voltage to 5.5v, then the current flowing from the ESC to the motor would have to DOUBLE to keep the sum of the power in and out of the ESC zero."

the esc doesn't have to double the battery peak current! If the duty cycle is 50%, the battery current will be half on half off so average is half of peak, the motor current will stay roughly battery current peak for both on and off due to inductance and freewheel diode path allowing the current to continue to circulate in the h-bridge. so average motor equal battery peak.

motor AVERAGE is two times battery AVERAGE. Motor AVERAGE voltage will be half battery voltage. this is true for brushed motors and sinusoidal AC motors, I don't see why this is different for brushless dc motors, as you say power can't just disappear

Although this chat has made me realise I need to read up on brushless motors and how they work.

This reply was deleted.

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