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Old 01-11-2011, 05:26 AM   #1
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Exclamation Lipo selecting confusion

Hey there, this is my first post, I知 not new to RC world but I was busy in other things for the past 10 years, a lot of things has changed !

I知 an electrical engineer and I値l try to apply my knowledge here a bit, btw the brushless motor is simply an AC 3 phase induction motor

Anyway, I just bought a brushless 1/16 car with a 3s battery and a brushless motor which I changed to Novak mangooso 9000kv motor .. ( I know !! ;P )

In my first run on the new motor, I found that the battery was cooked and realized that current capacity wasn稚 enough so I search the web for a way to select the Lipo. I found that a lot of posts talk about calculating the current capacity of the battery only which is rubbish ! all of us know that. At the end they select the biggest capacity and try to be on the safe side

so I made some studying and here is what I think:

Simply, current = V (voltage of the battery) / R (resistance of motor and ESC which is constant)

So the higher voltage you put will lead to a higher current drawn by the system and more power is consumed (higher speed).

If you have two 3s battery with a 20A current capacity each and you connect them in series then this will not increase the current capacity, it will remain 20 A. I値l explain more if you want

6s will only double your current demand because you doubled the voltage.

Bottom line, brushless system manufacturers should provide the current demand data (for the motor with ESC) on the battery side for different voltage levels (2s, 3s, 4s ), after that selecting a Lipo will be a piece of cake
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Old 01-11-2011, 05:36 AM   #2
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Thanks for adding to the confusion.
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Old 01-11-2011, 07:36 AM   #3
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Quote:
Originally Posted by zaidq8 View Post
Hey there, this is my first post, I知 not new to RC world but I was busy in other things for the past 10 years, a lot of things has changed !

I知 an electrical engineer and I値l try to apply my knowledge here a bit, btw the brushless motor is simply an AC 3 phase induction motor

Anyway, I just bought a brushless 1/16 car with a 3s battery and a brushless motor which I changed to Novak mangooso 9000kv motor .. ( I know !! ;P )

In my first run on the new motor, I found that the battery was cooked and realized that current capacity wasn稚 enough so I search the web for a way to select the Lipo. I found that a lot of posts talk about calculating the current capacity of the battery only which is rubbish ! all of us know that. At the end they select the biggest capacity and try to be on the safe side

so I made some studying and here is what I think:

Simply, current = V (voltage of the battery) / R (resistance of motor and ESC which is constant)

So the higher voltage you put will lead to a higher current drawn by the system and more power is consumed (higher speed).

If you have two 3s battery with a 20A current capacity each and you connect them in series then this will not increase the current capacity, it will remain 20 A. I値l explain more if you want

6s will only double your current demand because you doubled the voltage.

Bottom line, brushless system manufacturers should provide the current demand data (for the motor with ESC) on the battery side for different voltage levels (2s, 3s, 4s ), after that selecting a Lipo will be a piece of cake

Please explain what you are trying to say as you have said really nothing at all.
Not being rude but if what you say you are is true then your smarter than then basic crap you just said.
Talk smarter and not waste your time or others
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Old 01-11-2011, 08:14 AM   #4
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Simply over thought. A mongoose system is rated for 3s battery. The system will not draw more current then the Battery will allow ( 20C, 30C, 40C....) If you are running a 9000 system with a 3S Battery. I would check the gearing and find the Highest C rated Battery from a reputable manufacturer. Example below... Check the esc for current rating. I think the mongoose is 40 Amp per phase. So if you are running the 20C pack, you exceed the constant which will cause it to get warm.

- Typical measurement for 20C battery 18C continuous discharge rate of 36 amps with 50 amp bursts

Hope this helps.. Just read the spec sheets.
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Old 01-11-2011, 08:33 AM   #5
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Originally Posted by TAMAK View Post
Simply over thought. A mongoose system is rated for 3s battery. The system will not draw more current then the Battery will allow ( 20C, 30C, 40C....) If you are running a 9000 system with a 3S Battery. I would check the gearing and find the Highest C rated Battery from a reputable manufacturer. Example below... Check the esc for current rating. I think the mongoose is 40 Amp per phase. So if you are running the 20C pack, you exceed the constant which will cause it to get warm.

- Typical measurement for 20C battery 18C continuous discharge rate of 36 amps with 50 amp bursts

Hope this helps.. Just read the spec sheets.
Typical battery is ?? depends on what capacity the battery is. C= capacity
Resistance is everywhere including internal resistance in the battery itself without counting the wire, conections, solder joints and speed controls and the list goes on.
18c x what size battery are you talkin about??? must be small lol
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Old 01-11-2011, 09:08 AM   #6
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C is NOT capacity...... C (Coulomb) is discharge or charge...... Capacity is 2200mah, 5000mah, that is how long that battery can sustain C (discharge in our case)....

Last edited by TAMAK; 01-11-2011 at 09:26 AM. Reason: See post below
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Old 01-11-2011, 09:26 AM   #7
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Originally Posted by jjfo80 View Post
Typical battery is ?? depends on what capacity the battery is. C= capacity
Resistance is everywhere including internal resistance in the battery itself without counting the wire, conections, solder joints and speed controls and the list goes on.
18c x what size battery are you talkin about??? must be small lol
Yes the battery I was referencing was a 3 cell Lithium Polymer 2000mAh battery pack. 18C continuous discharge rate of 36 amps with 50 amp bursts make this pack perfect for use in the Losi Mini-T or a mongoose system (smaller motor then 9000)

Like I said C= Charge or "Coulomb"... You are right 18c x capacity = max current in milliAmp. But that is not true in some cases. Its best to look at the spec sheet for that particular Battery. Common Sense RC does a great job at that.
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Old 01-11-2011, 09:44 AM   #8
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Since when does knowing Ohm's law make you an electrical engineer? Also the resistance of the ESC and Motors are not constant. There is a huge difference between static and dynamic resistance, which will vary as the motor and ESC heat up. None of which really has to do with battery selection.
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Old 01-11-2011, 10:48 AM   #9
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first of all sorry for confusing some people , I come from the other half of world

current = amperage draw "A" , maybe that confused you guys

power capacity of battery is in mAh , the C is simply a factor you multiply by the mAh to get maximum amperage provided by the battery , they call it "capacity" for no clear reason ,

40 amp is the amperage draw on the motor side not the battery

and still higher voltage means higher amperage ! 40 on 2s is probably 60 on 3s

static or constant resistance, it will behave the same in different voltages !! and will give different amperage ,
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Old 01-11-2011, 10:48 AM   #10
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Quote:
Originally Posted by tunerjetta29 View Post
Since when does knowing Ohm's law make you an electrical engineer? Also the resistance of the ESC and Motors are not constant. There is a huge difference between static and dynamic resistance, which will vary as the motor and ESC heat up. None of which really has to do with battery selection.
Dont criticize a guy for asking a question. Take your sarcasm somewhere else. What you said did nothing to help the guy find the answer for his "battery" heating up.
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Old 01-11-2011, 10:59 AM   #11
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Originally Posted by zaidq8 View Post
first of all sorry for confusing some people , I come from the other half of world that's

current = amperage draw "A" , maybe that confused you guys

power capacity of battery is in mAh , the C is simply a factor you multiply by the mAh to get maximum amperage provided by the battery , the call it "capacity" for no clear reason ,

40 amp is the amperage draw on the motor side not the battery

and still higher voltage means higher amperage ! 40 on 2s is probably 60 on 3s
Please read: http://www.airthunder.com/lipoabc.aspx

And the 40 amp draw from the esc is the only thing that is drawing current from your battery. And, your battery has to be rated for that current draw. If the continuous current draw rating for a a Typical 18th scale battery is 36 amps, you are not meeting the required current requirement for that mongoose speed controller, "40 amps" to the motor.

And your statement regarding Higher voltage = higher amperage is false. The C (Coulomb) rating of a battery times that capacity is how you get MAX Current draw "roughly".

I believe you are overthinking this. Hope this helps to clarify
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