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ceong
04-02-2011, 02:12 PM
Hi Guys,

Apologize if the question asked already posted in this forum.

My question is - if the motor Current capacity : 35A/30s .. how do I know or calculate the right type of lipo (mAh & C rating) to match the motor?

I am thinking to get Turnigy TR 35-36C 1100kv Brushless Outrunner that run at 2 to 3 S lipo

Again, thanks for the help

Lieutenant Loughead
04-02-2011, 02:31 PM
Yes, this kind of question is asked all the time -- but not by YOU, or YOUR specific circumstance -- so no apology is necessary! :)

First of all, the motor current capacity is 35A for 30 seconds -- that is the BURST rating of the motor, and should NOT be used for your calculation. Somewhere on the motor specification sheet, the CONTINUOUS current rating will be listed -- that is what we need to use for the calculation.

However, for the sake of this discussion, I will go ahead and use 35 amps as the motor starting point.

So, you are looking for a LiPo to work with that motor. Each LiPo has two numbers that will mean something to you in this situation -- the mAH size, and the "C" rating.

Simply take the mAh size (2100 mAh, for example), and divide by 1000 -- that gives you the AMP-HOUR size of the battery. So, a 2100 mAh LiPo is a 2.1 Ah LiPo.

Then, take the "C" rating of your LiPo, and multiply it by the Amp-Hour (Ah) size of the LiPo -- so a "20C" 2.1 Ah LiPo would result in a 42 AMP maximum current draw from the LiPo.

Does that make sense? Did I answer your question?

In the end, you're going to want to choose a LiPo that will physically fit in your battery bay, AND that will supply enough current to your power system. Be sure to buy a WATT METER (I like the "Watts Up" brand) and test the system before flying -- just to make sure you're not drawing too many amps for the motor and/or LiPo.

See the attachment for more information -- I ran a class on electric power systems at a local club a few months ago. It's probably more than you need for this discussion, but it will be handy for you to have going forward. :)

ceong
04-02-2011, 02:41 PM
Yes, this kind of question is asked all the time -- but not by YOU, or YOUR specific circumstance -- so no apology is necessary! :)

First of all, the motor current capacity is 35A for 30 seconds -- that is the BURST rating of the motor, and should NOT be used for your calculation. Somewhere on the motor specification sheet, the CONTINUOUS current rating will be listed -- that is what we need to use for the calculation.

However, for the sake of this discussion, I will go ahead and use 35 amps as the motor starting point.

So, you are looking for a LiPo to work with that motor. Each LiPo has two numbers that will mean something to you in this situation -- the mAH size, and the "C" rating.

Excellent ..Thank you so much :)

Simply take the mAh size (2100 mAh, for example), and divide by 1000 -- that gives you the AMP-HOUR size of the battery. So, a 2100 mAh LiPo is a 2.1 Ah LiPo.

Then, take the "C" rating of your LiPo, and multiply it by the Amp-Hour (Ah) size of the LiPo -- so a "20C" 2.1 Ah LiPo would result in a 42 AMP maximum current draw from the LiPo.

Does that make sense? Did I answer your question?

In the end, you're going to want to choose a LiPo that will physically fit in your battery bay, AND that will supply enough current to your power system. Be sure to buy a WATT METER (I like the "Watts Up" brand) and test the system before flying -- just to make sure you're not drawing too many amps for the motor and/or LiPo.

Excellent ..Thank you so much :) sure I will get the watt meter from Hobby king

My last question assume if the motor Max. efficiency current is only 35 amp & if the lipo result in a 42 AMP maximum current sure going to kill the motor .. am i right ? Thanks

MustangMan
04-02-2011, 02:46 PM
A battery's "rated" MAXIMUM current is its capacity times the "C" rating.

For example a 1250 mAh, 20 C battery has a maximum current rating 1250 x 20 = 25000 mA = 25 Amps.

This calculation has nothing to do with how much current a particular power setup of battery, ESC, motor, and propeller will actually draw at full throttle (which is the only value that matters when determining if a particular setup is "safe"). It simply gives you a "DO NOT EXCEED" figure for the battery. If you overload a battery it will destroy itself trying to deliver more current than it can handle. Likewise the ESC current rating is a "DO NOT EXCEED" figure. Most ESC's cannot protect themselves from being overloaded and will destroy themselves if you overload them. Likewise the motor's maximum current rating is a "DO NOT EXCEED" figure. But, if you put too much propeller on a motor and/or use too high a voltage battery, the motor will TRY to handle the extra current and die due to overheating in the process. The ONLY protection you have against destroying one or more of your power system components is a balanced power setup design.

If you start with a motor selection, then the ESC and battery must be sized to handle AT LEAST as much current as the motor's maximum rating. Then you select a propeller which will load the motor, at full throttle with the battery you've chosen, to a current level that does not exceed the motor's current rating.

Remember also that if you operate your battery at a current corresponding to say, 20C ( 25 Amp in my example ), you will get at most 3 minutes of full throttle operation with that battery. A motor with a higher mAh capacity is needed if you want to fly longer.

As others will tell you, a watt meter, is an excellent tool for testing your power setup so that you know you're not exceeding the component ratings. If you want to experimentally determine the proper propeller size for a particular motor/ESC/battery combination, use a watt meter and start with an under size propeller. Then work your way up in small steps. A power setup with an under size propeller will be safe and happy. A few seconds with to0 much propeller can mean the death of expensive components.

Hope that helps. Welcome to the addiction of R/C aviation! :D

ceong
04-02-2011, 02:54 PM
A battery's "rated" MAXIMUM current is its capacity times the "C" rating.

For example a 1250 mAh, 20 C battery has a maximum current rating 1250 x 20 = 25000 mA = 25 Amps.

This calculation has nothing to do with how much current a particular power setup of battery, ESC, motor, and propeller will actually draw at full throttle (which is the only value that matters when determining if a particular setup is "safe"). It simply gives you a "DO NOT EXCEED" figure for the battery. If you overload a battery it will destroy itself trying to deliver more current than it can handle. Likewise the ESC current rating is a "DO NOT EXCEED" figure. Most ESC's cannot protect themselves from being overloaded and will destroy themselves if you overload them. Likewise the motor's maximum current rating is a "DO NOT EXCEED" figure. But, if you put too much propeller on a motor and/or use too high a voltage battery, the motor will TRY to handle the extra current and die due to overheating in the process. The ONLY protection you have against destroying one or more of your power system components is a balanced power setup design.

If you start with a motor selection, then the ESC and battery must be sized to handle AT LEAST as much current as the motor's maximum rating. Then you select a propeller which will load the motor, at full throttle with the battery you've chosen, to a current level that does not exceed the motor's current rating.

Remember also that if you operate your battery at a current corresponding to say, 20C ( 25 Amp in my example ), you will get at most 3 minutes of full throttle operation with that battery. A motor with a higher mAh capacity is needed if you want to fly longer.

As others will tell you, a watt meter, is an excellent tool for testing your power setup so that you know you're not exceeding the component ratings. If you want to experimentally determine the proper propeller size for a particular motor/ESC/battery combination, use a watt meter and start with an under size propeller. Then work your way up in small steps. A power setup with an under size propeller will be safe and happy. A few seconds with to0 much propeller can mean the death of expensive components.

Hope that helps. Welcome to the addiction of R/C aviation! :D


Wow..learning something new today :D

i guess back to my previous question if the motor Max. efficiency current is only 35 amp & if the lipo result in a 42 AMP maximum current sure going to kill the motor :eek:

MustangMan
04-02-2011, 03:06 PM
Wow..learning something new today :D

i guess back to my previous question if the motor Max. efficiency current is only 35 amp & if the lipo result in a 42 AMP maximum current sure going to kill the motor :eek:

NO! The battery's maximum means it can handle "Up to this much". The only battery attribute that could harm your motor is if you use too high a VOLTAGE (too many cells). This is because the motor will try to spin faster, thus delivering more power to the propeller, and demanding more current to try to get that power from the battery. This can lead to exceeding the motor's, and ESC's current ratings. This does not happen because the battery is capable of delivering the dangerous current, but because you changed the power system parameters to an operating point which exceeds the current rating of one or more components.

You could use a 6000 mAh, 50 C, rated battery with a motor rated at 2 Amps. The battery is capable of 300 AMPS, but, with the right propeller, the motor will only ask for 2 AMPS and everyone is happy. Of course (if it can actually fly, that's a lot of battery weight) it would be capable of flying for 3 hours!

If your motor is designed for operation with a 3-cell lipo then you can use ANY 3-cell lipo capable of deliver a maximum current at least as high as your motor will demand at full throttle with the propeller you've chosen.

ceong
04-02-2011, 03:09 PM
NO! The battery's maximum means it can handle "Up to this much". The only battery attribute that could harm your motor is if you use too high a VOLTAGE (too many cells). This is because the motor will try to spin faster, thus delivering more power to the propeller, and demanding more current to try to get that power from the battery. This can lead to exceeding the motor's, and ESC's current ratings. This does not happen because the battery is capable of delivering the dangerous current, but because you changed the power system parameters to an operating point which exceeds the current rating of one or more components.

You could use a 6000 mAh, 50 C, rated battery with a motor rated at 2 Amps. The battery is capable of 300 AMPS, but, with the right propeller, the motor will only ask for 2 AMPS and everyone is happy. Of course (if it can actually fly, that's a lot of battery weight) it would be capable of flying for 3 hours!

If your motor is designed for operation with a 3-cell lipo then you can use ANY 3-cell lipo capable of deliver a maximum current at least as high as your motor will demand at full throttle with the propeller you've chosen.

Thanks MustangMan.. understood :cool: it is clear now..million thanks

MustangMan
04-02-2011, 03:15 PM
You're welcome! Happy flying.

Feel free to use the THANKS button! ;););):D