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Lipos in KM16


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#1 ChevyDealer

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Posted 04 February 2009 - 11:51 AM

So the motor only pulls to a certain amperage based on the load that is placed on it, correct? Voltage is constant.  Well, I'm no electrical or battery engineer, but it seems like common sense to me that you want to provide a battery that has enough discharge capability that you can supply the amperage that the motor asks for. Amps is simply flow.

So has it been stated in a round-about way that if you provide too much flow the motor can't handle what it is asking for? It seems that a limit is being placed on the motor for it's own good by placing a stated limit on the battery.  That is fine as long as we are talking about Volts and Amps but storage capcity limitations I'm not following.  Or maybe I am not understanding this correctly.  mAh is mearly a storage capacity rating correct?

Has anyone come up with a constant and peak amperage rating for the motor in these KWA's? Don't we need to know this to properly select a Lipo?  Or what is the wattage rating of the motor since watts (volts multiplied by amps) are the measure of electrical power.  Sorry, but I have always had trouble with someone telling me I can't do such and such without giving me the real reasons and details why?  Thoughts?

Edited by ChevyDealer, 04 February 2009 - 11:53 AM.


#2 WiN&LeGeNd

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Posted 05 February 2009 - 05:21 AM

View PostChevyDealer, on Feb 4 2009, 02:51 PM, said:

So the motor only pulls to a certain amperage based on the load that is placed on it, correct? Voltage is constant.  Well, I'm no electrical or battery engineer, but it seems like common sense to me that you want to provide a battery that has enough discharge capability that you can supply the amperage that the motor asks for. Amps is simply flow.

So has it been stated in a round-about way that if you provide too much flow the motor can't handle what it is asking for? It seems that a limit is being placed on the motor for it's own good by placing a stated limit on the battery.  That is fine as long as we are talking about Volts and Amps but storage capcity limitations I'm not following.  Or maybe I am not understanding this correctly.  mAh is mearly a storage capacity rating correct?

Has anyone come up with a constant and peak amperage rating for the motor in these KWA's? Don't we need to know this to properly select a Lipo?  Or what is the wattage rating of the motor since watts (volts multiplied by amps) are the measure of electrical power.  Sorry, but I have always had trouble with someone telling me I can't do such and such without giving me the real reasons and details why?  Thoughts?

Think of current as water running through a pipe and voltage as kinda like a storage tank. Now suppose there is a pipe(which the water running through symbolizes current flow) and a valve at the end(which symbolizes the motor). No you open the valve partially, now even though that very large storage tank has enough water and gravity provides enough pull on the water to prove much higher flow rates the valve at end of the pipe limits the flow to only what is needed.

So an electric motor will only pull from the battery the current that is needed based on the torque loads it sees, that is typically 18~20 amps in KWA guns with stock gearboxes. More current = more torque, more voltage = higher RPM. The starting torque needed is going to be much higher than the running torque, so that means starting currents are going to be higher than running currents. I'd estimate 18~20 for starting and 10~15amps once the motor is turning.

Also remember that if your battery cannot supply enough current then the voltage is going to start dropping severely and your going to loose quite a bit of electrical energy in heat loss. Its a give and take system, once you exceed the sustainable current draw of a battery, be it LiPo, NiMH, 123A etc.. you will have to take away in an attempt to maintain the current draw, and what you take away is voltage. However if you are seeing those affects your already exceeding the capability's of the power source, its clear that a more capable source is needed or gear reduction in the gearbox in order to lessen torque loads. That is why torque up gears were made, before high current LiPo batteries were in use in airsoft world, only NiMH and NiCD were available, which have much lower max sustainable current capabilities.

Now with LiPo batteries as I explained in the previous thread about LiPo battery ratings in which you post, max continuous current draw is equal to C-rating x Capacity. So again a 4000mah(or 4amp/hrs)@15c = 60 amps continuous where 2000mah@15c = 30 amps continuous. That is because the internal chemical or physical structure of LiPo cells changes with capacity sizes, kinda like a bigger tank has a bigger thicker pipe and a smaller tank has a smaller thinner pipe. If your going to be running a systema magnum and M140+ you may be pulling anywhere from 35~45+ amps! So then a 2000mah@15c would be inadaquit to supply current, you will see your battery heat up, severe voltage sag, hot running motor, low RPM and sluggish response because the electrical system is being over taxed.

However it does not hurt to have higher power battery as long as its the same voltage, even though my zippy flight max can provide 80 amps cont.(4000mah x 20c), it will only give what the motor asks for, so affectively it will last much longer because it is not at its limits. Kinda like the difference between driving 20 miles at 100mph and 50 mph. Going faster requires a substantially greater amount of energy for the same distance, your only variable then is the time difference between the two which does not apply to this the electrical system in this case. So in essence you can't over current your motor unless you ask it to supply more torque than its rated for (i.e making it try to run your car transmission or something where it can't even rotate, in which case all the energy would turn into heat and melt the armature apart since it can't be turned into mechanical energy). But you can over volt it by making it turn so fast that the armature flies apart from centrifugal forces. Which we do have that problem in high powered R/C brush less AC motors running at 60,000 RPM, we simply wrap them in Kevlar belts ;-)

#3 ChevyDealer

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Posted 05 February 2009 - 11:07 AM

View PostWiN&LeGeNd, on Feb 5 2009, 07:21 AM, said:

Think of current as water running through a pipe and voltage as kinda like a storage tank. Now suppose there is a pipe(which the water running through symbolizes current flow) and a valve at the end(which symbolizes the motor). No you open the valve partially, now even though that very large storage tank has enough water and gravity provides enough pull on the water to prove much higher flow rates the valve at end of the pipe limits the flow to only what is needed.

So an electric motor will only pull from the battery the current that is needed based on the torque loads it sees, that is typically 18~20 amps in KWA guns with stock gearboxes. More current = more torque, more voltage = higher RPM. The starting torque needed is going to be much higher than the running torque, so that means starting currents are going to be higher than running currents. I'd estimate 18~20 for starting and 10~15amps once the motor is turning.

Also remember that if your battery cannot supply enough current then the voltage is going to start dropping severely and your going to loose quite a bit of electrical energy in heat loss. Its a give and take system, once you exceed the sustainable current draw of a battery, be it LiPo, NiMH, 123A etc.. you will have to take away in an attempt to maintain the current draw, and what you take away is voltage. However if you are seeing those affects your already exceeding the capability's of the power source, its clear that a more capable source is needed or gear reduction in the gearbox in order to lessen torque loads. That is why torque up gears were made, before high current LiPo batteries were in use in airsoft world, only NiMH and NiCD were available, which have much lower max sustainable current capabilities.

Now with LiPo batteries as I explained in the previous thread about LiPo battery ratings in which you post, max continuous current draw is equal to C-rating x Capacity. So again a 4000mah(or 4amp/hrs)@15c = 60 amps continuous where 2000mah@15c = 30 amps continuous. That is because the internal chemical or physical structure of LiPo cells changes with capacity sizes, kinda like a bigger tank has a bigger thicker pipe and a smaller tank has a smaller thinner pipe. If your going to be running a systema magnum and M140+ you may be pulling anywhere from 35~45+ amps! So then a 2000mah@15c would be inadaquit to supply current, you will see your battery heat up, severe voltage sag, hot running motor, low RPM and sluggish response because the electrical system is being over taxed.

However it does not hurt to have higher power battery as long as its the same voltage, even though my zippy flight max can provide 80 amps cont.(4000mah x 20c), it will only give what the motor asks for, so affectively it will last much longer because it is not at its limits. Kinda like the difference between driving 20 miles at 100mph and 50 mph. Going faster requires a substantially greater amount of energy for the same distance, your only variable then is the time difference between the two which does not apply to this the electrical system in this case. So in essence you can't over current your motor unless you ask it to supply more torque than its rated for (i.e making it try to run your car transmission or something where it can't even rotate, in which case all the energy would turn into heat and melt the armature apart since it can't be turned into mechanical energy). But you can over volt it by making it turn so fast that the armature flies apart from centrifugal forces. Which we do have that problem in high powered R/C brush less AC motors running at 60,000 RPM, we simply wrap them in Kevlar belts ;-)

Thank you very much for taking the time to type all that out.  I enjoyed reading it.  It validated all my thoughts and basic knowledge of the subject.  I have a Zippy FlightMax 4000 mAh 20C and a Zippy FlightMax 2200 mAh 15C that will be here before long.




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