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Can somebody help me figure out amps, c rating, and watts

#1
I plan on using a 250kv motor with the max power at 2360 watts and max loading 70amps.....what does this mean? I am using a 3blade 16x10 prop and in case you guys are wondering this is for a 10ft b-25. I want to use a 10s battery setup with it but I need to figure out what esc to use.....how do I calculate all of this?
 
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Balu

Moderator
Staff member
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#2
So if you motor want's to draw 70 A you'd need a 70 A (plus safety) ESC - around 85 A-ish.

There is no "10 V" battery setup. LiPo batteries voltage is defined by the number of cells. Each cell has a nominal voltage of 3.7 V. The closest you are going to get is a 3S (three cell) LiPo at 3 * 3.7 V = 11.1 V.

Now you need to decide how big of a battery your plane can handle. Size comes with weight of course. A somehow standard size is a 2200 mAh (or 2.2 Ah) LiPo. 70 A / 2.2 Ah means it has to have 32 C minimum.

But then your motor will probably not run at full power all the time, so you might get away with less C (but a burst / short term C of 32 as written above).

It's been a while that I have thought about this, so you might want to wait for someone else to confirm it :)

Can you tell us what plane you are going to use the 250 kv motor for?
 

DamoRC

Well-known member
Mentor
#3
I'll have a go.

"Max" -don't exceed these levels if you don't want to fry something. It has often been observed that high quality equipment from reputable manufacturers can exceed the stated "Max" levels and cheaper equipment barely makes it to these specs, so best to play it safe.

Power (Watts) = Voltage (volts) x Current (amps)

So if your max power is 2360 watts and max current is 70 amps, then you can run a max voltage of 2360 / 70 = 33.7 volts

If you are running 10S (nominal 37 volts, max 42 volts) then you are going to need to watch the amp draw of the motor carefully. It is possible that even at 10S, you could run a load (like a really big prop) that could take the motor to 70 amps so you should only use recommended prop range for the motor and check that you are not approaching the 70 amp mark.

If the max current the motor can handle is 70 amp, then typically your ESC needs to be able to handle 70 amps, plus some head room. It sounds like you are not going to overload the motor at 10 volts, so an 80 amp ESC should be fine. If at a later point you want to run higher voltages on the motor and may be approaching the 70 amp max for the motor, you may want to consider a 100A ESC (no point in killing the motor and the ESC).

Lastly for the C rating. C ratings help you calculate the amount of current batteries are capable of delivering. If you try to draw more current from a battery than it is rated for, you will damage it (from hot, to puffy, to explode).

Often the batteries have a C rating and a "Burst" C rating, which is a little higher. This means that the battery can deliver the burst level of current for a short (few seconds) period of time.

You use the C rating and the battery capacity to calculate the maximum current that the battery can deliver safely.

Lets say your 10S battery has a 20 C rating and is 3000 mAh. The calculation for the max current the battery can deliver is:

(3000 mAh * 20C) / 1000 (the divide by 1000 is to convert milli amps to amps) = 60 Amps. You can see from the calculation that higher (and more expensive) C rated batteries can deliver more current and also that bigger capacity batteries can deliver more current.

If your battery has a "Burst" rating of 30C then you can use the same calculation and find that your battery can deliver up to 90 amps for a short period of time.

Hope this helps.

DamoRC
 
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Bricks

Active member
#4
250kv motor with the max power at 2360 watts and max loading 70amps..

First you have to tell us at what voltage are these specs given for before we can go farther and what voltage the motor can be run at.
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#5
250kv motor at 10v is only what, like 2,500 rpms? Thats extremely slow. Even if you had a crazy pitch of 15 on your prop your talking max of like 35mph. Are you sure about your numbers?

At 250kv usually you are up around 8-12s (33v-50v)
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#7
The Amps/watts is a combination of several things. A larger prop is going to use more amps than a small diameter. A high pitch on the prop will take more amps than a small pitch. The motor has a maximum rating for amps and usually most manufacturers will give you a chart with suggestion battery voltage and props with the wattage and thrust they provide. If the motor you are looking at doesn't have this info i would suggest switching to a manufacturer that does, like emax or cobra.
One question i would ask is why are you set on wattage and voltage as your main constraints? Usually when building people are interested simply getting it off the ground (try and get 1:1 thrust), long range flights (lowest amps possible), or super duper fast (really high watt to weight ration).

Heres a good article on the watt to lb ratio for the type of flying you want
https://www.rc-airplane-world.com/watts-per-pound.html
 

rockyboy

Skill Collector
Mentor
#9
I have a 10s battery setup not a 10volt....my bad. I get what you mean though and the equations are helpful.
Umm... yeah - that's a little bit of a difference :p

Kids, don't go plugging a 10s battery into a 10 volt circuit. Bad things will happen. :black_eyed:
 

French

Construire Voler S'écraser Répéter
#10
What’s the capacity of your 10S battery? The C ratings are related to the capacity. But note, there’s no exact standard for C ratings.

For example if you have a 2200mah (2.2A) battery with a rating of 20C, the manufacturer suggests that you can safely draw 44amps from the battery without significantly overloading or sagging the battery. All batteries will sag under high loads, but higher C rates batteries theoretically should sag less. If it’s a 2200mah with a rating of 50C, it may be able to handle 110amp loads.
 

DamoRC

Well-known member
Mentor
#11
Just updated my earlier post to match the new information provided by Pigfarm1403

@Pigfarm - do you have the make / model etc of your motor? - would help a lot. The only motor that I could find that is close to these specs is the Turnigy Aerodrive SK3 - 6354-260KV Brushless Outrunner Motor. But it's 260 not 250kV.

DamoRC
 
#12
Thanks for the help!

I found an electronics setup that will work for my build. Thanks for the help guys I now understand what all of this means. I plan on making a 10ft b-25 Mitchel bomber and bringing it to flite fest. Hope some of you guys see it there!