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Jul 09, 2019, 10:53 AM
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Motor/Battery combo for long range


Hi,

I have a 6" quad built with 2206/1750kv motors and t-motor f55a esc. What would be the best battery to combine with those motors for efficient/long flight time? Would I be better to use a 3S/4000kv or to the other end and use something like a 6S/1800kv? Or should I choose a different motor completely? What flight time should I expect on a 6" quad?

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Jul 10, 2019, 09:48 AM
Registered User
Is not only about battery or motor, but the thinking of the platform.
Those intended for racing have other setups than those for endurance.
You don't make a Corvette more mileage than a Diesel van, only by increasing the tank capacity
Just look at Parrot Bebop 2 30 minutes machine, and try to find where is different than yours:
- 500 grams AUW
- 3350mAh 3s, about 200 grams pack
- 1600kv motors
- 3 blades 6" policarbonate props

Especially the parts weights ratios is what make a platform behave different than other.
The above design is what I call "rule of fifths": frame, payload and propulsion are each roughly 1/5 of the total weight, and battery is the rest, 2/5. Any platform, of almost any size, that obey this rule, will be capable of 30 minute flights, at about 30-40 kmph speed, which is the sweet spot for maximum translational lift effect, i.e. the speed where power consumed is smaller than hovering.

Not the last, forget recipes based on high cells count, as 6s, motors loves current to be efficient, not high voltage. The lower losses mantra is stupid, is about gaining 1% because is visible, but ignoring 10x more losses in magnetic fields, because can't be seen and measured easily, and need physics knowledge to be understood without being obvious, as are resistance losses.
Jul 10, 2019, 11:42 AM
We are not men, we are DEVO 7e
xanuser's Avatar
Quote:
Originally Posted by renatoa
Is not only about battery or motor, but the thinking of the platform.
Those intended for racing have other setups than those for endurance.
You don't make a Corvette more mileage than a Diesel van, only by increasing the tank capacity
Just look at Parrot Bebop 2 30 minutes machine, and try to find where is different than yours:
- 500 grams AUW
- 3350mAh 3s, about 200 grams pack
- 1600kv motors
- 3 blades 6" policarbonate props

Especially the parts weights ratios is what make a platform behave different than other.
The above design is what I call "rule of fifths": frame, payload and propulsion are each roughly 1/5 of the total weight, and battery is the rest, 2/5. Any platform, of almost any size, that obey this rule, will be capable of 30 minute flights, at about 30-40 kmph speed, which is the sweet spot for maximum translational lift effect, i.e. the speed where power consumed is smaller than hovering.

Not the last, forget recipes based on high cells count, as 6s, motors loves current to be efficient, not high voltage. The lower losses mantra is stupid, is about gaining 1% because is visible, but ignoring 10x more losses in magnetic fields, because can't be seen and measured easily, and need physics knowledge to be understood without being obvious, as are resistance losses.
wasn't it you that posted that spreadsheet for calculating all that stuff?

https://www.rcgroups.com/forums/show...3&d=1509112906
Latest blog entry: MY notes/ Ascent 2"build notes
Jul 10, 2019, 03:20 PM
Registered User
Thread OP
Quote:
Originally Posted by renatoa
Is not only about battery or motor, but the thinking of the platform.
Those intended for racing have other setups than those for endurance.
You don't make a Corvette more mileage than a Diesel van, only by increasing the tank capacity
Just look at Parrot Bebop 2 30 minutes machine, and try to find where is different than yours:
- 500 grams AUW
- 3350mAh 3s, about 200 grams pack
- 1600kv motors
- 3 blades 6" policarbonate props

Especially the parts weights ratios is what make a platform behave different than other.
The above design is what I call "rule of fifths": frame, payload and propulsion are each roughly 1/5 of the total weight, and battery is the rest, 2/5. Any platform, of almost any size, that obey this rule, will be capable of 30 minute flights, at about 30-40 kmph speed, which is the sweet spot for maximum translational lift effect, i.e. the speed where power consumed is smaller than hovering.

Not the last, forget recipes based on high cells count, as 6s, motors loves current to be efficient, not high voltage. The lower losses mantra is stupid, is about gaining 1% because is visible, but ignoring 10x more losses in magnetic fields, because can't be seen and measured easily, and need physics knowledge to be understood without being obvious, as are resistance losses.
Good info. thank you.

I am at 350g without a battery, that includes gps on a mast and 6040 tri-blades. The only place I could lose significant weight would be the frame and, at this stage in the game, that will have to wait for the next build. I will test out flight times on various batteries and see what works best for my setup.

Thanks again.
Jul 11, 2019, 03:38 AM
Registered User
Quote:
Originally Posted by xanuser
wasn't it you that posted that spreadsheet for calculating all that stuff?

https://www.rcgroups.com/forums/show...3&d=1509112906
The link led to a zip archive, containing a text file, not a spreadsheet.
The text is indeed an excerpt from one of my posts.
Is there anything that collide with the post above ?
Jul 11, 2019, 03:49 AM
Registered User
Quote:
Originally Posted by dahos
Good info. thank you.

I am at 350g without a battery, that includes gps on a mast and 6040 tri-blades. The only place I could lose significant weight would be the frame and, at this stage in the game, that will have to wait for the next build. I will test out flight times on various batteries and see what works best for my setup.

Thanks again.
GPS on a mast is one of the most stupid things first introduced by DJI, and herd followed by a lot of hobbyists.
Horrible aerodinamics, platform CG unbalancing, this is what they got, without any benefit.
Mast is not needed at all, GPS antena listen to the sky, anything that comes from bottom or surrounding is ignored. Especially in a deck style carbon frame made from plates, where GPS is on top of the upper deck, and electronics are buried between plates, the upper plate acts as a shield, cutting all the potential interference.
The 5.8G video stream is not an interference for GPS.
Actually, the real reason of the mast wasn't originally for GPS purpose, but for magnetometer/compass ! which was wrong placed in the same case with the GPS, they must be separated, and magnetometer placed on the platform bottom, not top !
Is one of the most horrible design mistakes of this kind, that influenced everything was been built for several years, until they finally separated the GPS from compass.
Jul 11, 2019, 05:20 AM
Registered User
Woga65's Avatar
Quote:
Originally Posted by dahos
Good info. thank you.

I am at 350g without a battery, that includes gps on a mast and 6040 tri-blades. The only place I could lose significant weight would be the frame and, at this stage in the game, that will have to wait for the next build. I will test out flight times on various batteries and see what works best for my setup.

Thanks again.
Just to illustrate the correlation between weight, propeller diameter and battery size.

Dry weight approximately 383g to 387g.
Propeller size 14.57 inch.
400 KV motors.

4s 450mAh: 16 mins
The first quadrocopter from the early 2000’s - the Intellicopter upgraded to BL in 2016 (1 min 26 sec)


It can hover 80 mins with a 4s2p 6000 mAh battery.

With a 4s3p 9000 mAh battery it can hovers just a little bit longer: 87mins

I did not test it, i guess the optimum would be 300KV motors and 3s3p or 3s 4p.
Jul 11, 2019, 08:01 AM
Registered User
Thread OP
Quote:
Originally Posted by renatoa
GPS on a mast is one of the most stupid things first introduced by DJI, and herd followed by a lot of hobbyists.
Horrible aerodinamics, platform CG unbalancing, this is what they got, without any benefit.
Mast is not needed at all, GPS antena listen to the sky, anything that comes from bottom or surrounding is ignored. Especially in a deck style carbon frame made from plates, where GPS is on top of the upper deck, and electronics are buried between plates, the upper plate acts as a shield, cutting all the potential interference.
The 5.8G video stream is not an interference for GPS.
Actually, the real reason of the mast wasn't originally for GPS purpose, but for magnetometer/compass ! which was wrong placed in the same case with the GPS, they must be separated, and magnetometer placed on the platform bottom, not top !
Is one of the most horrible design mistakes of this kind, that influenced everything was been built for several years, until they finally separated the GPS from compass.
I am not a fan of the mast either but the top plate only has room to hold my hd camera and the battery. I tried to put the gps on the arm but something, I am assuming the the hd camera since the gps worked again once I removed the camera, interfered with it enough to make it unable to acquire satellites. I could move the battery to the bottom plate, might be more practical...
Jul 12, 2019, 04:08 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Quote:
Originally Posted by renatoa
Not the last, forget recipes based on high cells count, as 6s, motors loves current to be efficient, not high voltage. The lower losses mantra is stupid, is about gaining 1% because is visible, but ignoring 10x more losses in magnetic fields, because can't be seen and measured easily, and need physics knowledge to be understood without being obvious, as are resistance losses.
This nonsense gospel of yours doesn't become any less wrong no matter how many times you repeat it.

After adjusting Kv for a target RPM, BLDC motor efficiency has zero correlation with input voltage. A motor wound for 500Kv will have identical performance on 6S as the same motor wound for 1000Kv running on 3S. Same prop, same RPM, same efficiency... 1/2 Kv, half the current, twice the voltage.

"10x more losses in magnetic fields"? Total BS! Get serious! You can't even offer a plausible mechanism for these mysterious voltage-driven losses, let alone a single iota of proof that they occur, because they only exist in your imagination. Distilled to its bones, your argument is that motor Kv has a determining influence on motor efficiency. This is the opposite of true. Ron van Sommeren would have some extremely stern words for you on this subject.

Designing the system around a higher input voltage means less current flow is required for any given power goal, allowing you to run smaller ESCs and lighter distribution wiring. Less weight = more endurance. The entire industry is shifting in this direction for a damn good reason, and that reason is basic electrical physics.

If high voltage is so inferior, and if high current is so great, why don't you build yourself a 5" race quad around a 2S system and use it to dominate the existing 6S setups?
Latest blog entry: Jeti ESC resto-mod
Jul 12, 2019, 06:46 AM
Registered User
The racer machines, either cars of flying saucers, aren't an example of efficiency, right ?
You are using 6s for oomph, not for efficiency.

Surely you are right here:
Quote:
A motor wound for 500Kv will have identical performance on 6S as the same motor wound for 1000Kv running on 3S.
Same prop, same RPM, same efficiency... 1/2 Kv, half the current, twice the voltage.
... but is not what I meant.
My rant is about an existing setup, where people is advised to go higher voltage, for the existing motor, without changing anything, because will be more efficient, this is what I call false.
Higher voltage means more RPM, so lower throttle for same thrust.
Ron will be happy to explain you why is better to obtain the desired thrust at 70% thr, than 40%... is a known thing since the beginning of electric propulsion, even if not many knows the inner reasons.

Though, regarding the above quote, there is a catch though... there are certain KV ranges favoured by the motor poles count.
You will never have a good efficiency 300kV motor built around a 14 poles core, the same being true for a high RPM motor using a 28 poles core.
Jul 12, 2019, 02:23 PM
Wake up, feel pulse, be happy!
Piece's Avatar
Quote:
The racer machines, either cars of flying saucers, aren't an example of efficiency, right ?
You are using 6s for oomph, not for efficiency.
Motor efficiency matters just as much for the snarling racers as it does for a sedate endurance flyer. Prop efficiency in racers is often sacrificed for the sake of other performance aspects, but motor efficiency governs the amount of input power the motor can tolerate before it melts into slag, and, to a lesser degree of importance, the percentage of that power that is ultimately pumped into the prop. It's true the racers are operating farther down the back side of the efficiency curve than the endurance flyers to achieve their high power-to-weight ratios, but in relative terms, the motor's efficiency is still of critical importance.

Regardless, my comment about a 2S racer was aimed at your fallacious comment about "motors lov[ing] current to be efficient" which is plainly untrue as any dope can see by graphing current versus efficiency, identifying max sustainable current versus max efficiency current on that curve, and then repeating this exercise for various RPM-equivalent combinations of Kv and voltage. The obvious drawback of a colossal ESC is a secondary concern, but also valid (and humorous - imagine a 5" quad with 4x 150A ESCs ).

Quote:
My rant is about an existing setup, where people is advised to go higher voltage, for the existing motor, without changing anything, because will be more efficient, this is what I call false.
OP specifically asked about changing both Kv and cell count to achieve longer flight time. When those variables are variable, a higher cell count is the way to go for all of the reasons I've already identified. I would partially agree with your rant if you qualified it as such in your post, but as it was originally written, you made no distinction between A) optimizing the whole drive system for HV, and B) simply slamming HV into an existing lower-voltage setup.

Quote:
Ron will be happy to explain you why is better to obtain the desired thrust at 70% thr, than 40%... is a known thing since the beginning of electric propulsion, even if not many knows the inner reasons.
Here you are confusing motor efficiency with ESC efficiency, and even then, the conventional knowledge about part-throttle operation does not apply to modern ESCs using synchronous rectification to mitigate the usual drive FET body diode losses. The difference between 100% throttle and 75% throttle has become negligible on the ESC side, and our iron-rich outrunners already do an excellent job of smoothing the PWM voltage within themselves.

Quote:
Though, regarding the above quote, there is a catch though... there are certain KV ranges favoured by the motor poles count.
You will never have a good efficiency 300kV motor built around a 14 poles core, the same being true for a high RPM motor using a 28 poles core.
Even right here in this paragraph, you've conflated Kv with RPM. Different pole counts favor different RPM ranges (with some correlation to physical size/mass), but that's RPM, not Kv. A two-pole design would be a poor choice for a low RPM/high torque motor, just like a 28-pole motor would be a poor choice for high RPM, but there's still no mechanism that allows Kv to directly influence motor efficiency.
Latest blog entry: Jeti ESC resto-mod


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