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Old May 04, 2013, 08:16 PM
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derpron's Avatar
Australia, WA, Perth
Joined Aug 2012
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why use more cells?

i was just thinking,

a 1 meter long cessna with a 1500kv motor, 8x6 prop and a 3 cell

or a 4500kv motor, same prop but with 1 cell

it would have the same KV right?, so whats the point of having more battery cells?
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Old May 04, 2013, 08:38 PM
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Evan D's Avatar
Charlotte, NC
Joined Jan 2004
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You are correct, try it...
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Old May 04, 2013, 08:40 PM
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Kimber's Avatar
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You will need three times the amps to get the same wattage.
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Old May 04, 2013, 08:42 PM
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C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
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Quote:
it would have the same KV right?, so whats the point of having more battery cells?
It wouldn't have the same Kv; it would have the same final RPMs at the prop. The Kv is a physical constant of the motor.

To answer your question, though, it would take three times as much current to get the same RPMs with the 1-cell setup. If your 1500Kv setup pulls 20A on 3S, then the 4500Kv setup would be turning the prop at the same RPMs but it'd be drawing 60A (assuming the motor is identical with Kv being the only difference). You'd need a much bigger/heavier ESC and thicker supply wiring to sustain the greatly increased current. There are also issues related to the logic circuitry of most ESCs since they tend to require more than 4.2V to function - you couldn't just plug a beefy single cell into a normal ESC and make it all work.
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Old May 04, 2013, 09:03 PM
darn you, kakka carrot cake.
derpron's Avatar
Australia, WA, Perth
Joined Aug 2012
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Quote:
Originally Posted by C₄H₁₀ View Post
It wouldn't have the same Kv; it would have the same final RPMs at the prop. The Kv is a physical constant of the motor.

To answer your question, though, it would take three times as much current to get the same RPMs with the 1-cell setup. If your 1500Kv setup pulls 20A on 3S, then the 4500Kv setup would be turning the prop at the same RPMs but it'd be drawing 60A (assuming the motor is identical with Kv being the only difference). You'd need a much bigger/heavier ESC and thicker supply wiring to sustain the greatly increased current. There are also issues related to the logic circuitry of most ESCs since they tend to require more than 4.2V to function - you couldn't just plug a beefy single cell into a normal ESC and make it all work.
ah alright. thanks for that... so it would get a lot hotter too im guessing
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Old May 04, 2013, 09:58 PM
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Joined Dec 2012
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I don't think you would want to spin the prop that fast on a Cessna type airplane also where would you get a single cell battery with that much amperage.
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Old May 04, 2013, 10:04 PM
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Originally Posted by colj00 View Post
ah alright. thanks for that... so it would get a lot hotter too im guessing
probably. amps= heat.

You have to remember, its not volts or amps that a plane needs a certain amount of to fly, its watt's.
You wont hear someone say that you need "12 amps per pound" to fly a certain way. (that really wouldn't tell you anything) Its watts,
Say you have a plane that needs around 200 watts to fly well. You have to decide how to get there.
We know that Volts x amps= watts, so you can choose,

a 1500kv motor on 3 cells, for 11.1v x 20 amps = 222 watts
a 3000lv motor on 2 cells, for 7.4v x 30 amps = 222 watts
a 4500kv motor on 1 cell, for 3.7 x 60 amps = 222 watts

It seems at first glance that any way you go is as good as another, but remember, more amps = more heat, and heat = wasted power.... so the 3 cell setup is more efficient.
Also, a 3 cell battery than can supply 20 amps is going to be cheaper than a 2 cell that can do 30 amps, or a 1 cell that can do 60 amps....
You can also use smaller gage wiring on higher voltage/lower amp setups, saving weight, since more amps require heavier gage wiring to keep from heating up, while you can up the voltage a LOT without creating extra heat... that's one of the reasons they send 10,000 volts over high tension power wires, rather than 110 volts.... the size of the wire needed to handle the current would be.... unmanageable....

When you see a really high powerd 700 size helicopter, you wont see it flying on a 3 cell battery, becuase the required C rating and wiring size would be enormus. its will more likley be flying on 12+ cells, to keep the required C ratings and wiring size to a managable level...
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Old May 04, 2013, 10:32 PM
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Quote:
I don't think you would want to spin the prop that fast on a Cessna type airplane also where would you get a single cell battery with that much amperage
The prop speed would be identical with both setups. The Kv has tripled, yes, but the voltage has been reduced by the same factor of 3. RPM is determined by the Kv AND the voltage. As for the high-current cell, see below for the explanation.

Quote:
so it would get a lot hotter too im guessing
Quote:
It seems at first glance that any way you go is as good as another, but remember, more amps = more heat, and heat = wasted power.... so the 3 cell setup is more efficient.
No, it wouldn't. The 4500Kv motor would have one-third as many turns and three times the effective winding cross-section, so its phase resistance is far lower. Overall copper losses within the motor (heat due to current) would actually end up being about the same. If we were taking the 1500Kv motor and simply jamming three times as much current into it then yes, it would get a heck of a lot hotter, but that's not what's going on here.

Quote:
Also, a 3 cell battery than can supply 20 amps is going to be cheaper than a 2 cell that can do 30 amps, or a 1 cell that can do 60 amps....
Again, no. If we were simply tossing out two cells then this would be accurate, but that would cut the total energy storage down to 1/3 of what we started with which obviously isn't desirable (regardless of the actual output abilities of the cells). Getting that energy storage back means we need a single cell with three times the capacity, which is electrically identical to taking the 3S1P pack we started with and simply resoldering all the cells into a parallel configuration... And, accordingly, the pack's maximum current output would be tripled.
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Old May 04, 2013, 11:12 PM
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3Daddict's Avatar
ohio
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The question I have for you is why? Why do you want to use a 1-cell instead of a perfectly good 3-cell setup?
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Old May 04, 2013, 11:23 PM
darn you, kakka carrot cake.
derpron's Avatar
Australia, WA, Perth
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Originally Posted by 3Daddict View Post
The question I have for you is why? Why do you want to use a 1-cell instead of a perfectly good 3-cell setup?
its an idea.
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Old May 04, 2013, 11:40 PM
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C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
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The question I have for you is why? Why do you want to use a 1-cell instead of a perfectly good 3-cell setup?
To me it didn't sound so much like he was planning to actually use a one-cell system as much as just wondering why multiple cells were used... It's a very valid question, and personally I like to see that there are people who do think about this stuff.

A guy I know actually did the exact opposite of this. He used something like a nine-cell system in a smaller warbird with a very, very low-Kv motor. It kicked out all the power he needed while drawing only a few amps, but the lack of high-voltage, low-current ESCs out there makes it a little impractical at that level. For proof of concept, though, it certainly did work great.
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Old May 04, 2013, 11:45 PM
darn you, kakka carrot cake.
derpron's Avatar
Australia, WA, Perth
Joined Aug 2012
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Quote:
Originally Posted by C₄H₁₀ View Post
To me it didn't sound so much like he was planning to actually use a one-cell system as much as just wondering why multiple cells were used... It's a very valid question, and personally I like to see that there are people who do think about this stuff.

A guy I know actually did the exact opposite of this. He used something like a nine-cell system in a smaller warbird with a very, very low-Kv motor. It kicked out all the power he needed while drawing only a few amps, but the lack of high-voltage, low-current ESCs out there makes it a little impractical at that level. For proof of concept, though, it certainly did work great.
the battery would of weighed a ton though
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Old May 04, 2013, 11:49 PM
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C₄H₁₀'s Avatar
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the battery would of weighed a ton though
How do you figure?
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Old May 05, 2013, 01:09 AM
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Rich in ILM's Avatar
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If we are talking higher voltage into the same resistance doubling the voltage will produce 4 times the power. P=V(sq)/R

4 volts squared = 16/2 ohms = 8 watts
8 volts squared =64/2 ohms = 32 watts

Try it with a wattmeter and a 3s and 4s battery with the same motor and prop. Power goes up around 70%

I know this is a bit off the discussion but I don't know how many times I have heard people expect the power to go up only 25% when they go from a 3s to 4s battery on the same motor and prop.
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Old May 05, 2013, 01:31 AM
darn you, kakka carrot cake.
derpron's Avatar
Australia, WA, Perth
Joined Aug 2012
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Quote:
Originally Posted by C₄H₁₀ View Post
How do you figure?
well cause its 9 cells. it would weigh 9 times a single cell for the same amperage
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