


Discussion
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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To answer your question, though, it would take three times as much current to get the same RPMs with the 1cell 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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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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A guy I know actually did the exact opposite of this. He used something like a ninecell system in a smaller warbird with a very, very lowKv motor. It kicked out all the power he needed while drawing only a few amps, but the lack of highvoltage, lowcurrent 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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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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