Thread: Build Log Wicked powered SR-71
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Old Jun 27, 2012, 12:19 AM
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United States, WA, Ravensdale
Joined Jan 2011
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Originally Posted by Aros View Post
My confusion is that I would think 4S trumps 3S any day of the week on this setup, even at a lower C rating...
I realized that I didn't mention anything about the differences between 3S and 4S.

Here's how I do my calculations -- first, you've got to look at your motors and the kV rating. If you have a 4000kV motor, that means that you are "supposed" to get 4,000rpm out of that motor for each volt of power supplied to the motor.

On LiPo batteries, a fully charged single cell sits at 4.2V. A fully charged 3S battery would max out at 12.6V and a 4S battery would get you up to 16.8V. Obviously, as soon as you throttle up the motors, that voltage starts to drop a bit... which is why you see Don's test data on some of his motors list the voltage that he took his readings at. He also is very clear with the specific fan that he tests his motors with -- and you see that the same motor may have higher Watt or Amp numbers with just a different fan.

Watts = Volts x Amps. So... a motor that's "rated" at 800W on a 4S setup, should pull no more than 47.6A (Amps = 800 Watts / 16.8 Volts) -- but the fan that's attached to the motor is a variable that will change that math a bit. Think of the fan as a drag device that's slowing the rotation of the motor... the number of blades on the fan, or the pitch of the individual blades can cause the motor to "work" harder to achieve the charted KV... which will result in higher power draw (increased Amp draw from the battery).

So... 4S should always result in a higher motor RPM than a 3S setup... but will also result in a higher Amp draw from the battery. With that said, replacing a 3S 2200mAh 20C battery with a 4S 2200mAh 20C battery isn't always going to work. Both batteries can supply 44A continuously... but the 4S battery will spin the motor faster (due to the KV rating of the motor), and increase the Amp draw. If the 3S setup was pulling 42A, and the 4S setup is now pulling 48A, the 4S 2200mAh 20C battery will not be big enough.

Does that help at all???

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