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Old 03-29-2011, 02:35 AM
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Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952

Originally Posted by RCFlyer44 View Post
I am building this Great Planes Super Sportster EP and I purchased the recommended RimFire .25 42-40-1000 Brushless Outrunner Electric Motor and the Silver Series ESC 45A Brushless with 5V/2A BEC.

Both the ESC and Motor say anything from a 2S to 4S works great, and the plane recommends a 3S 3500mAh battery.

Haven't purchased the battery yet and I have plenty of 2200mAh batteries but I was wondering if I am correct in assuming if I use a 4S I would basically increase the RPMs by 3400 RPMs since I am increasing voltage by 3.4 volts and the motor has a 1000kV rating??

Also I know Volts X Amps = Watts and this motor is rated for 666watts according the manual but at 11.1 volts I am going to be right around 500 watts and they are getting their 666Watts using a 4S battery, obviously.

So any reason I should not just use a 4S battery vs. a 3S and what this the right formula for figuring the mAh rating if I do switch?

Also a 4S 2200 mAh battery seems to be a little cheaper (not enough to make a difference to me though) than a 3S 3500 mAh battery.

Or am I out in left field and done jumped the fence thinking this way???
Just a quick note:
Going from a 3 cell battery to a 4 cell battery, and not changing the motor or prop will result in the current input to your motor increasing by perhaps 50%, and your power input to the motor about doubling! (It's that RPM ratio raised to the third power factor)

As others in this thread have indicated, be absolutely certain to monitor everything with a wattmeter while checking out different batteries.
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