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Old Feb 02, 2013, 03:49 PM
JimmyKudo is offline
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Originally Posted by Dr Kiwi View Post
Amp draw is determined by prop size.. in your computation, increase your prop size 'til you get to the amp draw you need (but stay within the motor's limit).

ESC's don't limit amps.. they let through what they are capable of letting through... exceed that and they fry.

The low current to the motor indicates that the pack cannot deliver what that calculator suggests is what the motor/prop would demand, without dropping voltage.

NiMh cells (1.2v-1.4v HOC) typically can maintain only about 1v under load... in the same way Lipo cells (4.2v HOC) will be at 3.5v-3.8v under load.
Thanks for your help!

Originally Posted by Lnagel View Post
The calculator shows the following data:
Battery pack voltage = 12.5 volts
Cell Resistance = 0.022 ohms -- ten cells in parllel would be 0.22 ohms
ESC resistance = 0.0065 ohms
Motor current = 14.4 amps

The battery resistance and the ESC resistance are in series with the battery and the motor so the current through them is the same as the motor current and the voltage drop across them will be subtracted from the battery voltage. The voltage drop across the battery resistance and the ESC is calculated by multiplying their combined resistance by the current. So their combined voltage drop would be 0.2265 ohms x 14.4 amps = 3.26volts. Subtracting that from the 12.5 volts battery voltage leaves 9.24 volts to the motor.

The same applies. with 8 cells battery voltage would be around 11 volts. 0.1825 ohms times 11.8 amps = 2.15 volts leaving 8.6 volts or so to the motor.

As long as the motor is drawing current the voltage being applied to it will always be lower than the battery full voltage because of these series resistances. The higher the current, the greater the difference and the higher the battery and ESC resistances, the greater the difference.

Thank you for the clarification Larry; I'll keep resistances in mind when shopping for ESC's and batteries, if that even applies. Much appreciated!
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