Thread: Discussion Why isn't my motor drawing the voltage my battery supplies? View Single Post
Jan 31, 2013, 06:12 PM
Senile Member
Moab, Utah, USA
Joined Apr 2003
6,685 Posts
Quote:
 Originally Posted by JimmyKudo What is the reason for the low voltage draw?
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.

Quote:
 Originally Posted by JimmyKudo When I reduce the pack size to 8 cells, which is still above 9.24V it's "currently" drawing, the voltage goes down to 8.6V and the current goes down 11.8 A give or take.
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.

Larry