Check my math please

[Ben Diss]

I have a 20 cell battery that shows 29V with no load. Running full throttle the voltage drops to 20V and it pulls 62A. Am I correct that the total resistance of this drive train is 145mOhm?

-Ben

Edit: Corrected typo.

[Viper Pilot]

Ben,

I don't know if the math is correct, but the unit of resistance is ohms.

VP

[Viper Pilot]

R=E/I

R=20/62

R=0.323 ohms

VP

[Ollie]

Resistance is in ohms. The units of current is amperes. There is not enough information to calculate the resistance of the battery, wiring, ESC, and motor.

[Ben Diss]

Ollie- I thought the difference in loaded, vs unloaded voltage would get us there. What more would be needed?

-Ben

[steve lewin]

No. I think you're confusing 2 things. It is possible to calaculate roughly the resistance of the battery by taking the voltage at 2 different load currents. That's what you calculated but it's not a good idea to use zero as one of your load points. It's a lot more accurate to use say 10A and 50A.

However, as Ollie has suggested, that won't tell you anything about the effective resistance of the overall load side of the circuit (motor, esc, wiring etc). The simplest way think of that is just to assume the loaded voltage and divide by the load current as VP has shown.

So the external resistance at that current is approximately 320 milliohms and the effective battery resistance may be somewhere near 145 milliohms, if you're lucky.

HTH - Steve

[Ollie]

The resistance of a running motor is not so simple to measure because the motor generates a counter EMF and there are iron losses to account for in addition to the pure resistance of the windings. Also, there are brush losses if it is a brushed motor, friction losses and there is the power that is transmitted to the propeller. All these losses plus the power output sum to the (voltage across the motor minus the counter EMF) times the current through the motor, if I remember correctly the course in electric motors that I took about 40 years ago.

[Ben Diss]

I see your point. I was measuring voltage with a wattmeter. I think that in itself limits me to measuring the battery alone.

[steve lewin]

For a simple DC motor V/I gives you an accurate *effective" resistance number for a particular load. The point that people miss is that the resistance calculated that way is only accurate at that particular load. I.e. as Ollie points out it is really a complex impedance, nothing like pure resistance.

In particular you can't use that value in a simple Ohms law calculation for different load/voltage points and expect it to be accurate. Just because it takes 62A at 20V doesn't mean it will take 31A if you reduce the volatge to 10V.

If you're really interested in how to calculate motor loads etc there is an excellent set of articles by Jim Bourke on this website. Go to the Ezone main page and search for "Understanding Electric Power Systems". But I should warn you, it contains mathematics.

Steve