50A Accurate Current Sensing Circuit Design
I can not find any circuit that achieves this kind of accuracy.


In practice it is very difficult to get that kind of accuracy (0.01A at 50A is 0.02%) but it isn't necessary. 0.01A resolution is useful at lower currents, and is not difficult to achieve. I did it with a 0.001&ohm; shunt, precision op amp, and an MCU with 10 bit ADC.

Voltage will sag as the battery discharges, so to get accurate efficiency you need to measure Amps, Volts, rpm and torque simultaneously. However all these readings will have ripple, so they should be averaged over a period long enough to integrate the variations that occur at motor commutation speed.

With all the error sources combined you will be lucky to get better than 1% efficiency measurement accuracy, so don't worry if you can only get a current measurement accuracy of 0.2% (0.1A at 50A).