measuring current draw by motors and brain

Good Morning,

As part of my son’s STEM project, we would like to calculate battery life. As part of the project we would like to use a digital multi meter to measure the amps that a VEX smart motor draws while in use, and also the amps used by the brain. This will be the experimental data that we can compare with our calculated estimate.

Of course, we do not want to destroy the brain, or a smart motor, or anything else by taking these measurements.

Is there a way to cut into a cable to connect a multimeter, or some safe way to take these measurements ? Or is another approach better ?

A private message response is fine with me.


Jeff W

OK, thanks, I may try a more simple approach. Jeff

Something simpler might be charging the battery and then running your program until the battery is dead. You know the power rating of the battery in mAh, you can then use that to find the average current draw per unit of time (maybe hour or seconds) and do it that way.

You could also look at stall current for the motors to get an idea there.

Another way to measure the electrical current draw of the Smart Motor is to use the current sensor built into each motor. Using Modkit or RobotC, the electrical current is available as an input value. Attached is a screenshot of a sample Modkit program that displays the electrical current for the specific motor.


Art, does the controller or any of the sensors offer that functionality as well or just motors?

Currently on the Smart Motors have a built-in current sensor. The other sensors have very low and predictable current draw in comparison to motors, so an alternative method would be needed to measure their current draw.

OK, I programmed the blocks that you kindly posted Art. I loaded the Modkit program into slot 4 of the Vex brain, and I played the program. The Vex brain displays “amps 0.00” but the controller does not drive the robot anymore. What can I do to remedy this?

thanks, Jeff W

never mind. I figured it out. The amps display very nicely. thanks, Jeff W.

OK, we have measured the amps drawn by the smart motors. about 150 milli amps, sometimes more, depending on the load. Now the question has come up - what is the battery discharge rating. While the main VEX battery has a 2000mAH rating and 7.2 volts, there should be a discharge rating as well.

What we find experimentally is that when the battery gets depleted, the brain still operates, and the robot still functions, but it begins to act strangely. Some motors work well while others slow down. I suspect that at this point the battery still can deliver electrons, and it still delivers about 7.1 volts, it can no longer deliver the current required by all of the robot components.

Does anyone know the discharge rating of the VEX main battery? Or help explain this ?

thanks, Jeff