paper: Battery Voltage in Robot Drivetrain Simulation and Modeling

[apalrd]

Originally Posted by Al Skierkiewicz

Andrew,
Nice paper! If you look at the battery voltage graph and compare to acceleration you will get an idea on what might be taking place. As the load changes, the current drawn by the motors change, and so to the voltage drop across the internal resistance of the battery. Of course, teams should also be aware that using other motors in the robot (at the same time as driving), also modifies the voltage drops and available current. The length and wire size also have a great effect on the current available to the motors. Longer and smaller diameter wire, adds significant resistance to the individual motors. At 100 amps, #10 wire will drop 0.1 volt per foot. So if the motor is wired with two feet of wire, the drop will be 0.4 volts since you must take into account both the red and black paths. If a team uses #12 for the motor path, double that loss.

I eventually measured (sorta) the resistance of a battery using the Battery Beak. It told me the exact battery I measured (plus it's half of the cable and connector) had an internal resistance of 0.021 ohms. I had already picked 0.03 because that number matched the actual data fairly well. I think the extra 0.009 ohms accounts for some of the voltage drop in electrical components, as we would see ~0.004 ohms from the #10 wire alone (by Al's numbers). The simulation uses 12v as the base battery voltage, a good moderately charged battery is usually higher also.

Al, at 400a, what are the losses of a #6 vs #4 battery cable? We always keep the length short (never adding length to the 12" COTS cable and using a ~6" wire between Main Breaker and PD), and wondered if switching to #4 on one or both ends would help.