[BBray_T1296]

Also note that while the motors are not turning (or struggling to turn), their effective resistance approaches ~0.1ohms (at perfect 12v stall draws 133 amps V=IR). All of the wiring adds some resistance, but basically it means there is significant resistance inside (citation needed) of the battery compared to outside between the poles. This, according to Ampere's law, means that regardless of the voltage difference in the battery's cells, the voltage difference across the terminals is (at least instantaneously) much less than 12v. This all means that the CIMs indeed stall at much less current than you think because batteries are not perfect emf suppliers.

On another point, The same exact robot, under the exact same (light duty!) conditions, with a 4cim drive vs 6cim drive, will pull (ok, slight inefficiencies, sue me) the same exact net current! This is because regardless of the number of motors (with the same robot weight), it take the same amount of energy to maintain a speed, so the total current draw is the same, but the current/motor is less.


That being said, nobody simply adds another motor to their 4CIM setup, they change the gearing for higher speed, etc. This forces the CIMs to work roughly as hard each as each motor in a 4cim drive, with 3/2 the available torque being used to push harder/accelerate faster, etc. This does put considerable strain on the 120 amp main breaker, but not necessarily the 40 amp ones, compared to a 4 CIM variant.

With a reasonably geared 3CIM drive, and perhaps some clever "ramp up" coding, you will have no trouble with the 120amp breaker. On our robot we used a 3CIM ballshifter from VexPro, geared to final speeds of 5.5 and 15.5fps. We were very happy with it, and although the top speed may not have been as high as it could have been, it sure reached that speed quickly, and I would be worried about the decreased acceleration time in a faster drive. Our low gear was also burly enough to deliver the torque we needed, and usable speeds, while keeping the main breaker cool. We had no trouble all season, despite playing an unfortunate amount of defense