[Al Skierkiewicz]

Chris,
I like to calculate worse case items. Most people forget that stall current on motors occurs when the motor has electrical power connected and the motor is not moving. (Like when the motor is starting) It is generally accepted that most teams can wire a CIM such that it can draw between 100 and 116 amps in stall per motor. That number will not tell you how long your battery will last but it will tell you a few things about the electrical system. I have talked about the "wire foot" for some time now. My definition is 1 foot of #10 wire drawing 100 amps will drop 0.1 volt. Basing on that, #12 is about twice (actually 1.6) the series resistance so 1 foot of #12 is 2 wire feet. #6 is about half of the series resistance so 1 foot of #6 is 0.5 wire feet (wf). #14 is almost 3 times the #10 per foot.
The critical factor here is the available voltage then determines the actual current supplied to the load. The 116 amps above is calculated on the loss of a standard run of 4 feet of #10 and the series resistance of the speed controller, battery and #6 wiring. Without the series resistance the motor is speced at 131 amps stall at 12 volts.
To make calculations on how long your battery will last, you will have to come up with a model of how the robot is driven. You might for instance, guess that the robot will normally drive for 10 seconds at 30 amps per CIM then stop for 10 seconds while it is trying to score. Then turn for two seconds and drive for ten seconds and then stop to pickup or defend. Doing worse case again, assume that the 18 amp hour battery under our conditions actually will derate to 15 AH or less. You might be able to back into a calculation on battery life. I have seen teams actually do spreadsheet analysis for this and come pretty close to predicting the actual robot behavior. The critical factor is keeping parts of the control system at levels that will prevent them from rebooting. In the previous control system, 5.5 volts was the critical battery level. At that point, the digital side car would cease functioning and all motor control would stop. At 4.5 volts the power supplies would stop functioning and the cRio and/or the radio would reboot. This drop is due to the series resistance of the #6 wiring, and the internal resistance of the battery (11mohms at full charge).
BTW we had a great time in Marceline. Everyone we met was friendly and helpful.