Lithion-Ion Batteries

[Al Skierkiewicz]

OK,
I think it is time to enter a few real world numbers to the discussion both on motors and batteries since a lot of data has been quoted from various sources.

As to the motor controllers now in use, while rated at 40 amps, these devices regularly survive 130 amp peaks when driving CIM motors at or near stall. The Victors are a little better at this than the Jags due to the intelligent monitoring in the Jag for faults. The Jag monitor tries to limit over current by faulting.
While everyone likes to look at the limits of the electrical system via breaker size, all FRC breakers withstand 600% overload for a short periods of time (seconds) without trip and easily handle 200% for several seconds.
First has always attempted to keep some simulated real world limits in the rules to force creative thinking using a defined set of parts. It is for this reason that many "legal parts" choices are made. If a team were to gain an advantage over other teams by simply finding an esoteric part, that while stocked was not generally known or available, that team would have gained a real advantage over other competitors. While this can be viewed as "holding back" some teams, it allows everyone to compete using the same components for producing mechanical power on the robot.
As to the battery choice, the AGM SLA batteries now in use are 1/4 the price of other battery technologies while providing more than twice the peak output current at a fraction of the series resistance of these other technologies. One item that many teams ignore or forget is that our batteries have a defined life (charge/discharge) of 400 cycles. This life is shortened by severe use resulting from high current demands. Robots designed with inefficient mechanical systems will regularly exhaust a battery in one match. While I have no real data, it would seem that these teams shorten that 400 cycle life by 1/3-1/2. While the lithium technologies have some interesting specifications, please look at the series resistance or impedance. Then calculate the internal voltage drop for say a 400 amp spike and the 12 volt battery Li suddenly is making 0 volts at full charge instead of the 8 volts that will be available from the SLA battery now in use.
As to the use of brushless motors, it would be nice to make some types legal in the future, I agree. However they have serious limitations in general use as several people have already discussed. The teams need to be informed on their best use and implementation.