Thanks Don for getting in with an early answer.
Although the MK battery is a Sealed Lead Acid we must remember that it is also an AGM battery so that changes things a little bit. One needs to look at the entire data sheet when evaluating this battery and the significant data is life expectancy vs. depth of discharge. For our applications, there should be another (somewhat intangible) variable added that fudges life expectancy for current maximum draw.
I think we can all agree that this year's game produced the lowest current draw of any game thus far due to the lack of friction for the drive train. However, in those years where robots drive on carpet, we must consider the teams who choose an operating speed that by design draws excessive amounts of current. The battery fully charged is capable of 600+ amps (albeit for short periods of time) and yes teams do approach that figure. A well designed electrical system is capable of delivering near the stall current for each motor on the robot. A CIM motor stall current is 129 amps. Four motors in a drive system=? In a pushing match with manipulators it is not unheard of for teams to deplete a battery in a two minute match. It is for this reason that IFI included a backup battery to keep the control system functional even when the current load pulled the battery below the operating voltage of the control system. Please be advised that sound mechanical design leads to good electrical performance. The new PD designed for 2009 included regulators that are designed to accommodate these voltage fluctuations.
Please also note the amp hour capacity of these batteries is based on discharge rate. 18 AH at 1.8 amps discharge or 6.4 AH at 54 amps. Interpolate that data as you might and a two minute match of average 200 amps will deplete the battery and shorten it's life.
MK is working on a new design for our competition and hoping it will prove to be the FRC battery in the future. Some teams have samples and will be working on testing in the next couple of months.
Capacitor banks simply are not designed for or capable of sustained high current demands without significant reduction in terminal voltage. Using the data in the application notes and product guide, the cap solution at 200 amps would last about 60 seconds. We still need to remember that caps in series cannot simply be added. Five 3000 farad caps in series would result in about 600 farad equivalent capacitance with a five times increase in series resistance. Again using the application notes equations (
http://www.tecategroup.com/app_notes...re-1007239.pdf) we would need an equivalent capacitance of 3,000 farads total or five banks of 5 caps in parallel to achieve the power needed for our robot at 88 amps average. So 25 caps at 0.5 kg is 12.5 kilos or twice the weight and five times the size.
In ham radio battery operations, a common practice is to average battery use by "key down" percentages or the time an operator may actually transmit (high current) to the time one is just receiving (low current). Say in our case, we have a normal 40% key down condition of 200 amps (that is about 45 amps/motor) coupled with a 20 amp key up. We could calculate this to about 88 amps average current which interpolating from the MK graph would still give us 2-3 minutes. With careful driving, software ramp up for speed and more efficient designs a team might be able to get that figure down to 150 amps key down and the result would be 68 amps average which would slide that back towards 4 minutes on the discharge curves. Each time the average current is reduced so is the required charge time. This could be significant in the finals.