[Al Skierkiewicz]

As Richard pointed out the West Mountain tester(CBA-II) will give a graphical display of the discharge characteristic of a battery. It can only discharge the robot batteries at about 7.5 amps but that is enough to show you the state of charge and it gives you the amp-hour rating of the battery. The beauty of these little devices is that for about $100 you get a nice little USB device that will save the data and print battery labels in a 3" cube. West Mountain has a new product (I think at the urging of the FIRST community) that will discharge at much higher currents but runs about $500. The tester will show if individual cells have reduced capacity and has given many teams an indication of a defective battery out of the box. If you are looking for internal resistance measurements there are fairly expensive devices that will do that. I know of a SnapOn product that tests state of charge and internal impedance in a few seconds of testing. It is about the size of a small brief case and the cost is pretty high.

If you are trying to model the discharge of the robot battery in use by somehow comparing or calculating from the amp-hour rating, that will be nearly impossible. There are so many factors that are involved in battery chemistry, discharge rate, peak current, etc. during a match that calculations are fruitless. Wildstang built a current test fixture that we used on several robots during the 2002-2003 seasons. It was able to measure current from several probes on the robot and main battery terminal voltage. The data was ported out through the dashboard port along with time marks provided from the RC so we could correlate current demand against time while watching replays of a the match in which the data was being generated. The data was surprising and we learned a lot about robot current draw but nothing that could lead us into a predictable battery life curve. We could accurately predict that certain robots would fail at some times during a match due to low battery voltage and we could also predict that certain robots would drain the battery by match end. Armed with that info, teams still took no action to correct fatal errors in their design. Thankfully, IFI saw the need to have a backup battery on the RC which didn't correct all problems but reduced the time it took for a robot to recover from a low voltage condition.
One thing to keep in mind at all times, the internal resistance of the battery is 11 milliohms, draw max current (400 amps) from the fully charged battery and the terminal voltage will drop to 7.5 volts. A robot that will draw high current throughout the match will cause that voltage to drop to perhaps as low as 4 volts later in the match when the battery is discharged. 4 volts=robot dead! Take that same battery out of the robot and measure with a DVM and it will read 12 volts, no load, but won't run another match.
Ken, you ought to look into the CBA-II it is a pretty cool device.