Quote:
Originally Posted by Al Skierkiewicz
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. |
very interesting, I have some
experience with the maxwell ucaps and you both are correct that they would not be acceptable under weight, size or cost constraints. Though I should say I am working with a company who has an electrolyte chemistry that allows their ultracapacitor nominal cell voltage to be 4.1V (vs 2.5-2.7V) so the number in series is reduced to 3. Additionally, their manufacturing process allows for a range of Ah size Ultracaps by varying their specific area in the cell pouch. They are prismatic cells that should allow for a more compact construction.
That all aside you all may still be correct that the cost may be too great.