[Al Skierkiewicz]

Jose,
The current density on small solar cell arrays is very small so your device would need to be in sun for quite a while to charge the caps. You could use either a relay or an electronic switch that would change over to the fuel cell after the caps discharge. You could make the control using a timer or a comparator. You need to see how long the car can run on just the caps and then a 555 timer might work just fine. I am just brainstorming here so it might not work but you get the idea. A 5 volt relay across the series combo of the caps and fuel cell would drop out at 3-4 volts. Just be sure to get a low current coil on the relay.