capacitor
If it can, what kind would I get? Two potatoes if that helps. I know you have to have a fair amount of potatoes to power an LED, so outside of having eight potatoes wired up, I thought maybe two could charge a capacitor.
I thought if I could charge a capacitor with a potato or two, then I could let the capacitor charge up and light the LED, since a continuous flow from a couple of potatoes isn't enough to power the LED alone (I have to have a bunch of taters for that.)
You can connect multiple charging sources to the battery using isolating diodes, which prevent current from back-feeding into lower voltage sources. Then you just need to make sure that the combined charging current does not exceed the the battery's rating.
For the mains charger a simple DC power supply and series resistor will do the job. A 7 cell "8.4V" NiMH battery charges up to about 10.5V at the 10 hour rate (200mA for a 2000mAh battery) so you will need at least 12V to maintain a reasonably constant charging current through the resistor and diode.
This circuit shows how it could all be connected:-
simulate this circuit – Schematic created using CircuitLab
Note: most AA NiMH cells are only good for about 2~3A max. 4 servos alone could draw up to 2A, and the motors may draw a lot more. If you must use AA cells then I recommend Sanyo Eneloops. They have a higher discharge current rating than most high capacity NiMH cells, and hold their charge for much longer.
The biggest capacitor I saw was one at 300 Farads, similar to the one below: -
At full charge, the voltage is 2.7 volts. This means it can store an energy of: -
Energy = \$\dfrac{CV^2}{2}\$ - plug in 2.7 volts and 3000F and you get 10935 joules.
This is equivalent to consuming 10.9 watts for 1003 seconds. In reality this won't be achievable due to non-ideal power convertors. You might get half this time before things are pretty much spent and the robot won't move.
This is just an example calculation for you to plug in your own values.
Best Answer
Yes, that should work. Use a large value electrolytic capacitor, like 2200uF or 3300uF (voltage common value 16V, but any voltage would work.) Let it charge for half a minute.
You'll need at least two taters wired in series, and use a red LED, since those only need about 1.7V or so. (Green and blue LEDs need much higher forward voltage.)